The World Within Micro-organisms in the Digestive Tract: Friends, Foes, and Visitors Janice M. Joneja, Ph.D 2002
The Internal Landscape 2
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 3
Digestive Enzymes Mouth : Salivary α -amylase Lingual lipase Stomach : Acid hydrolysis Gastric pepsins Small intestine : Gall bladder : Bile salts Small intestine : Pancreatic α -amylase Lipase Colipase Trypsin Chymotrypsin Small intestine : Elastase Brush border : Carboxypeptidases Lactase (ß galactosidase) Glucoamylase ( α -glucosidase) Sucrase-isomaltase Amino-oligopeptidases Dipeptidyl-peptidase Large bowel Microbial metabolism 4
Microbial Colonisation Mouth : Saliva Microbial colonisation Esophagus : Micro-organisms present Stomach High acidity Usually sterile Small intestine Neutral or slightly alkaline No resident microbial population Micro-organisms populate lower ileum Large bowel Dense microbial population Mostly anaerobic organisms Rectum Faeces Dense microbial population 5
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 6
Colonisation by Microorganisms The 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 7
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 8
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 9
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 10
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 11
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 12
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 13
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) 14
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 15
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 16
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 17
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 18
Micro-organisms in the Large Bowel Inter-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 19
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 20
Micro-organisms in the Large Bowel Nutrient 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 21
Microbial Use of Material in the Large Bowel: Microbial Use of Material in the Large Bowel: Carbohydrates 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 22
Carbohydrates Carbohydrates � Dietary fibre � Structural parts of plants � Have beta-glycosidic linkages between molecules � Indigestible by human enzymes � Includes: � Pectin � Cellulose � Gums � Beta-glucans � Fructans 23
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 24
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