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tCanning the process of sealing food in a~ slosed container and heating to destroy~" living organisms

rOOd infection disease caused by active in-0 fection resulting from ingestion of

Ii pathogen-contaminated food'ilifood poisoning (food intoxication): disease caused by the ingestion of food

L "',that contains preformed microbialtoxins~()ol(lspoilage a change in the appearance,", smell, or taste of a food that makes it un-

acceptable to the consumer

29.1 . MICROBIAL GROWTH AND FOOD SPOILAGE. 951

Irradiation the exposure of food to ionizingradiation for the purpose of inhibitinggrowth of microorganisms and insectpests, or to retard growth or ripening

Listeriosis gastrointestinal food infectioncaused by Listeria monocytogenes that maylead to bacteremia and meningitis

Lyophilization (freeze-drying) the proc.-ess of removing all water from frozen fooc!under vacuum

Nonperishable (stable) foods foods oflow water activity that have an extendedshelf life and are resistant to spoilage bymicroorganisms

Perishable foods fresh foods generally ofhigh water activity, that have a very shortshelf life due to potential for spoilage bygrowth of microorganisms

Pickling the process of acidifying food toprevent microbial growth and spoilage

Salmonellosis enterocolitis caused by anyof 1400 serotypes of Salmonella spp.

Semiperishable foods foods of interme-diate water activity that have a limitedshelf life due to potential for spoilage bygrowth of microorganisms

Water activity (awJ the availablity ofwater for use in metabolic processes

icroorganisms are important factors in ourfood supply. A variety of foods that we con-

~ume are produced or enhanced by microbial action. ForIXffillple, dairy products such as cheese, buttermilk, sour"ream, and yogurt are all produced by microbial fer-jonentation. Sauerkraut is a fermented vegetable food.i1~qt products including certain sausages, pates, and~ver spreads are produced using microbial fermentationJechniques. Cider vinegar is produced by lactic acid bac-~~r!a (~ Section 30.10), and alcoholic beverages are

..roduced by fermentation processes using yeast (001::)~i)ection30.13). We discuss the use of microorganisms to_1i9sluceedible foods on an industrial scale in Chapter 30." In this chapter, we will concentrate on the negative~.spects of microbial growth in food. Uncontrolled and

~\N'anted microbial growth destroys vast quantities of~ood, causing significant economic loss as well as a~:jjremendousloss of nutrients. Consumption of food con-'~J.Ilinated with particular microorganisms or microbial1roducts can also cause serious illnesses such as food, 'ections or food poisoning.

FOOD PRESERVATIONAND MICROBIAL GROWTH

!,;~"(:ticroorganismsare ubiquitous in our environment and

~;~nbe found in water, air, and especially in food. Freshlood, most prepared foods, and sometimes even pre-~erved foods are contaminated with microorganisms.~~irstwe discuss the microorganisms that are important

spoilage agents in foods and we then present a varietyof methods used to control their growth and preservethe food supply.

.. Microbial Growth and FoodSpoilage

A wide variety of microorganisms colonize and grow oncommon foods. Many foods provide a suitable mediumfor the growth of microorganisms, and microbial growthusually reduces food quality and availability.

Food SpoilageFood spoilage is any change in the appearance, smell, ortaste of a food product that makes it unacceptable to theconsumer. Spoiled food is not necessarily food unsafe toeat, but in some cases pathogenic organisms may causespoilage. Spoiled food is generally regarded as unpalat-able and will not be purchased or consumed. Foodspoilage causes economic loss to producers, distributors,and even consumers in the form of higher prices andconstricted supply.

Since foods are organic materiat they provide nutri-ents for the growth of a wide variety of chemoorgano-trophic bacteria. The physical and chemical characteristicsof the food determine its degree of susceptibility to mi-crobial activity. With respect to spoilage, foods can be clas-sified into three major categories: (1) perishable foods,including many &esh foods; (2) semiperishable foods,such as potatoes and nuts; and (3)stable or nonperishablefoods such as flour and sugar (Table29.1).These food cat-egories differ largely with regard to moisturecontent,whichis related, as we saw in Section 6.12, to water activity, aw.Nonperishable foods have low water activity and can

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952 .Chapter 29 .FOOD PRESERVATION AND FOODBORNE MICROBIAL DISEASES

Highly perishable Meats, fish, poultry, eggs, milk, mostfruits and vegetables

Potatoes, some apples, and nutsSugar, flour, rice, and dry beans

SemiperishableNonperishable

generally be stored for considerable lengths of time with-out deterioration. Perishable and semiperishable foodsare those with higher water activity. These foods must bestored under conditions that slow or stop microbialgrowth.

Fresh foods are spoiled by a variety of bacteria andfungi, and each type of fresh food is typically colonizedby particular microorganisms (Table 29.2). Because thechemical properties of foods vary widely, different foodsare colonized by the indigenous spoilage organisms thatare best able to use the available nutrients.

For example, enteric bacteria such as Salmonella,Shigella, and Escherichia,all potential pathogens that livein the gut of animals, are rarely implicated in fruit orvegetable spoilage, but often contaminate and spoilmeat. At slaughter, intestinal contents, including theliving bacteria, can leak and contaminate the meat. Like-wise, lactic acid bacteria, the most common microor-ganisms in dairy products, are the major spoilers of milkand milk products. Pseudomonas species are found inboth soil and animals and are thus widely involved inthe spoilage of fresh foods.

Microbial growth in foods follows the standard pat-tern for bacterial growth (~ Sections 6.3 and 6.4). Thelag phase may be of variable duration in a food, depend-ing on the contaminating organism and its previousgrowth history. The rate of growth during the exponentialphase depends on the temperature,the nutrient value of thefood, and other conditions of growth. The time required for

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the population density to reach a significant level in agiven food product depends on both the size of the initialinoculum and the rate of growth during the exponentialphase. Only when the microbial population density reach-es a substantial level are spoilage effectsusually observed.Throughout much of the exponential growth phase, pop-ulation densities may be so low that no effect can be ob-served. Only the last doubling or two leads to observablespoilage (~ Section 6.4).Thus, for much of the period ofmicrobial growth in a food, there is no visible or easilydetectable change in food quality.

./ 29.1 Concept Check

Foods often spoil due to contamination by microorganisms.Foods vary considerably in their sensitivity to microbialgrowth, depending on their nutrient content and water con-tent. Individual categories of food have specific spoilagepatterns and spoilage organisms. Many food spoilage mi-croorganisms are also potential pathogens.

.I List the major categories of food and define them withrespect to water content.

