the world within micro-organisms in the digestive tract: friends, foes, and visitors janice m....
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The World Within
Micro-organisms in the Digestive Tract:Friends, Foes, and Visitors
Janice M. Joneja, Ph.D 2002
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TheInternal Landscape
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The Digestive Tract
Each site within the digestive tract is designed for optimal function: Digestion of food Protection against invading disease-causing microorganisms Maintenance of healthy balance (homeostasis)
In the lower bowel, micro-organisms play an active role in all these functions
Sometimes, conditions favour colonisation by microorganisms; others are hostile to their survival
The proper functioning of the resident microflora is essential to the health of the body
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Digestive Enzymes
Stomach:Acid hydrolysisGastric pepsins
Mouth:Salivary -amylaseLingual lipase
Small intestine:Pancreatic-amylaseLipaseColipaseTrypsinChymotrypsinElastaseCarboxypeptidases
Small intestine:Gall bladder:Bile salts
Small intestine:Brush border:Lactase (ß galactosidase)Glucoamylase (-glucosidase)Sucrase-isomaltaseAmino-oligopeptidasesDipeptidyl-peptidaseLarge bowel
Microbial metabolism
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Microbial Colonisation
Mouth:SalivaMicrobial colonisation
Esophagus:Micro-organisms present
StomachHigh acidityUsually sterile
Small intestineNeutral or slightly alkalineNo resident microbial populationMicro-organisms populate lower ileum
Large bowelDense microbial populationMostly anaerobic organisms
RectumFaecesDense microbial population
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Microbial Colonisation of the Digestive Tract
Factors allowing micro-organisms to live: Body defences (immune system)
Determines who stays, who goes
Environment: Acidity and alkalinity (pH) Level of oxygen present
Diet: Provides nutrients for microbial growth and
reproduction
Interactions between different types of micro-organisms Survival of the fittest
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Colonisation by MicroorganismsThe Mouth
Micro-organisms enter through the mouth from the external environment
Nutrients and salivary secretions in the mouth allow colonisation: Crevices around the teeth Pockets in oral tissues Bacterial plaque on the surface of teeth
Numbers and persistence of micro-organisms depends on: Available nutrients Hygiene Speed of transit of contents
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Micro-organisms in the Esophagus
Micro-organisms pass with the oral contents through the esophagus
The environment of the esophagus is the same as in the mouth, but it is a conduit, not a “vessel”
Material passes through, but does not remain in location, and therefore micro-organisms have no opportunity to colonise the area
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Micro-organisms in the Stomach
In the healthy individual the stomach is sterile The process of eating triggers release of
gastric secretions and acid After a meal the pH can be as low as 3.0 Most micro-organisms cannot survive this Gastric secretions and hydrochloric acid kill
off most micro-organisms passing from the esophagus
Rate of flow of food through stomach also influences microbial survival
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Micro-organisms in the Stomach
Low acidity (higher pH) allows some micro-organisms to survive
Conditions that may allow bacteria to live: Achlorhydria (lack of gastric acid),
especially in the very young, and the elderly Neutralizing substances that reduce acidity
of contents, e.g.: sodium bicarbonate other antacids
Most common pathogen: Helicobacter pylori
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Survival of Micro-organisms in the Stomach
Rapid movement of food material through before pH is low enough to kill them: Before a meal, pH of stomach is 4-5 Drops to pH 3 while eating
Rate of flow of stomach contents influenced by: Composition of meal:
Fat passes through slowly Liquid passes through quickly
Micro-organisms that survive through the stomach pass into the small intestine
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Micro-organisms in the Small Intestine
Very few micro-organisms live in the first part of the healthy small intestine
Numbers increase as the digesta passes into the terminal ileum
Conditions that influence microbial multiplication: Rate of flow of digesta:
Flow rate greatest at the beginning Slows as material reaches distal end Normal length of time food material takes to transit small
intestine: 3-4 hours
Water is absorbed Consistency is more solid and allows organisms to
stay in place long enough to multiply
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Micro-organisms in the Small Intestine
Under normal circumstances several processes inhibit adherence and colonization in the small intestine, and kill micro-organisms surviving from the stomach: Mucus coats bacteria and disallows contact
with the intestinal wall Antibodies, especially secretory IgA,
neutralize bacteria Lysozyme in secretions is bactericidal (kills
bacteria) Bile salts are bactericidal
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Micro-organisms in the Small Intestine
Micro-organisms can colonise the small intestine and cause infection if they can adhere to the intestinal wall
Usually, contents pass through too rapidly to allow this
