Enteric Microbiotica

Introduction

300-500 species of bacteria in human stool.

Dermis 10¹² bugs, body 10¹³cells, GI 10¹⁴ bugs.

Majority (>99%) of colonic bacteria are obligate anaerobes. The small amount of facultative anaerobes play an important role in eliminating all oxygen, allowing other bugs to survive.

Obligate anaerobes are not pathogenic because they cannot survive in the oxygen tension present in tissues. The exceptions to this are: Bacteroides fragilis (because can grow symbiotically with facultative bugs in abscesses), Clostridium difficile and other toxin producing bugs – pathogenic bacteria are almost always facultative anaerobes

In 1^st week of life there is a huge growth of bugs in the infant. Two sources are: vaginal flora and food.

Ecosystem Diversity and Population Dynamics

As in all ecosystems, a few species predominate while a large number of species are represented by a few individuals. – this allows genetic diversity. If main bug population is wiped out (by antibiotics for example) some in the minority will not be affected and will reproduce.

Bacterial populations in enteric ecosystems are comparatively stable – bug generation rates and total numbers are limited by the rate of entry of utilizable substrates ("chemostat" system). Generation rate = rate of loss (death, defecation, etc.)

Diet – nature of dietary intake affects types and numbers of enetric bugs. Basic principle: nutrients escaping digestion and absorption by host are available for bugs. Includes foreign cmpds like drugs, dyes (aka xenobiotics). Carbohydrates that escape digestion and absorption by host are major carbon and energy sources of enteric microbiota.

Efforts to alter enteric bacterial populations by altering the diet – "Probiotics are a live microbial feed supplement which beneficially affects the host animal by improving its microbial balance." Ex: antibiotic associated diarrhea due to Clostridium difficle overgrowth can be treated by yogurts because they contain other bugs.

Host genes affect enteric bacterial populations – twins have similar enteric bug populations as compared to non-twins.

Implantation, Distribution, and Controlling Factors

Implantation – at birth infant is bacteriologically sterile. Inoculation during delivery, and later with solid foods.

Regional Distribution –

While mouth and colon have huge numbers of bugs (10¹⁰-10¹¹), stomach and small intestine doesn’t (<10⁵). Bug growth in these areas is the 1^st step in pathogenesis of diarrhea.

Chronic contamination of small intestine with colonic bugs Þ "bacterial overgrowth syndrome" Þ bugs compete for dietary CHOs, vitamin B₁₂, etc.

Host controls against small bowel contamination:

Gastric acid secretion – pH<5 inhibits growth.
Bile salt detergents – secreted in bile, lyse bugs.
Antibiotic polypeptides – lysozymes and defensins secreted by Paneth cells.
Secretory IgA – by intestinal mucosa.
Gut motility – peristalsis keeps bugs from migrating into small intestine from colon.
Ileocecal valve – ineffective barrier in most people.

"Colonization Resistance" – competitive exclusion of outside bugs.

Indigenous bugs in a stable community adapted to compete for intralumenal nutrients in mutual associations with each other, outside bugs don’t.
"Ecological Ectocrines" – stuff secreted by dominant bug strains that inhibits colonization by other bugs.
"Bacteriocins" - Many enteric bugs produce peptide antibiotics that kill other bugs.

Metabolic Activities of Enteric Bacteria

Anaerobic Fermentation of Carbohydrates

Rumen and Colon Physiology – colonic fermentation in humans is analogous to rumen fermentation in cows

Rumen fermentation uses energy in the following ways:

Microbial Utilization – 10% (that’s all they get, not very efficient!)
Heat – 10%
CH₄ Production – 10% (wasted energy, belched out)
Short Chain Fatty Acids (FAs) – 70% (not used by bugs, but by cows)

Rumen bugs furnish host’s protein and amino acid requirements. UreaÞ ammoniaÞ proteins

Rumen fermentation also makes B vitamins. Principal source of B12 for humans who eat meat!

How is colon different than rumen? No protozoa, methanogenic bugs only in 1/3 of population. The more the methanogenic bugs a person has the less the sulfur reducing bugs. Humans absorb almost all FAs produced by bugs = 150-200 kCal/day. FAs are the principal anions excreted in fecal water. In malabsorption syndromes the increased CHOs reaching colon leads to more FAs in the stool.

Adverse effects of bacterial fermentation in the colon – excessive flatus production and D-lactic acid acidosis (unusual condition leading to blood acidosis. Caused by overgrowth of bugs that produce D-lactic acid as part of fermentation).

Clinical application: hydrogen breath test – H₂ in bloodÞ breath comes only from bugs. Therefore, this test checks for overgrowth indirectly.

Gastrointestinal Gas – arises from swallowed air and bacterial fermentation of undigested and unabsorbed CHOs

Absorption of vitamins produced by enteric bacteria – little of the vitamins synthesized by colonic bugs are absorbed. Exception: Vitamin K, and possibly folate.

Other metabolic reactions

Hydrolytic Reactions –

Breakdown of proteins by bacterial proteases, and breakdown of saccharides by bacterial glycosidases;

Urea hydrolysis and hepatic encephalopathy – bacterial urease breaks down urea into ammonia and carbon dioxide. Some of the ammonia is used by the bug to make amino acids and proteins, rest goes back and converted to urea in liver. In liver disease urea synthesis is impaired and Ý [ammonia] reaches brain causing altered cerebral function. Administration of non-digestible lactulose increases the bacterial biomass Þ more bugs use the ammonia. Also, lactulose ß pH by Ý lactic acid, thus shifting reaction right: NH₃+H⁺Þ NH₄⁺.

Another treatment is antibiotic neomycin sulfate, kills strains of enteric bacteria that produce urease.

Deconjugation – enterohepatic recycling occurs when bugs deconjugate conjugated cmpds (glucuronides, etc.).

Ex: bacterial deconjugation of bile salts to bile acid results in loss of detergent efficiency (occurs in bacterial overgrowth syndrome) Þ impaired fat absorption.

Reductions –

Inactivation of digoxin by Eubacterium lentum.
Reductive cleavage of azo bonds by the bacterial enzyme azo reductase.
Bacterial nitrate reductases form mutagenic/carcinogenic nitrosoamines (R,R'N-N=O) from dietary nitrate (R,R'NH)

Dehydroxylations – trihydroxy Þ dihydroxy Þ monohydroxy bile acids (lithocholic acid: poorly soluble, no detergent action).

Hydroxylations – when bile is missing and can’t form micelles, unabsorbed oleic acid enters colon and is hydroxylated by bugs into 10-hydroxy stearic acid – a structure related to castor oil.

Formation of Carcinogens and Mutagens – Fecapentaenes – mutagenic polysaturated ether lipids formed by Bacteroides strains from unsaturated phosphoglycolipids.

Bugs may also inactivate cmpds that have antitumor effects, ex: butyrate produced by anaerobic glycolysis.