.I Identify at least three bacterial genera that cause bothfood spoilage and human disease.

Ell Food Preservation

We now examine a number of food storage and preser-vation processes that inhibit or stop microbial growth infood, stopping the growth of spoilage microorganismsand human pathogens.

TemperatureBesides moisture, one of the most crucial factors affectingmicrobial growth in food is temperature (OC!:)Section 6.9).In general, a lower storage temperature results in a re-tarded spoilage rate. A number of psychrotolerant (cold

Erwinia, Pseudomonas, Corynebacterium (mainly vegetable pathogens; rarely

spoil fruit)

Aspergillus, Botrytis, Geotrichium, Rhizopus, Penicillium, Cladosporium, Alternaria,

Phytophora, various yeastsAcinetobacter, Aeromonas, Pseudomonas, Micrococcus, AchrorilObacter,

Flavobacterium, Proteus, Salmonella, Escherichia, Campylobacter, ListeriaCladosporium, Mucor, Rhizopus, Penicillium, Geotrichium, Sporotrichium,

Candida, Torula, Rhodotorula

Streptococcus, Leuconostoc, Lactococcus, Lactobacillus, Pseudomonas, ProteusClostridium, Bacillus, Flavobacterium

Saccharomyces, Torula, Penicillium

Fruits and vegetables Bacteria

Fungi;i"I] Fresh meat, poultry, and seafood Bacteria

Fungi

Milk

High-sugar foods

BacteriaBacteria

Fungi

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aThe organisms listed are the most commonly observed spoilage agents of fresh, perishable foods. Genera in bold face include possible human pathogens,

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29.2 . FOOD PRESERVATION. 953

)lerant) microorganisms, however, can survive androw at refrigerator temperatures. Therefore, storage oferishable food products for long periods of time (greaterlan several days) is possible only at temperatures below:eezing. Freezing and thawing alter the physical struc-He of many foods. Therefore, freezing is not an accept-ble preservation method for many fresh foods, but isridely used for the preservation of meats and many~uitsand vegetables. Freezers providing a temperaturef -20°C are most commonly used. At -20°C, storage)r weeks or months is possible, but microbial growthlay still occur in pockets of liquid water trapped with-1the frozen mass. For long-term storage, temperaturesllch as -80°C (dry ice temperature) are necessary. Main-~nance of such low temperatures is expensive and con-equently is not used for routine food storage.

lcidity,nother major factor affecting microbial growth in food; pH or acidity. Foods vary somewhat in pH, but mostre neutral or acidic. Microorganisms differ in their abil-:y to grow under acidic conditions, but conditions ofH 5 or less inhibit the growth of most spoilage organ-,ms (~ Section 6.11). Therefore, acid is often used in)od preservation, a process called pickling. Vinegar,rhich is dilute acetic acid, is usually added in the pick-ng process (vinegar is a fermentation product of thecetic acid bacteria; its industrial production will be dis-ussed in Section 30.10). In addition to vinegar, pickling1ethods usually include the addition of large amounts,fsalt or sugar to decrease water availability (see below)nd further inhibit microbial growth. Common pickled)ods include cucumbers (sweet, sour, and dill pickles),'eppers, meats, fish, and fruits. In some cases, acid canlevelop in the food as a result of microbial action, andhe product is called a fermented food, such as in sauer-:raut (cabbage), yogurt, cheese, and sour cream. The mi-roorganisms involved in food fermentations includehe lactic acid bacteria, the acetic acid bacteria, and thelropionic acid bacteria. These bacteria do not grow'elow about pH 4, so the food fermentation is a self-lim-ting process.

Vater activityVater activity (aw) is the availability of water for use'y microorganisms in metabolic processes. Because mi-roorganisms do not grow under condition of low waterctivity (low water availability), microbial growth can'e controlled by lowering the available water content ofhe food by drying or by adding high concentrations of,solute such as salt or sugar (~Section 6.12). Natur-IIor artificial heat is often used for drying, but the leastlamaging physical method used to dry foods is the>rocessof lyophilization (freeze-drying), where foodslIe frozen and water is removed under vacuum. Milk,

meat, fish, vegetables, fruit, eggs, and other economi-cally important foods are commonly preserved by someform of drying.

A number of foods are preserved by addition of saltor sugar to reduce water activity. Foods preserved byaddition of sugar are mainly fruits (jams, jellies, and pre-serves). Salted products are primarily meats and fish.Sausage and ham are preserved by salt, although indi-vidual products vary in water activity depending onhow much salt is added and how much the meat hasbeen dried.

CanningCanning is a process in which a food is sealed in a con-tainer such as a can or glass jar and heated to kill allliv-ing organisms, or at least to ensure that there will be nogrowth of residual organisms. When the can is proper-ly sealed and heated, the food should remain stable andunspoiled indefinitely, even when stored in the absenceof refrigeration.

The temperature-time relationships for canning de-pend on the type of food, its pH, the size of the contain-er, and the consistency or density of the food. Becauseheat must penetrate completely to the center of the foodwithin the can, heating times must be longer for largecans or very dense foods. Acid foods can often be cannedeffectively by heating just to boiling, 100°C, whereasnonacid foods must be heated to autoclave temperatures(121°C). Unfortunately, heating times long enough toguarantee absolute sterility of every can (~ Section20.1)would change the food so greatly that it would like-ly be unpalatable and lose nutritional value. Therefore,properly canned foods may not be sterile.

The environment inside a can is anoxic, and mi-crobial growth in a canned food is frequently the resultof the growth of organisms that produce extensiveamounts of gas. This can cause pressure build-up in-side the can, resulting in bulges or, in severe cases, evenan explosion of the can (Figure 29.1). Because some ofthe anoxic bacteria that grow in canned foods are toxinproducers of the genus Clostridium (OCt)Sections 21.10and 29.5), food from a can that is visibly altered shouldnever be eaten'oHowever, the lack of obvious gas pro-duction is not an absolute guarantee that the food issafe to consume.

Chemical Food Preservation

A number of chemicals are used commercially to con-trol microbial growth in food. These are classified by theU.S. Food and Drug Administration as "generally rec-ognized as safe" and find wide application in the foodindustry (Table 29.3).Many of these chemicals, like sodi-um propionate, have been used for many years with noevidence of human toxicity. Others, like nitrites (carcino-gen precursor), ethylene or propylene oxides (mutagens;

954 . Chapter 29 . FOOD PRESERVATION AND FOODBORNE MICROBIAL DISEASES

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IIim!EII Changes in cans as a result of microbial spoilage.(a) Normal can; the top of the can is slightly indented due to negative

pressure (vacuum) inside. (b) Swelling resulting from minimal gas-pro-

duction. Note that the top is slightly distended. (c) Severe swelling due

to extensive gas production. (d) The can shown in (c) was dropped and

the gas pressure resulted in a violent explosion, tearing the lid apart.

