updateMicrobialE. coli

12 International Food Hygiene — Volume 24 Number 1

Perhaps of all foodborne bacteria,Escherichia coli is the most famous. Ithas been isolated and researched con-

tinuously for nearly 130 years, but it stillposes great problems and can cause largeunexpected outbreaks of food poisoning.Theodor Escherich was a pioneering paedi-atrician, who studied bacteriology andbelieved that a study of gut micro-organismscould potentially solve many infant illnesses.His work took him to laboratories aroundEurope, and he attended large outbreaks ofillness such as the 1884 cholera epidemic inNaples. In 1885 Escherich published a paperdescribing ‘Bacterium coli commune’ – abacterium that he isolated from the stools ofinfants suffering from enteritis. Escherich died in 1911 and never saw hisown name attached to this now famousorganism, as this only occurred in 1920when Castellani and Chalmers first describedthe Genus Escherichia, named in honour ofEscherich.The Genus contained the single species E.coli for 54 years, before it was joined by E.blattae in 1973, this being an odd organismisolated from the gut of a cockroach.In the 1980s three more species were

identified E. hermannii, E. vulneris and E. fer-gusonii. The final species E. albertii wasadded in 2003 being isolated from the stoolsof children in Bangladesh.But of course, the best known of all speciesin the Genus remains E. coli, known byschool children and students as probably thefirst bacterium they ever cultured, and byreaders of the popular press as the cause ofmany outbreaks of food poisoning.

What is it?

E. coli is a Gram negative, non-sporeforming,rod shaped bacterium and a member of theFamily Enterobacteriaceae. They are usuallymotile and have an optimum growth tem-perature of 37°C, but can grow over a widetemperature range from approximately 15-45°C. The organism is a facultative anaerobegrowing equally well under either aerobic oranaerobic conditions. The organism is widelydispersed in nature, its origin is usually the

Dr Roy Betts, Head of Microbiology,Campden BRI, Chipping Campden,Gloucestershire, GL55 6LD, UK.Produced as a service to the food industry by Oxoid, part of ThermoFisher Scientific.

gut of animals, but from there it will spreadto various soils and waters through animalfaeces, and is often considered to be a‘marker of animal faecal contamination.

What does it cause?

E. coli is a very common micro-organism.The intestinal flora of man contains an abun-dant flora of Enterobacteriaceae, and ofthese E. coli is the predominant species. Theorganism can be found in the gut shortlyafter birth and soon colonises the large intes-tine. It is a saprophytic commensal organismand is believed to play an important role invarious physiological functions of the gut.There are, however, many different strainsof E. coli recognised and, whilst a vast major-ity are not just harmless but necessary forthe good functioning of the gut, some arepathogens and will cause illness. The types of illness caused by E. coli is veryvaried, all will usually result in some form ofdiarrhoea that begins approximately 24-48hours after consuming the organism, but theseverity of the resulting illness will vary froma mild and short lived problem, to a majorlife threatening disease. The different pathogenic forms of E. coliare noted below:

l Entero-pathogenic E. coli (EPEC).This group produces watery diarrhoea withvomiting and fever. It is usually self limiting.This group are usually associated with infantsand young children and have in the pastcaused large outbreaks in hospitals. Therehave been few reported outbreaks in Europeor the USA since the 1970s but it can be animportant illness for children in developingcountries.

l Entero-toxigenic E. coli (ETEC).This group produces watery diarrhoea withabdominal cramps, fever, malaise and vomit-ing. In very severe cases the disease canresemble the symptoms of cholera. It is a

major cause of illness in children in develop-ing countries and the major cause of trav-ellers’ diarrhoea.

l Entero-invasive E. coli (EIEC).This group produce acute watery diarrhoeawith a fever and abdominal cramps.Sometimes the stools can become bloodyand mucoid.