Some situations may predispose to colonization: Motility disorders that interfere with the
normal passage of material through Material becomes lodged within tissue
pockets (diverticulae)
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The Large Bowel
Most of the micro-organisms that colonise the human body live and thrive in the large intestine
Digesta from small intestine enters the caecum where microbial activity begins in earnest
As the contents pass from the caecum to the rectum, microbial numbers increase dramatically
Adult eating typical Western diet: Total contents: 220 grams dry weight Bacterial dry matter: 18 grams
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Micro-organisms in the Large Bowel
Contents of the large bowel pass from the body as faeces
Micro-organisms in faeces same as in terminal part of large bowel
Bacteria in faeces: Approximately a trillion per gram dry weight The longer the material remains in the colon,
the greater the number of micro-organisms Several hundred different microbial species About 99% of these belong to only 30-40
species
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Micro-organisms in the Large Bowel
Food material remains in the colon approximately 70 hours
Inter-individual variation: 20 - 120 hours Many species multiply rapidly: some double
every twenty minutes Type and species of micro-organisms is
surprisingly stable for each individual Even when infection changes the nature of the
species, after pathogens are removed, microflora tends to revert to its original composition
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Micro-organisms in the Large Bowel
Conditions that influence type and numbers of micro-organisms: Amount of oxygen available (many are strict
anaerobes and are killed by exposure to oxygen) Competition for nutrients Type of nutrients available Type of micro-organisms present:
Organisms that can break down food material and use nutrients fastest will multiply fastest
Confined space Organisms that multiply fastest, crowd out
others
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Micro-organisms in the Large BowelInter-Species Competition
Space and nutrients are limited Species that break down and use available
nutrients most efficiently achieve the highest numbers
Advantage to species that can: Use substrates most other species cannot
process Use waste products of other species, e.g.
Hydrogen sulphide Organic acids
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Source of Nutrients in the Large Bowel
Material that has not been completely digested and absorbed in the small intestine: Food matter consumed in diet Cells and tissues sloughed off from digestive tract Enzymes and other material from body processes
such as: saliva intestinal secretions such as mucin blood cells
Dead micro-organisms
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Micro-organisms in the Large BowelNutrient Substrates
Most important nutrient substrates are: Carbohydrates
Starch Plant storage material
Non-starch polysaccharides (dietary fibre) Plant structural material
Oligosaccharides (long chain sugars) From partial digestion of carbohydrates
Sometimes disaccharides (sugars) Most are broken down in small intestine
Proteins Diet Body secretions, including digestive enzymes Dead micro-organisms
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Microbial Use of Material in the Large Bowel:
Carbohydrates
Majority of bacterial species in the large bowel act on carbohydrates
Carbohydrates entering the colon of the average adult eating a Western diet per day include: Dietary fibre…………………………12 grams Undigested starch…………………30-40 grams Material from the digestive tract (mucins, enzymes and dead micro-organisms)
…………………...……….3-4 grams
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Carbohydrates
Dietary fibre Structural parts of plants Have beta-glycosidic linkages between
molecules Indigestible by human enzymes
Includes: Pectin Cellulose Gums Beta-glucans Fructans
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Dietary Fibre
Usually separated into two types depending how it interacts with water:
Soluble fibre: Forms gel or gum
Insoluble fibre: Remains unchanged in water
Both types present in plants, e.g in legumes: Hard outer skin is insoluble type Inner “pulse” higher in soluble type
Cooking and processing does not change the nature of fibre
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Carbohydrates
Starches Previously thought all starch was digested
and absorbed in the small intestine Enzymes break alpha-glycosidic linkages
between molecules Recent research shows 15%-20% of dietary
starch passes undigested into the colon from high starch foods such as: potato pasta rice banana grains (wheat, corn)
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Starch
Undigested starch is called “resistant” starch
Starch that is readily digested and absorbed in the small intestine is called “non-resistant”
Resistant starch is resistant to digestive enzymes Passes into the colon where it is fermented by gut
microflora Unlike fibre, resistant starch is affected by cooking
and processing
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Resistant Starch
Process of digestion in the small intestine can be speeded up by cooking - starch is gelatinized
Cooling causes a process of crystallization (retrogradation) that renders the molecules non-digestible by enzymes
Undigested material passes into the large bowel
Freezing and drying can also cause changes in starch that makes it resistant to digestion