Sodium or calcium propionateSodium benzoate

Sorbic acid

Sulfur dioxide, sulfites, bisulfites

Formaldehyde (fromfood-smoking process)

Ethylene and propylene oxidesSodium nitrite

Bread

Carbonated beverages, fruit,fruit juices, pickles,margarine, preserves

Citrus products, cheese,pickles, salads

Dried fruits and vegetables;wine

Meat, fish

Spices, dried fruits, nutsSmoked ham, bacon

~ Section 10.4), or antibiotics (development of an-tibiotic-resistant pathogens; ~ Section 20.12), aremore controversial food additives because of evidencethat these compounds may be detrimental to humanhealth.

Because of lengthy and costly testing programs for

any new chemical proposed as a food preservative today,it is unlikely that new compounds will be added to thelist of chemical food preservatives in Table 29.3 in thenear future.

Irradiation

Irradiation of food using ionizing irradiation is now astandard method for reducing contamination by bacte-ria, fungi, and even insects (~Section 20.2). Table 29.4lists some foods for which radiation treatment has beenapproved. Foods such as spices are routinely irradiated.In the United States, fresh meat products such as ham-burger and poultry can now be irradiated to limit con-tamination by Escherichiacoli0157:H7 and other entericpathogens (hamburger) and Campylobacterjejuni (poul-try). For food irradiation, gamma rays are generatedfrom 60Coor 137Cssources. The food products receive acontrolled radiation dose. This dose varies considerablyby each food category and purpose. For example, a doseof 44 kilo Grays (kGys) is used to sterilize meat prod-ucts used on United States NASA space flights and isnearly 10 times higher than the 4.5-kGy dose used forcontrol of pathogens in hamburger (Table 29.4). In theUnited States, a consumer product information labelmust be affixed to foods that are irradiated.


Food microbiology deals with methods for limiting spoilageand the growth of disease-causing microorganisms in foodduring processing and storage. Foods vary considerably intheir sensitivity to microbial growth, depending on theirnutrient content, water availability, and pH. The growth of mi-croorganisms in perishable foods can be controlled by refrig-eration, freezing, canning, pickling, dehydration, chemicalpreservation, or irradiation.

,/' Outline at least four methods of food preservation. Howdoes each method limit growth of microorganisms?

,/' Are-food spoilage microorganisms also pathogens? Giveexamples to support your answer.

II FOODBORNEDISEASES

Failure to adequately decontaminate and preserve foodmay allow the growth of pathogens, resulting in diseaseswith significant morbidity and mortality. Like water-borne diseases, foodborne diseases are common-source

diseases; a single contaminated food50urceJfor instal1\:e

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Dry or dehydrated enzyme preparations (e.g., meat tenderizer)Frozen, uncooked poultry and poultry products

29.4 . STAPHYLOCOCCAL FOOD POISONING. 955

Control of Trichinellaspiralis parasiteInhibition of growth and maturation (ripening)Microbial disinfection

Control of foodbome pathogensExtension of shelf lifeSterilization

Microbial disinfection

Control of foodborne pathogens

aConsumer labeling laws in the United States require that all irradiated foods must be conspicuously labeled "Treated with radiation" or "Treated by irradia-tion" in addition to information required by other regulations.

[at a food-processing plant or a restaurant, may affect a!'number of individuals.

Foodborne Diseasesand Microbial Sampling

A summary of the most prevalent foodborne diseasesand the microorganisms that cause them is shown in[Table29.5. These common diseases can be separated into'.two categories, foodpoisoningand foodinfection.Special-'ized microbial sampling techniques are necessary to iso-late the pathogens responsible for foodborne diseases.

,Food Poisoning"Food poisoning or food intoxication is disease that re-tsults from ingestion of foods containing preformed mi-~crobial toxins. The microorganisms that produced the!toxins do not have to grow in the host and are often not'alive at the time the contaminated food is consumed.;

"Theillness is due to ingestion and subsequent action of~preformed bioactive toxin. We previously discussed"some of these toxins, notably the exotoxin of Clostridium'"botulinum(000 Section 21.10) and the superantigen tox-ins of Staphylococcusaureus (~ Section 22.14).

!Food Infection

tFood infections are active infections resulting from in-1'gestion of pathogen-contaminated food. In addition tothe passive transfer of microbial toxins, food may contain;sufficient numbers of viable pathogens to cause infec-,bon and disease in the host. Food infection is a very com-':montype of foodborne illness (Table 29.5), and Salmonella'food infection is a typical example (see Section 29.6).'Many of these infectious agents also cause waterborne'diseases (~ Chapter 28).

Microbial Sampling of Foods[Asdiscussed in Section 29.1, microorganisms are always!presentin fresh foods. Because pathogens may be present

along with many harmless organisms, methods have beendeveloped to detect important pathogens such asEscherichiacoli 0157:H7, Salmonella, Staphylococcus, andClostridium botulinum. We discussed in Section 24.13 the

use of nucleic acid probes for the detection of specific food-borne pathogens. For growth studies of nonliquid foods,preliminary treatment is usually required to suspend mi-croorganisms embedded or entrapped within the food.The most suitable method is high-speed blending. Thefood should be examined as soon after sampling as possi-ble; ifexamination cannot begin within 1h of sampling, thefood should be refrigerated. A frozen food should bethawed in its original container in a refrigerator and ex-amined or cultured as soon as thawing is complete. Sam-ples can be inoculated onto emiched media, followed bytransfer to differential or selective media for isolation and

identification, as we described for human pathogens (~Section 24.1), or probed directly for pathogen presenceusing nucleic acid-based methods such as the polymerasechain reaction (PCR) (0Ci::ISection 24.13).

./ 29.3 ConcepfCheck

Foodbome diseases include food poisoning resulting from theaction of microbial toxins and food infections due to the growthand invasion of microorganisms in the body. Specialized tech-niques are used to sample microorganisms in food.

./' Distinguish between food infection and food poisoning.

./' Describe the sampling of a solid food such as meat for thepresence of microorganisms.

Ell Staphylococcal Food Poisoning

A very common food poisoning is caused by the gram-positive coccus, Staphylococcusaureus.