l Entero-haemorrhagic E. coli (EHEC) (alsoknown as Verocytotoxin producing E. coli(VTEC) or Shiga Toxin producing E. coli(STEC).Also commonly known as VTEC or in theUSA as STEC, this group were firstdescribed in 1977, but it was only in 1982that they were first recognised as a cause ofhaemorrhagic colitis (HC) and haemolyticuremic syndrome (HUS). The key strain of this group is E. coliO157:H7 but others (O111, O26, O103,O121, O45, and O145) can cause a similarillness. These strains cause the most severeform of food poisoning produced by E. coli,with symptoms of severe bloody diarrhoeaand blood in the urine. Some of thoseaffected suffer kidney failure and death fromthese infections can sometimes occur. In recent years there has been consider-able interest in non-O157 VTEC and in theUSA there are now regulations in place thatrequire all processed raw meats to be testedfor serotypes O26, O45, O103, O111,O121, and O145 (known in the USA as the‘big 6’) as well as O157. In Europe the large outbreak of food poi-soning associated with E. coli O104:H4sprouted seeds in Germany and France in2011 has resulted in new microbiological cri-teria being developed. These criteria willcome into force mid way through 2013 andrequire seeds used for sprouting andsprouted seeds to be tested for VTECserotypes O157, O26, O111, O103, O145and O104:H4. There are a limited number of methods forthe detection of these serotypes, a numberof kit manufacturers have produced methodsthat cover either VTEC in general, or the USbig 6 serotypes, but at present there are nokits covering the serotypes in the Europeancriteria. So in Europe we are left with anISO Technical Specification (ISO/TS13136:2012) covering O157, O26, O111,O103, O145, but not O104:H4. In the future we will see increased testingfor non-O157 VTEC serotypes as weincreasingly recognise their public health sig-nificance.

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l Entero-aggregative E. coli (EAEC orEAggEC).These strains have been associated with per-sistent diarrhoea lasting for many days (up to14 days) usually watery, with vomiting, dehy-dration and abdominal pain.

Where does it come from?

E. coli is an organism that will always origi-nate in the animal gut, research has shownthat most animals carry E. coli and in amajority of cases it causes no ill effects to theanimal host. However, whilst individualstrains cause no ill effects to their immediatehosts, if that strain were introduced to a dif-ferent ‘host’ then illness could result.In foods, contamination will always have adistant link to animals, this may be passed onthrough soil, plants (through close contactwith soils or contaminated irrigation water),water itself (contamination occurringthrough field run off, or animals being pre-sent in water sources) or even human con-tact with food where poor hygienic practicesare followed. General E. coli measurements are oftenused as a hygiene indicator in food produc-tion systems, as they will tend to show upareas that have been improperly cleaned, orwhere the process has failed to eliminate orreduce contaminants to an acceptable level.E. coli is sometimes considered a ‘faecal indi-cator’ which is not strictly true. Whilst theorganism will probably have always origi-nated from a faecal source, it is naturallyfound in many soils and other ‘natural’ envi-ronments. The Health Protection Agency (HPA) giveguidance on levels of general E. coli in readyto eat foods, they note that less than 20/g issatisfactory, whilst between 20-100/g is con-sidered ‘borderline’ and should result inreview of processing and cleaning proce-dures and potentially more investigative sam-pling and testing. A level greater than 100/gis considered unsatisfactory according toHPA guidance.Common sources for the most pathogenicforms of E. coli (VTEC) are raw meats,underprocessed or post process contami-nated milk, uncooked salad items/vegeta-bles, raw waters, unprocessed fruit juices,unprocessed flour. However, the impor-tance of direct contamination should not bemisjudged. There have been cases of VTEC illnesstraced to person to person contamination,animal to person contamination (from zoosand animal parks) and even cases originatingfrom children playing in a paddling pool thathad become faecally contaminated.

