The Human Body as an Ecosystem

The normal body flora and its role in health and disease

The protective effect of normal flora

The incredible array of TV commercials, advertising disinfectant sprays, mouth washes, germ killing toothpastes, colon cleansers, hand antiseptics, toilet bowl and shower cleansers would lead us to accept the prejudice of most people, that the “only good bug is a dead bug”.  Their aim would seem to be the eradication of every last microbe inhabiting our homes and bodies. This multi-million dollar industry is aimed at microbial genocide, and it is only the hopelessness of their task, which guarantees the survival of our species. 

Stanley Falkow, an infectious disease specialist from Stanford University,  put it this way:

“Although we are prey to a small number of frankly pathogenic microorganisms, we are host to countless commensal bacteria, fungi, protozoan’s and minute insect species.  For most commensal bacterial species, their replication on or within us is essential to their survival.  Moreover, our own survival is likely dependent on the presence of those microorganisms.   S. Falkow. 1997. ASM News 63: 359-365.     

Acquisition of the normal body flora

Prior to birth, most humans are essentially sterile microbiologically.  There are viruses and bacteria that can cross the placental barrier, but these occurrences are rare.  Our first exposure to large numbers of microorganisms is occasioned by our short trip down the birth canal, although this exposure is delayed a bit for the nearly 30% of babies now born by Caesarean section.  This initial exposure to the rich microbial population of the mothers birth canal rarely causes problems although it does result in the inoculation of the newborn skin, nasopharynx and gut with a variety of microorganisms, not all of which persist in the tissues of the baby.  Handling, cuddling, cleansing and nursing contribute more representatives to the cast of microbial species gaining a foot-hold (biofilm hold?) in thenew-borne.

The diet of the new borne plays a decisive role in the microbial population of the gut with the flora of breast-fed infants being dominated by organisms such as Bifidobacteriumbifidus.  Transition to cows milk or milk based formula encourages the growth of gas producing coliforms, a transition commonly noted by parents as an olfactory change when diaper changing is necessary. Continued dietary changes take place as the diet becomes increasingly complex eventually taking on the characteristics of the adult gut with a predominance of such organisms as Bacteroides spp, Fusobacterium spp, anaerobic cocci, Enterococci, yeasts and protozoa.

A similar ecological succession occurs in virtually every colonized area of the body, although the rates of change and the specific microbial representatives differ dramatically from one organ system to another.

In the mouth for instance, the first appearance of teeth is coincident with the appearance of many species of streptococci including S. sanguis.  This organism becomes a major component of the adult oral flora.But S. sanguis is joined by a host of other inhabitants, which coaggregate with one another in a series of exquisitely precise pairings leading to an ecological succession mimicking in many respects the succession of plants in a newly cleared forest tract.  In one of the most thorough investigations of oral microorganisms, Paul Kolenbrander  has identified as many as 600 different species that are found in the mouth (Reference needed).