Biology and EpidemiologyStaphylococcusaureusand other members of the genusare small,gram-positivecocci(0;10Section12.19).As we

956 . Chapter 29 . FOOD PRESERVATION AND FOOD BORNE MICROBIAL DISEASES -

Bacteria

Bacillus cereus

Campylobacter jejuniClostridium perfringensEscherichia coli 0157:H7

Other enteropathogenic Escherichia coliListeria monocytogenesSalmonella spp.Staphylococcus aureusStreptococcusYersinia enterocoliticaAll other bacteriaTotal bacterial

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Parasites

CryptosporidiumparvumCyclosporacayetanensisGiardialamblia

ToxoplasmagondiiTotal parasites

VirusesNorwalk-like virusesAll other virusesTotal viruses

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Total Annual Foodborne Diseases

27,0001,963,000

248,00063,000

110,0002,500

1,340,000185,00050,00087,000

102,0004,177,500

Rice and starchy foods, high-sugar foodsPoultry, dairyCooked and reheated meats and meat productsMeat, especially ground meatMeat, especially ground meatMeat and dairyPoultry, meat, dairy, eggsMeat, dessertsDairy, meatPork, milk

30,00016,000

200,000113,000359,000

Raw and undercooked meat

Fresh produceContaminated or infected meatRaw and undercooked meat

9,200,00082,000

9,282,000

13,818,500

Shellfish, many other foods

app, food poisoning: PI, food infection. Estimates are based on data provided by the Centers for Disease Control and Prevention, Atlanta, GA, USA.

discussed in Section 26.9, staphylococci are found on theskin and in the respiratory tract of nearly all humans,and are often opportunistic pathogens. Staphylococcusaureus is frequently associated with food poisoning be-cause, as it grows, this organism produces several heat-stable protein enterotoxins (~ Section 21.11). Theenterotoxins are released into the surrounding mediumor food; if food that contains toxin is ingested, gas-troenteritis characterized by nausea, vomiting, and di-arrhea, occurs within 1-6 h.

Each year, an estimated 185,000 cases of staphylo-coccal food poisoning occur in the United States (Table29.5). The foods most commonly involved are custard-and cream-filled baked goods, poultry, meat and meatproducts, gravies, egg and meat salads, puddings, andcreamy salad dressings. If such foods are kept refriger-ated after preparation, they usually remain safe, becauseS. aureus growth is significantly reduced at low temper-atures. Foods of this type, however, are often kept inkitchens at room temperature or outdoors at picnics. Thefood, if inoculated with S. aureus from an infected foodhandler, supports rapid bacterial growth and enterotox-in production. Even if the toxin-containing foods arereheated again before eating, the toxin is relatively heat-stable and may remain active.

PathogenesisStaphylococcusaureusproduces seven different enterotox-ins: A, B,C1, C2, C3, D, and E. Enterotoxin A, a super anti-gen, is most frequently associated with staphylococcalfood poisoning (~Section 22.14). Superantigens workby stimulating large numbers of T cells, which in turn re-lease intercellular mediators called cytokines, activatinga general inflammatory response in the intestine that re-sults in gastroenteritis, including massive loss of fluidsfrom the intestine.

S. aureus enterotoxin A is a small single peptide of30,000 molecular weight that is encoded by a chromo-somal gene. Cloning and sequencing of this gene, theentA gene, and of several other S. aureus enterotoxingenes, show that the toxins are genetically related. Al-though the entA gene is on the bacterial chromosome,type Band C S.aureusenterotoxins may be encoded onplasmids, transposons, or lysogenic bacteriophages. Wediscussed the importance of accessory genetic elementssuch as plasmids and bacteriophages as vectors for toxinproduction in Sections 21.10 and 21.11.

Diagnosis, Treatment, and PreventionSeveral assays based on the detection of either entero-toxin (ELISAdetection of enterotoxin; ~ Section 24.11)

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or S.aureusexonuclease (an enzyme that degrades DNA),. are available to detect dangerous levels of S. aureus in

food. However, these rapid tests are qualitative, con-firming only the presence or absence of S. aureus abovethe detection limits of the assay. To obtain quantitative

."data and determine the extent ofbacterialcontamination,bacterial plate counts are required. For staphylococcalcounts, a high-salt medium (either sodium chloride orlithium chloride at a final concentration of 7.5%) is used.Of the organisms present in foods, staphylococci are theonly common ones tolerant of such levels of salt.

S. aureus food poisoning can be quite severe, but isself-limiting, usually resolving within 48 h after onset.Severe cases may require treatment for dehydration.Treatment with antibiotics is not useful because thedisease is caused by a preformed toxin, not an activeinfection.

Staphylococcal food poisoning can be prevented by,. careful sanitation and hygiene measures both in pro-~.duction and food preparation steps and by storage of

foods at low temperatures to inhibit bacterial growth.Foods susceptible to colonization by S. aureus and kept

rfor several hours above 4°C (refrigerator temperature)should be discarded rather than eaten.

29.4 Concept Check

Staphylococcal food poisoning results from the ingestion ofr preformedenterotoxinA, a superantigen produced by Staphyl-ococcusaureuswhen growing in foods. In many cases, S.aureuscannot be cultured from the contaminated food.

,f Explain the symptoms of staphylococcal food poisoningand the action of enterotoxin A.

,f Will antibiotics effect the outcome or the severity ofstaphylococcal food poisoning? Explain.

!- ClostridialFoodPoisoningiBoth Clostridium perfringens and Clostridium botulinumcause serious food poisoning. Members of the Clostri-,odiumgenus are anaerobic spore formers. Canning andcooking procedures kill living organisms but do not killtspores. Under appropriate anaerobic conditions, thespores germinate and toxin is produced.

Clostridium perfringens Food PoisoningiClostridium perfringens is an anaerobic, gram-positive';spore-forming rod commonly found in soil (Figure 29.2).IJtalso lives in small numbers in the intestinal tract of

:many animals and is therefore found in sewage (00::1Section 12.21). C. perfringens is the most prevalent re-

orted cause of foodpoisoning in the United States, with.anestimated 248,000 annual cases (Table 29.5).

29.5 . CLOSTRIDIAL FOOD POISONING. 957

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The disease results from the ingestion of a large doseof Clostridium perfringens (> 108 cells) in contaminatedcooked and uncooked foods, especially meat, poultry,and fish. Large numbers of C. perfringens can grow inmeat dishes cooked in bulk (heat penetration in thesesituations is often slow and insufficient) and then left at20-40°C for short time periods. Spores of C. perfringensgerminate under anoxic conditions, such as in a sealedcontainer, and grow quickly in the meat. However, thetoxin is not yet present.

