MICROBIAL INFECTIONS OF HUMANS(HUMAN MICROBIOLOGY)
Ignoring a low rate of mutation as a source of heterogeneity, bacterial division results in clonal
expansion, with the daughter cells considered to be similar if not identical to the parent cells.
Collectively, these cells are called a population. However, bacteria rarely exist as a single species
within any one habitat but, instead, are usually found as collections of different species called
communities, where each particular species will exist in a particular niche but may well contribute to
the maintenance of the entire community (e.g. syntrophism).
Interactions between microbial communities may have a negative (e.g. competition) or positive (cooperation) outcome. The interactions will be between both a single population (i.e. between members of
the same species) or between members of different populations. Typically, co-operation will occur at
low population densities, whereas competition will dominate at high population density. The net effect
will regulate the population size to an optimum, depending on the selection pressures.
It is of interest that bacteria that infect humans proliferate in a clonal manner seemingly independent of
other organisms. The invading organism is thus growing as a population and nota community. Whilst
there are examples of synergistic infections, where more than one species of organism is needed to
initiate the infection, this appears to be the exception rather than the rule. As we shall see later, rather
than depending on the microbial community in the host, an invading pathogen is often inhibited from
proliferating by the presence of microbial populations residing normally in the host.
INTERACTIONS BETWEEN ORGANISMS:
NEUTRALISM: Neither organism benefits, but neither is harmed. It is suspected that such a state is rare in nature.
COMMENSALISM: Literally ‘eating at the same table’, cohabitation is the association of two different organisms where neither is disadvantaged but, unlike symbiosis, the mutual dependence is minimal. Whilst neither organism has any strict dependency on the other (e.g. no nutritional needs), commensalism usually benefits one of the partners whilst having no impact on the other.