Research on contents of ileostomy bag
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Comparison of Dietary Starch
A comparison of dietary starch:a) Fedb) Recovered after digestion in the small intestine
Food StarchFed(grams)
StarchRecovered(grams)
PercentageStarchRecovered(%)
White bread 62 1.6 3
Oats 58 1.2 2
Cornflakes 74 3.7 5
Banana (raw) 19 17.2 89
Potato freshly cooked cooled reheated
454747
4.5 5.8 3.6
312 8
Englyst and Kingman 1994
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Factors Affecting Amount of Starch in the Colon
Physical accessibility Cell walls of plant cells entrap starch Prevents its swelling and dispersion Delays or prevents digestion by enzymes Includes whole grains, nuts, seeds:
vegetables with “skins”: sweet corn, peas, beans
partly milled grains and seeds: “whole grain” breads and cereals
If the rigid structures of the plant are physically removed, more of the alpha-glycosidic bonds of the starch are exposed to the action of enzymes in the small intestine
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Factors Affecting Amount of Starch in the Colon
Cooking Disrupts starch granules Facilitates digestion by enzymes in saliva and the
small intestine When foods with a high level of resistant starch
are eaten raw, more undigested starch passes into the colon e.g. Banana
Retrograded starch increases on cooling: eat foods with high level of resistant starch when it is hot
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Factors Affecting Amount of Starch in the Colon
Chewing Amylase (ptyalin) in saliva is first enzyme to
start process of starch digestion The more the food is chewed, the greater the
exposure of the starch to enzymes in the mouth and the small intestine
Speed of transit of food The faster the food transits the small intestine,
the less exposure to enzymes High fat slows transit High fluid (water with the meal) speeds the
transit
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Oligosaccharides
Polymers of glucose 3 - 8 hexose units in length Exist in plant materials as oligosaccharides Or are derived from partial digestion of
starches Trisaccharides are most “notorious”
Raffinose Stachyose
Principally in legumes such as dried peas, beans, lentils
Proficient in generating excessive amounts of intestinal gas and flatus
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Oligosaccharides
Fructo-oligosaccharides Polymers of fructose - called inulins Made by plants such as:
onions garlic artichokes chicory
Appearing as “health foods” Resist human digestive enzymes Promote growth of Bifidobacteria in the large bowel Tend to reduce growth of “undesirable” bacteria
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Fructo-oligosaccharides and Bifidobacteria
Bifidobacteria are beneficial because they: Stimulate immune function Enhance synthesis of B vitamins Restore normal microbial flora after antibiotic
therapy Prevent colonization by potential pathogens,
especially Clostridia
Fructo-oligosacchardies: Reduce triglyceride and cholesterol levels in
rats and diabetic humans
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Disaccharides
Principally: Lactose; sucrose; maltose
Usually broken down to monosaccharides (“single sugars”) and absorbed in the small intestine
When enzymes deficient, disaccharides pass undigested into the colon
Have several effects: Change osmotic pressure Act as substrate for microbial fermentation
Results in symptoms typical of lactose intolerance; Diarrhea Abdominal bloating Gas Pain
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Products of Microbial Fermentation of Carbohydrates
Any carbohydrate entering the colon acts as substrate (nutrient) for microbial fermentation
Principal products are short-chain fatty acids (SCFAs): Acetic acid Propionic acid Butyric acid
These three account for 85-95% of SCFAs in the colon
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Other Sources of SCFAs
A smaller percentage of SCFAs come from proteins Up to 40% of SCFAs are derived from protein,
depending on the diet
Branched chain amino acids are converted to branched chain fatty acids
Contribute to the total SCFAs in the colon resulting from microbial activity
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Products of Microbial Fermentation of Carbohydrates
In converting the carbohydrates to these SCFAs intermediate products are formed: Lactate Succinate Ethanol
Most do not accumulate, but are converted to SCFAs in the colon
However, occasionally ethanol may accumulate: Results in “autobrewery syndrome”
resembling alcohol intoxication
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Function of SCFAs
SCFAs absorbed into the body through the colonic membrane (wall), and can be measured in blood
SCFAs serve a variety of functions in the colon: Provide source of energy Preserve the integrity of the colonic mucosa (lining) Stimulate absorption of water and sodium Reduce intestinal pH Aid in protection against bacterial infection Butyrate thought to be particularly important in
protection against colon cancer May also protect against inflammatory bowel diseases
such as ulcerative colitis
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Proteins in the Colon
12 – 13 grams of protein enter the large bowel each day
Material comes from: Diet (even a vegan diet) Secretions from the digestive tract Dead bacteria Tissue cells
Much of the material is digested by pancreatic enzymes in the small intestine
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Proteins in the Colon