After consumption of the contaminated food, theliving C. perfringens begins to sporulate in the intestine,triggering production of the perfringens enterotoxin(OQ:)Table 21.4). The enterotoxin alters the permeabili-ty of the intestinal epithelium, leading to diarrhea and in-testinal cramps, usually with no fever or vomiting. Theonset of perfringens food poisoning begins about 7-15 hafter consumption of the contaminated food, but usual-ly resolves within 24 h, and fatalities are rare.

Diagnosis, Treatment, and PreventionDiagnosis of perfringens food poisoning is made by iso-lation of C. perfringens from the gut or, more reliably, bya direct enzyme-linked immunosorbent assay (ELISA)to detect C. perfringens enterotoxin in feces (~ Sec-tion 24.11). Because C. perfringens food poisoning is self-limiting, treatment is usually not necessary, although an-titoxins are available (~ Section 22.11). Prevention ofperfringens food poisoning requires measures to pre-vent contamination of raw and cooked foods and control

of cooking and canning procedures to ensure proper heattreatment of all foods.

Botulism

Botulism is a severe food poisoning; it is often fatal andoccurs following the consumption of food containingthe exotoxin produced by the anaerobic, gram-positive

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rod Clostridium botulinum. This bacterium normally in-habits soil or water, but its spores may contaminate rawfoods before harvest or slaughter. If the foods are prop-erly processed so that the C. botulinum spores are re-moved or killed, no problem arises; but if viable sporesare present, they may initiate growth and toxin produc-tion. Even a small amount of the resultant neurotoxin

can be poisonous.We discussed the nature and action of botulinum

toxin in Section 21.10 (~ Figure 21.19).Botulinum toxinis a neurotoxin that causes flacid paralysis, usually af-fecting the autonomic nerves that control body functionssuch as respiration and heart beat. At least seven dis-tinct types of botulinum toxin are known. The toxins aredestroyed by high heat (80DCfor 10 min), and so thor-oughly cooked food, even if contaminated with toxin,may be harmless.

Most cases of botulism occur as a result of eatingfoods that are not cooked after processing (Figure 29.3a).For example, nonacid, home-canned vegetables (e.g.,home-canned corn and beans) are often used withoutcooking when making cold salads. Similarly, smokedand fresh fish, vacuum packed in plastic, are often eatenwithout cooking. Under such conditions, C. botulinumspores germinate, and the resulting cells produce toxin.If these foods are consumed, then ingestion of even asmall amount will result in this severe and highly dan-gerous type of food poisoning.

Infant botulism occurs when spores of Clostridiumbotulinum are ingested, often from raw honey (Figure29.3b). If the infant's normal flora is not well developedor if the infant is undergoing antibiotic therapy, the sporesmay germinate and C. botulinum cells may grow and re-lease toxin. Most cases of infant botulism occur between

the first week of life and 2 months of age; infant botu-lism is rare in children older than 6 months when the nor-mal intestinal flora is more developed (~ Section 21.4).

All forms of botulism are quite rare and usually lessthan 150 total cases occur each year in the United States,but up to 25% are fatal. Death occurs from respiratoryparalysis or cardiac arrest due to the paralyzing action ofthe botulinum neurotoxin (~Section 21.10).

Diagnosis, Treatment, and PreventionDiagnosis is by demonstrating toxin in patient serum, orby finding toxin or Clostridium botulinum in suspectedfood products. Laboratory findings are coupled with clin-ical observations including neurological signs of local-ized paralysis (impaired vision and speech) beginning18-24 h after ingestion of contaminated food. Treatmentinvolves administration of antitoxin (~ Section 22.11)and mechanical ventilation. Prevention requires main-taining careful controls over canning and preservationmethods and heating susceptible foods to destroy spores,or boiling for 20 minutes to destroy the toxins.

-- Laboratory-confirmedcases- NationalNotifiableDiseases

Surveillance System (NNDSS) data

enQ)encuu-0Q)t0Q.Q)a:

110

10090807060504030201001979 1984

Outbr~ak caused bysauteed onions, IL

j

Outbreak causedby fermented

"hit'" Pi"C'" AK

Outbreak caused bybaked potatoes, TX

+

19941989 1999

Year

(a) Foodborne botulism

enQ)encuu-0Q)t0Q.Q)a:

110

100

90

80

70

6050

40

30

20

10

01979 1984 1989 1994 1999

Year

(b) Infant botulism

~e incidence of botulism in the United States. (a)Foodborne botulism. In years with high numbers of cases, major out-breaks that account for the increase are indicated. (b) Infant botulism.

More than half of the cases of infant botulism in the United States

occur in California. Data are from the Centers for Disease Control and

Prevention, Atlanta, GA, USA.

In infant botulism, C. botulism and toxin are oftenfound in bowel contents. Infant botulism is usually sel£-limiting and most infants recover with only supportivetherapy, such as assisted ventilation. Occasional deathsmay occur due to respiratory failure. Honey may be asource of C. botulinum spores. Therefore, feeding honeyto children under 2 years of age is generally discouraged.


Clostridiumfood poisoning resultsfromingestionof toxinspro-duced by microbial growth in foods or due to microbial growthand toxin production in the body. Perfringens food poisoningis quite common, and is usually a self-limiting gastrointesti-nal disease. Botulism is a rare but very serious disease, with

significant mortality.

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40,000 al

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./ Describe the events that lead to perfringens food poison-ing. What is the likely outcome of the poisoning?

./ Describe the events that lead to botulism. What is thelikely outcome of botulism?

Ell Salmonellosis

Although sometimes called food poisoning, salmonello-sis is a gastrointestinal disease due to foodbome Salmo-nellainfection. Symptoms begin only after the pathogencolonizes the intestinal epithelium.

Biology and EpidemiologySalmonella spp. are gram-negative facultative aerobicrods related to Escherichiacoli, Shigella spp., and otherenteric Bacteria (~ Section 12.11). Salmonella normallyinhabit the gut of animals and are thus found in sewage.Virtually all Salmonellaare pathogenic for humans: One,S. typhi, causes the serious human disease typhoid fever,

~.but is fortunately very rare in the United States, withmost of the 500 foodbome cases imported from othercountries. However, a number of Salmonella speciescause foodbome gastroenteritis. In all, over 1400 sero-

. types of various Salmonellaspecies are known to be path-, ogenic for humans. S. typhimuriumis the most commoncause of salmonellosis in humans. The incidence and

prevalence of reportedsalmonellosis has been very steady! over the last decade, with about 45,000 documentedleases each year (Figure 29.4). However, less than 4% ofithe total cases of salmonellosis are probably reported,

30

c:0

~ 251'5

a.0a.0 2000

1'0'.0~:: 15

Q)a.mQ)gj 10

it)

n~'.t8. 5Q)a:

.- Outbreakcausedby contaminatedpasteurized milk, IL

01969 1974 1979 1984 1989 1994 1999

Year

.~ The prevalence of reported cases of salmonellosisin the United States. The total number of reported cases is between

40,000 and 45,000 per year. Epidemiologic investigations suggest that10nlyabout 3% of all cases of salmonellosis are properly identified andreported. Data are from the Centers for Disease Control and Preven-tion, Atlanta. GA, USA.