Controls for E. coli

E. coli is not a difficult organism to control. Itis not particularly heat resistant and can beprevented from growing by reducing pH orwater activity of foods. But it is important

that food producers consider the risks of E.coli in their own production environments,and introduce appropriate controls ifrequired. Adequate cooking will kill E. coli, and in theUK a process of 7°C for two minutes at theslowest heating point within the product, isconsidered appropriate to reduce the risk ofthe organism considerably from all products(note: milk pasteurisation treatments arecovered by legislation and must be fol-lowed). Adjusting product pH, water activity ordecreasing the temperature will prevent thegrowth of the organism. However food pro-ducers using such techniques must remem-ber that whilst growth may be prevented, ifthe organism is present in the food or ingre-dient, it will remain viable. Unlike salmonella,where all types are considered pathogenicand must be eliminated from ready to eatfoods, relatively few strains of E. coli causeillness, and this has implications when consid-ering microbial criteria for foods. Pathogenic strains (E. coli O157) shouldnot be present in ready to eat foods, andspecific test methods for this type can beused to check for absence of the organism in25g of product if required. With other (non-pathogenic) E. coli, how-ever, whilst the absence of the organism isalways the target, the presence of very lowlevels can be accepted, as long as suitablechecks are done to eliminate or reduce theproblem from future production.Key control factors that can be used to

prevent the growth of VTEC and most othertypes of E. coli are pH <4.4, water activity<0.935, temperature <7°C. Combinationsof these factors well above the values notedpreviously may be equally as inhibitory togrowth, but these should be establishedusing predictive microbiology or challengetesting techniques before they are used. At one time E. coli O157 were thought tobe more acid resistant that other types, butthese concerns do appear to have beengreatly reduced, and acid resistance in theorganism is not now a major concern.

Detection and enumeration

Methods used to test foods for E. coli canbroadly be split into three types:l Direct plating methods for standard E. colithat use a specific diagnostic medium. Thesemethods will tend to have a lower limit ofdetection of 20 cells/g and are simple to use.l Broth based methods for standard E. coliusing one or more broth media to detect orenumerate (via MPN) levels of E. coli lessthan 20/g.l Specific enrichment based methods todetect pathogenic E. coli O157. These tendto be based on enrichment in a selectiveenrichment broth, followed by a separationphase using magnetic particles coated withantibodies specific to O157 (this is known asImmunomagnetic separation). The particlesare finally spread onto selective and differen-

tial agar plates. These methods are designedto detect very low levels (a single organismin 25g of product) of E. coli O157.There are a variety of new ‘rapid methods’for E. coli. Some instrument based methodshave been designed to automate testing forgeneral E. coli, whilst there are a number ofnew tests for the O157 types that utiliseimmunoassays, or the polymerase chainreaction (PCR). These tests can reduce testtimes for the organism by one to two days.

Conclusions

E. coli is a very interesting organism. It isundoubtedly the best studied of all the bac-teria we know. It is found in large numbers inthe human intestine, and some consider itessential for good operation of a healthy gut.However, some strains can be very harmful,and give rise to life threatening illnesses. Infoods, general ‘non-pathogenic’ E. coli areoften used as indicators of hygiene. The presence of very low levels is undesir-able and requires investigation, but it is notconsidered a major risk to food safety.However, the presence of even very lowlevels of E. coli O157 (and similar VTECorganisms) is a major risk to the consumer.nb maureen.sanders@thermofisher.com

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Referencesl US Food and Drug Administration,Center for Food Safety and AppliedNutrition. Bad Bug Book – FoodbornePathogenic Micro-organisms and NaturalToxins Handbook – Escherichia coliO157:H7.www.cfsan.fda.gov/~mow/chap15.html

l Microbiology of Food and AnimalFeeding Stuffs – Horizontal Method forthe Detection of E. coli O157, EN ISO16654.

l Chapter 35 Pathogenic E. coli (2001).In Compendium of Methods for theMicrobiological Examination of Foods,4th Edition. Downes, F. P & Ito, K.APHA.

l Death of Professor Escherich. TheLancet, 1911, 1: 626.

l Pathogenic E. coli. C. Bell and A.Kyriakides. In ‘Foodborne Pathogens’Eds. C. Blackburn and P. McClure.Woodhead Publishing.

l Escherichia coli (2000). G. Willshaw,T. Cheasty and H. Smith. In ‘TheMicrobiological Safety and Quality ofFood. Eds: B. M. Lund, A. C. Baird-Parker and G. Gould. Aspen Publications,Gaithersburg, MD, USA.

l ISO /TS 13136:2012. Horizontalmethod for the detection of STEC andthe determination of O157, O26, O111,O103, O145 serogroups.

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