Pancreatic enzymes continue digestion in the large bowel as they pass in with the digesta from the small intestine
Bacterial enzymes actively attack the undigested proteins
Bacterial species Bacteroides are particularly active in this process
These species are also the most active degraders of fibre in the colon
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Protein breakdown in the Colon
Proteins are first broken down to polypeptides
Some bacteria use these directly as nutrients Other bacteria produce enzymes to break
down the polypeptides into dipeptides Dipeptides are then broken down further into
single amino acids 20 amino acids make up all dietary proteins
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Amino Acids in the Colon
Bacteria utilize the amino acids in a variety of ways: Deamination to ammonia Decarboxylation to amines and carbon
dioxide
Both systems are important in maintaining a healthy colon
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Ammonia in the Colon
Large quantities almost always present in the colon High levels can be toxic Can be a risk factor in the development of colon cancer Colon bacteria use ammonia as a source of nitrogen in
their metabolism These strains are important to maintain a healthy colon These bacteria use carbohydrate, and especially fibre
as a course of energy Fibre in the diet thus aids in growth of the ammonia-
utilizing bacteria, which is thought to reduce the risk of colon cancer
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Biogenic Amines in the Colon
Sometimes the amines are detrimental to a person’s health, e.g. Histamine:
Migraine headaches Symptoms resembling allergy
Hives Tissue swelling (angioedema) Rhinitis(“stuffy nose”) Itching Reddening and flushing Increased heart rate
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Biogenic Amines in the Colon
Tyramine Migraine headaches Hypertensive crisis
Serotonin Piperidine Pyrrolidine Cadaverine Purescine
Have adverse effects only in excess and in sensitive individuals
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Fate of Microbial Products
Most microbial products enter circulation by being absorbed through the colon wall
Taken to the liver Cleared and excreted in the urine Examples:
Phenol and p-cresol from amino acid tyrosine in proteins
50-100 mg per day in the healthy adult urine Level increases with increase in protein in the diet Decreases when bran added to the diet – bran acts as
energy source for bacteria that use tyrosine to build bacterial protein
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Fate of Microbial Products
Products of microbial activity normally cleared in the liver and excreted in the urine without adverse effects
Scientific data about the fate of many by-products of microbial metabolism is presently lacking in many cases
There is suspicion that in sensitive individuals some “psychological disturbances” following ingestion of certain food materials might be caused by these microbial by-products
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Protection Against Invading Pathogens
Because of its ideal environment, the large bowel may be the site of invasion by disease-causing microorganisms
Various factors protect against this: Resident microflora protect their own space SCFAs act as antagonists to many pathogenic
micro-organisms: Salmonella Shigella (dysentery) Vibrio (cholera) E.coli (enteritis)
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Invading Pathogens
Antibiotics taken by mouth kill off many of the resident species Less SCFAs are produced pH rises
Pathogens can now invade and colonize more readily
Takes time for the resident micro-flora to re-establish
Symptoms of irritable bowel syndrome not uncommon following enteric infections
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Protection Against Invading Pathogens
Diarrheal diseases also decrease SCFAs Microbial infection Lactose intolerance Magnesium-based laxatives
Fibre increases level of SCFAs because bacteria that produce them also use fibre as a substrate, which increases bacterial numbers
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GAS
Fermentation always leads to production of various types of gases
80% of the gas from fermentation is released as flatus
20% is absorbed into the body and excreted in breath
Volume of gas depends on composition of diet: from 0.5 to 4 litres per day in the adult human
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Gas
Healthy people pass flatus an average of 14 times per day
25 – 100 ml on each occasion Can rise to 168 ml per hour when >50% of
the diet is in the form of non-starch fibres and non-absorbable sugars: Beans Whole grains Some vegetables Some fruits
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Gases in Breath
Principal gases in breath are: Hydrogen Carbon dioxide
Small quantities: Methanediol Ethanediol Ammonia Hydrogen sulphide
Occasionally Methane
Type of gas depends on the presence of the specific bacteria capable of producing it
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Colonic Gases
Some bacteria use gases for their metabolism: Hydrogen metabolized to:
Methane Hydrogen sulphide Acetate
These may be: utilized by micro-organisms excreted as flatus passed into circulation and breath
Amount of hydrogen even from the same amount of substrate is not constant: it depends on: Type of micro-organisms present Speed of fermentation Utilization by other micro-organisms
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Hydrogen Breath Test for Lactose Intolerance