29.6 . SALMONELLOSIS. 959'"

and estimates place the actual number at over 1,300,000cases of salmonellosis every year (Table 29.5).

The ultimate sources of the foodbome salmonellasare the intestinal tracts of humans and warm-blooded

animals. The organism may reach food by fecal contam-ination from food handlers. Food production animalssuch as chickens and cattle may also harbor Salmonellastrains that are pathogenic to humans and may pass thebacteria to finished fresh foods such as eggs, meat, anddairy products. Salmonellafood infections are often tracedto products such as custards, cream cakes, meringues,pies, and eggnog made with uncooked eggs. Other foodscommonly implicated in salmonellosis outbreaks aremeats and meat products such as meat pies, cured butuncooked sausages and meats, poultry, milk, and milkproducts.

PathogenesisThe most common salmonellosis is a Salmonella-induced

enterocolitis.Ingestion of food containing 105-108 viableSalmonella results in colonization of the small and largeintestine. Onset of the disease occurs 8-48 h after inges-tion. Symptoms include the sudden onset of headache,chills, vomiting, and diarrhea, followed by a fever thatlasts a few days. The disease normally resolves withoutintervention in 2 to 3 days. However, even after recovery,patients shed Salmonellain feces for several weeks. Somepatients recover and remain asymptomatic, but shedorganisms for months or even years, resulting in a chron-ic carrier condition (~ the box, the Tragic Case of Ty-phoid Mary, Chapter 25).

Salmonellosis may also cause septicemia, a bloodinfection, enteric fever, or typhoidfever, a disease characr-terized by systemic infection and high fever lasting sev-eral weeks. Mortality can approach 15% in untreatedtyphoid fever.

Diagnosis, Treatment, and PreventionDiagnosis is made by observation of clinical symptoms,history of recent food consumption, and by culture ofthe organism from feces. Several selective media areavailable (~ Section 24.2), and tests for the presenceof Salmonellaare commonly done on animal food prod-ucts, such as raw meat, poultry, eggs, and powderedmilk, because Salmonella from production animals is theusual source of food contamination.

For enterocolitis, treatment is usually unnecessary,and antibiotic treatment does not shorten the course ofthe disease or eliminate the carrier state. Antibiotic treat-

ment, however, significantly reduces the length andseverity of septicemia and typhoid fever. Mortality dueto typhoid fever can be reduced to less than 1% with ap-propriate antibiotic therapy.

Cooked foods heated to 70°C for at least 10 minutes

are considered safe if consumed immediately, or if held

..


at 50°C or stored at lOoCor less. Cooked or canned foods

that become contaminated by an infected food handlercan support the growth of Salmonellaif the foods are heldfor long periods of time without heating or refrigeration.Salmonella infections are more common in summer than

in winter, probably because warm environmental con-ditions are more favorable for growth of microorgan-isms in foods (Figure 29.4).

Although local laws and enforcement vary, becauseof the lengthy carrier state, infected individuals are oftenbanned from work as food handlers until their feces arenegative for Salmonella in three successive cultures.

,/ 29.6 Concept Check

Salmonellosis, an extremely common foodborne infection, re-sults from infection with ingested Salmonella spp. Salmonellacan enter the food chain via production animals or foodhandlers.

./ Describe the three kinds of salmonellosis food infection.Which is most common?

./ How might Salmonellacontamination of production ani-mals be contained?

Ell Pathogenic Escherichia coli

Several strains of Escherichiacoliare potential foodbornepathogens. All pathogenic strains act first on the intestineand several are characterized by their ability to producepotent enterotoxins (~Section 21.11).

Biology, Epidemiology, and PathogenesisEscherichiacoli is a common inhabitant of the animal gut.The short, gram-negative rods are classified as entericBacteria (GQ::;,Section 12.11). There are about 200 knownpathogenic E. coli that can cause life-threatening diar-rheal disease and urinary tract infections. The patho-genic strains are divided into several categories, basedprimarily on the toxins they produce and the diseasesthey cause.

Enterohemorrhagic E. coli(EHEC) produce verotoxin,an enterotoxin similar to one produced by Shigelladysen-teriae, the Shiga toxin (0fJt:\Table 21.4). After ingestion offood or water containing one particular EHEC strain, E.coli 0157:H7, the organism grows in the small intestineand produces the verotoxin. Verotoxin causes both hem-orrhagic (bloody) diarrhea and kidney failure. E. coli0157:H7 causes at least 60,000 infections and 50 deathseach year from foodborne disease (Table 29.5) and is aleading cause of kidney failure in children. The mostcommon cause of this infection is the consumption ofcontaminated uncooked or undercooked meat, particu-larly mass-processed ground meat. In several major out-breaks involving E. coli 0157;H7-infected ground beef,

regional distribution centers were the source of the con-taminated meat and the infected product caused diseasein several states. Another outbreak involved processed,cured, but uncooked beef in ready-to-eat sausages. Themajor source of contamination was the beef, and the E.coli 0157:H7 probably originated from the slaughteredbeef carcasses. Since E. coli O.157:H7 grows in the in-testines and is found in fecal material, it is also a poten-tial source of waterborne disease. There have been several

cases of serious E. coli 0157:H7 infections from fecallycontaminated public swimming areas.

Diarrheal disease often occurs in children in develop-ing countries. It also occurs as "traveler's diarrhea," an ex-tremely common enteric infection causing watery diarrheain travelers to developing countries. The primary causalagents are the enterotoxigenicEscherichiacoli(ETEC).TheETEC strains make two heat-labile diarrhea-producing en-terotoxins. In studies done with U.S. citizens traveling inMexico, the infection rate with ETEC is often greater than50%.The prime vehicles are foods such as fresh vegetables(for example, lettuce in salads) and water. The very high in-fection rate in travelers is due to contamination of localpublic water supplies. The local population is usually re-sistant to the infecting strains, undoubtedly because theyhave lived with the agent for a long period of time. Secre-tory antibodies (~ Section 22.8)present in the bowel mayprevent successful colonization of the pathogen in localresidents, but the organism readily colonizes the intestineof a nonimmune person, causing disease.

