Impact of symbiosis on health and disease

Influence of symbiosis on the body

Physiology and anatomy can be influenced by the presence of mutualistic bacteria. Examples of this are the development of the intestine and immune system. Evidence for the former was obtained using germ free mice:Gnotobiotic mice are raised and observed to have underdeveloped intestinal epithelium and immunity. These mice are obtained by cesarian section in sterile conditions - isolator. In human neonates, the intestine is colonised by facultative anaerobes. During weaning, obligate anaerobes predominate and the intestinal epithelium develops an extensive capillary network ensuring abundant blood supply - this correlates to colonisation by bactericides. Immune system defects exhibited by gnotobiotic mice:  higher infection susceptibility  diminished cytokine levels  reduced innate cells levels - DC reduced serum Ig levels  reduced CD4 cells 

Logic behind gnotobiotic deprivation 

The body will not produce what it does not require. In sterile conditions, the body does not require an immune system, and an intestinal system void of commensal metabolism will be nutritionally deprived and thus require lesser vascularity. Bacteroides inhabit the intestinal tract. Upon inoculation with the commensal bacteria there is both increased vascularity in the intestinal epithelia, and restored CD4 presence. This is due to bacterial metabolism providing nutrients to the host, and the presence of PSA antigen on the bacteroides polysaccharide capsule. The benefit to the bacteria resides in the presence of a healthy immune system preventing pathogens killing the host and prolonging the life of bacteroides. 

Alterations to GI microflora contributing disease 

Obesity can be caused by alterations to gut microflora. In humans, obese people have a lower proportion of bacteroidetes and higher proportion of firmicutes in the intestine. Upon restriction of calories, the proportion of firmicutes decreased, and bacteroidetes increased.This is only founded on correlated statistics, thus it may be related to obesity. In mice, it is observed that conventionally reared mice have a higher fat concentration to gnotobiotic mice. Transplantation of microbial community from conventional mice into gnotobiotic mice increases fat content. This establishes a cause and effect relationship. The Firmicutes:bacteroidetes ratio stands true in mice. Inoculation of GF mice with obese mouse microflora results in greater fat content than inoculation with conventional mouse microflora. Hypothesis: the bacterial community in obese individuals may be more efficient in  utilizing otherwise indigestible polysaccharides.This is not definitive, as studies among humans have been small and more research is to be done.  

Niche exclusion 

Two organisms are competing for the same resource. One will predominate and the other will diminish in number and be removed from the niche. Niche exclusion by commensals can protect against pathogens by means of selective harm. Antibiotic colitis can occur as a product of antibiotic treatment. Pseudomembranous colitis  is caused by toxins A and B killing colonic mucosa, causing inflammation and dead cell accumulation - pseudomembrane. This occurs as a product of commensal bacteria being susceptible to an antibiotic (ampicillin) that clostridium is resistant to. Thus reducing the commensal population and allowing clostridium to overpower the niche. This is treated by vancomycin antibiotics - clostridium sensitive.