Results of hydrogen breath test used in the diagnosis of lactose intolerance varies depending on type of micro-organisms in the bowel
Rationale for test: If lactose is not digested by brush-border lactase, it
passes into the large bowel Here it will be fermented by the resident micro-
organisms, with the production of hydrogen The hydrogen is absorbed, taken in blood to the lungs
where it is excreted Amount of hydrogen collected from breath is measured
and used as an indication of the degree of lactase deficiency
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Methane
Methane-producing bacteria convert hydrogen to methane 30-50% of healthy adults have methane-producing
bacteria in their colon Gas is excreted in the breath Not detectable in children under the age of two years In methane-producers, adult level of methane reached
by age 10 years Tends to be familial Methane production does not vary with diet May be associated with:
large bowel cancer intestinal polyps ulcerative colitis
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Hydrogen Sulphide
Sulphate-reducing bacteria in the colon convert hydrogen to hydrogen sulphide
Methane-producing and sulphide-producing bacteria compete for hydrogen in the colon
When the diet is high in foods that contain sulphates, hydrogen-sulphide producing bacteria have an advantage
Another source of sulphate is body secretions such as mucins that contain sulphated glycoproteins
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Sulphate-Containing Foods
Sulphates may occur naturally: Some fruits Some vegetables
Sulphates may be used as clarifying agents and stabilizers in manufactured foods, such as: Cheeses Egg products Pickles Candied and glazed fruit Flours; breads; cereals; pastas Sugars Wine; beers Nutritional supplements Laxatives; homeopathic remedies; medications
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Acetate If methane-producing and sulphide-producing
bacteria are absent, bacteria may convert hydrogen and carbon dioxide to acetate
The extent to which this occurs is unknown Acetate may be used by the body as a source of
energy in certain metabolic processes The type of gases excreted as flatus or in breath
depends more on the species of micro-organisms colonising the bowel than on the composition of the diet
Components of the diet determine the amount of gas produced
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Vitamins Produced by Bacteria
Bacteria not only break down food material (catabolism), they synthesise nutrients (anabolism) from these building blocks
Vitamin K Required in blood clotting Menaquinone component of the vitamin is derived
from bacterial action on vegetable material mostly in the ileum from where it is absorbed
Taken to the liver, where it is complexed with prothrombin
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Vitamins Produced by Bacteria Vitamin B12
Made solely by micro-organisms in ruminant digestive tract
Absorbed through small intestine Passes into meat and milk of the animals Human bacteria (Pseudomonas and Klebsiella) also
synthesise B12
5 mcg excreted in feces daily Site of synthesis in humans is large bowel but absorption
from here is poor Some people have micro-organisms capable of
synthesising B12 in the small intestine
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Vitamins Produced by Bacteria
Biotin Synthesised by bacteria in animals and humans Absorbed in lower ileum Antibiotics can reduce biotin levels in urine, indicating
significant reduction in biotin synthesis when bacteria are killed
Folic acid Thiamine
Produced by bacteria, especially in the large bowel Amount absorbed is inadequate alone, and the vitamins
must be provided in food to avoid deficiency
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Changing the Microbial Flora of the Bowel
Diet has very little influence on the types of micro-organisms that colonise the digestive tract
Attempts to alter the gut microflora by direct dietary manipulation tend to be frustrating
Differences in types and numbers in the bowel of one individual compared to another in the same community, eating the same diet
Microflora can be changed by use of oral antibiotics
Microflora tends to return to pre-antibiotic types over time
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Probiotics
Food supplement containing live bacterial culture
Trials in disease situations such as : Diarrheal diseases Re-establishment of normal flora after antibiotic
therapy Inflammatory bowel diseases Fungal disease (e.g. candidiasis) Cancers Cholesterol lowering
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Probiotics
Examples of bacteria: Lactobacilli Bifidobacteria
Examples of food supplements containing live culture: Yogurts Fermented milks Fortified fruit juice Powders Capsules Tablets Sprays
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Prebiotics
Non-digestible food ingredients that selectively stimulate a limited number of bacteria, to improve health
Examples: Fructo-oligosaccharides Lactulose Galacto-oligosaccharides
Provided in: Beverages and fermented milks Health drinks and spreads Cereals, confectionery, cakes Food supplements
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Synbiotics
Combine prebiotics and probiotics Prebiotic substrate should enhance
survival of probiotic bacteria Example:
Bifidobacteria + fructo-oligosaccharide
In order to establish the new species, need to continue to provide live culture, and appropriate substrate