EnteropathogenicE.coli (EPEC)cause diarrheal dis-eases in infants and small children, but does not cause in-vasive disease or produce toxins. Enteroinvasive E.coli(EIEC) strains cause invasive disease in the colon, pro-ducing watery to bloody diarrhea. The cells are takenup by phagocytes, where they escape lysis in the phago-lysosomes (~ Section 22.2), grow in the cytoplasm,and move into other cells. This invasive disease causes

diarrhea and is common in developing countries.

Diagnosis, Treatment, and PreventionThe general pattern established for diagnosis, treatment,and prevention of infection by Escherichiacoli 0157:H7reflects the current procedures used for all of the variouspathogenic strains. Diagnosis of infection by Escherichiacoli0157:H7 involves culture from the feces and identi-

fication of the 0 and H antigens and toxins by serology(~ Sections 4.9 and 24.7). Subtyping of strains is alsodone using molecular methods such as restriction frag-ment length polymorphism (RFLP) and pulse field gelelectrophoresis (~ the box, DNA Fingerprinting,Chapter 31).Escherichiacoli0157:H7 illness is a nation-ally reportable infectious disease (~ Table 25.3).

Treatmentof allpathogenicE.coliinfectionsinvolvessupportive therapy and, in severe cases, antimicrobialdrugs to shorten and eliminate infection.

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Campyidomes'

'-,29.9 . LISTERIOSIS. 961

The most effective way to prevent infection withhoodbome enteropathogenic E. coli 0157:H7 is to make

~sure that meat is cooked thoroughly, which means that~it should app~ar gray or brown a~d juices shoul~ be'clear. As we dIscussed above (SectIon 29.2), the Umted~States has approved the irradiation of ground meat as~an acceptable means of eliminating or reducing food in-~fectionbacteria, largely because E. coli0157:H7 has beenI'implicated in several foodbome epidemics. Penetrating

~radiation is considered the only effective means to en-

I

,sure decontamination after the grinding process because;grinding may distribute the pathogens throughout themeat, not simply on the surface. In general, proper food

mhandling, water purification, and hygiene habits will~prevent the spread of pathogenic E. coli.Traveler's diar-

i;rhea can be prevented by avoiding local water sources, and fresh foods.

,/ 29.7 Concept Check

,Enteropathogenic Escherichiacolican cause serious food infec-hions. Specific measures, such as radiation of ground beef, have, .tcbeenlmplemented to curb spread of these pathogens.

Describe the pathology of enteropathogenic Escherichiacoli food infection. What is the likely outcome?

How might Escherichia coli contamination of productionanimals be prevented?

IEII Campylobacter

!Campylobacter spp. cause the most prevalent bacterialfood-JJ10rneinfections in the United States.

!Biology and Epidemiology

~Campylobacterspecies are gram-negative, motile, curved~rodsthat grow at reduced oxygen tension, that is, as mi-~croaerophiles (~ Section 12.14). Several pathogenic!species, Campylobacterjejuni, C. coli,and C.fetus, are rec-

fpgnized. C. jejuni and C. coliaccount for nearly 2 million~annual cases of bacterial diarrhea (Table 29.5). Campylo-!Pacterfetus is economically important because it is a major[~auseof sterility and spontaneous abortion in cattle and!,sheep.

Campylobacteris transmitted to humans via contam-~inatedfood, most frequently in poultry, pork, raw clams,land other shellfish, or in surface waters not subjected to

~

"

,

F

.

"

",

hlorination. C. jejuni is a normal resident in the intesti-, al tract of poultry, and virtually all chicken and turkey[:arcasses contain this organism. Beef, on the other hand,is rarely a vehicle. Campylobacter species also infect do-

estic animals such as dogs, causing a milder form of di-farrhea than that observed in humans. Infant cases of

fF,

..'

"

ampylobacterinfection are frequently traced to infected~domesticanimals, especially dogs.

PathogenesisAfter ingesting food contaminated with at least 104Campylobacter, the organism multiplies in the small in-testine, invades the epithelium, and causes inflamma-tion, resulting in disease. The symptoms of Campylobacterinfection include a high fever (usually greater than 104°For 40°C), headache, malaise, nausea, abdominal cramps,and profuse diarrhea with watery, frequently bloody,stools. The disease subsides in about 1 week, and re-

covery is complete and spontaneous.

Diagnosis, , Treatment, and PreventionDiagnosis requires isolation of the organism from stoolsamples and identification by growth-dependent tests orimmunological assays. Because of the frequency withwhich C.jejuni infections are observed in infants, a varietyof selective media and highly specific immunologicalmethods have been developed for positive identificationof this organism. Treatment of infections with ery-thromycin do not shorten the acute diarrhea, but mayshorten the time during which patients shed Campylobacterin their feces. Personal hygiene, proper washing of un-cooked poultry (and any utensils coming in contact withuncooked poultry), and thorough cooking of the meateliminate the possibility of Campylobacterinfection.


I

I

I

L....

Campylabacterinfection is by far the most prevalent foodbornebacterial infection. Though usually self-limiting, this diseaseaffects nearly 2 million people per year.

.I ,Describe the pathology of Campylobacter food infection.What is the likely outcome?

.I How might Campylabacter contamination of productionanimals be controlled?

~Listeriamonocytogenesis emerging as an important food-borne pathogen. L. monocytogenescauses listeriosis,a gas-trointestinal food infection that may lead to bacteremiaand meningitis.

Biology and EpidemiologyListeria monocytogenes is an acid-tolerant, psychrotolerant(cold-tolerant), facultatively anaerobic, and salt-tolerantbacterium. It is a short, gram-positive, nonspore-formingrod (~ Section 12.19) (Figure 29.5). It is found widelyin soil and water, and virtually no fresh food source issafe from possible L. monocytogenescontamination. Freshfood can become contaminated at any stage during foodgrowth or processing. Methods such as refrigeration,which ordinarily slow microbial growth, are ineffective


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~ Gram stain of Listeria monocytogenes. The short,gram-positive bacilli are about 0.5 IJm in diameter.

in limiting growth of this psychrotolerant organism.Thus, meat, dairy products, and fresh produce can becontaminated with this pathogen. Common sources oflisteriosis outbreaks are ready-to-eat processed foodssuch as meat products and unpasteurized dairy prod-ucts that are stored for long periods at refrigerator tem-perature (4°C).

PathogenesisListeria monocytogenesis an intracellular pathogen. It en-ters the body through the gastrointestinal tract after in-gestion of contaminated food. Uptake of the pathogenby phagocytes results in growth and proliferation of thebacterium, lysis of the phagocyte, and spread to sur-rounding cells. Immunity to 1. monocytogenes is mainlycell-mediated via TH1cells (0Ct:iSection22.7).Individu-als having weakened cellular immunity, induding the el-derly, neonates, patients undergoing immunosuppressivedrug treatment (e.g., steroid treatment), or those whohave immunosuppressive diseases such as AIDS, haveincreased susceptibility to listeriosis (0Ct:iSection 26.14).

Although exposure to 1. monocytogenes is undoubt-edly very common, acute listeriosis is quite rare. Theacute disease is usually characterized by bacteremia andmeningitis and has a mortality rate of about 20%. Al-though there are only about 2500 cases of acute listerio-sis each year, about 500 cases end in death. Nearly allrequire hospitalization.

Diagnosis, Treatment, and PreventionThe diagnosis of listeriosis is accomplished by culturingListeria monocytogenes from the blood or spinal fluid.L. monocytogenes can be identified in food by direct cul-ture or by a variety of molecular methods such as ribo-typing (0Ct:i Section 11.9) and the polymerase chainreaction (PCR) (~ Section 10.17, Section 24.13, and

Table 24.9). Antibiotic treatment with trimethoprim-sulfamethoxazole is effective.

Prevention measures include recalling contaminatedfood and taking steps to limit 1. monocytogenescontami-nation at the food-processing site. Since 1. monocytogenesis susceptible to heat and radiation, raw food and foodhandling equipment can be readily decontaminated.However, without sterilizing the finished food product,the risk of food contamination cannot be completelyeliminated because of the widespread distribution of thepathogen. Individuals who are immunocompromisedshould avoid nonpasteurized dairy products and paycareful attention to expiration dates when consumingready-to-eat processed foods.


Listeriamonocytogenesis ubiquitous in the environment. In nor-mal individuals, Listeria causes no infection. However, in im-munocompromised individuals, Listeria can cause seriousdisease and even death.

,/ Describe the pathology of Listeria food infection. What isthe likely outcome in normal individuals?

,/ Why are immunocompromised individuals extremelysusceptible to life-threatening Listeria infections?

Other Foodborne InfectiousDiseases

A number of other microorganisms and infectious agentscontribute to foodbome diseases.

BacteriaTable 29.5 lists several other bacteria that cause human

foodbome disease. Yersinia enterocolitica is commonlyfound in the intestines of domestic animals and caus-es foodbome infections due to contaminated meat anddairy products. Y.enterocolitica causes enteric fever, asevere life-threatening infection. Bacillus cereus pro-duces an enterotoxin that causes diarrhea and vomit-

ing. B. cereus grows in high-carbohydrate foods suchas rice. Spores of this gram-positive rod germinateand, as the organism grows in food that is left at roomtemperature, pathogenic amounts of toxin are pro-duced. Reheating may kill the B. cereus, but the toxinremains active. Shigellaspp. cause nearly 100,000 casesof severe foodborne invasive gastroenteritis calledshigellosis each year. Several members of the Vibriogenus cause food poisoning after consumption of con-taminated shellfish.

Viruses

The largest number of annual foodbome infectionsare thought to be caused by viruses. In general, viral

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oodborne illness consists of gastroenteritis character-zed by diarrhea, often accompanied by nausea andomiting. Recovery is spontaneous and rapid, usuallyrithin 24-48 h ("24-hour flu"). Norwalk-like viruses~ Table 25.5 and Section 28.8) are responsible for most)fthe foodborne infectious disease in the United StatesTable 29.5), accounting for over 9 million of the esti-nated 13 million cases of food infection per year. Ro-avirus, astrovirus, and hepatitis A (~ Section 26.11)ollectively cause 100,000 cases of foodborne disease.ach year. These viruses inhabit the gut and are oftenransmitted to food or water with fecal matter. As with

any foodborne infections, proper food handling, hand-ashing, and a source of clean water to prepare fresh

oods are essential to prevent infection.

arasites

nportant foodborne parasitic diseases are listed in Table9.5.Parasites including Giardialamblia,Cryptospordiumarvum, and Cyclosporacayetanensiscan be spread viaood, presumably contaminated by fecal matter in un-:reated water used to wash, irrigate, or spray crops.~resh foods such as fruits are often implicated as theiource of these parasites. We discussed giardiasis and:ryptosporidiosis in the previous chapter (~ Section~8.6).Cyclosporiasis is an acute gastroenteritis and is anmportant emerging disease. Most cases appear to be the'esult of eating imported fresh produce.

Toxoplasma gondii is a parasite spread through cateces, but also found in raw or undercooked meat. In.ost individuals, the toxoplasmosis infection causes aild, self-limiting gastroenteritis. However, prenatal in-

ection can lead to a variety of complications, includingHndness and stillbirth. Immunocompromised patientsIso exhibit signs of acute toxoplasmosis.

REVIEW QUESTIONS. 963

Prions

Prions are proteins, presumably of host origin, that adoptnovel conformations, inhibiting normal protein functionand causing disruption in neural tissue. The foodbornevariety of prion disease in humans is known as "variantCreutzfeldt-Jakob Disease" (vCJD). VCJD is a slow-act-ing degenerative nervous system disorder and has af-fected several hundred people in the United Kingdomand other European countries (~Section 9.13). Thedisease appears to be spread by eating meat productsfrom cattle afflicted with bovine spongiform encephali-tis (BSE),a prion disease commonly called mad cow dis-ease. Although the mechanism of replication is notentirely clear, BSE prions consumed in the meat prod-ucts from affected cattle somehow trigger structurallyand functionally related human proteins to assume an al-tered conformation, resulting in protein dysfunction anddisease. BSE has not yet been discovered in cattle norhas vCJD been observed in humans in the United States.In Europe, all cattle known or suspected to have BSEhave been destroyed. Bans on feeding cattle with ani-mal meal appear to have stopped the development ofnew cases of BSE in Europe.


Over 200 different infectious agents cause foodborne disease.Viruses cause the vast majority of foodborne illnesses. A num-ber of other bacteria, parasites, and prions also cause food-borne illnesses.

,/' Identify the viruses most likely to be involved in food-borne illnesses.

,/' How might prion contamination of production animals beprevented in the United States?

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~i · microbial growth and food spoilage. 951 ... of methods used to control their growth and...

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