Functions of Resident Flora
It should be appreciated that microbes serve a useful purpose in their human hosts. The normal microbiota maintains a pro-tected environment that prevents colonization with potentially pathogenic organisms. For example, Clostridium difficile pro-duces gastrointestinal disease when the normal intestinal flora have been reduced or removed by antibiotics. The production of proteolytic enzymes by microbes augments host factors in the digestion of food. Intestinal bacteria can also synthesize vita-mins and other biological products (e.g., biotin, pantothenic acid, pyridoxine, riboflavin, vitamin K, etc.). Colicins produced by some bacteria of normal flora prevent harmful effects of the bacteria.
The normal microbial flora is more or less constant for each mammalian species and is broadly divided into residents and transients. The interaction between microbes and humans can result in the following general outcomes: (a) disease, (b) transient colonization, and (c) prolonged colonization.
Disease results when the interaction between microbe and human host results in a pathological process. This process is mediated by microbial factors or by the host’s immune response to the presence of the organism. The other outcome of microbe and host interaction is colonization, either transient or prolonged.
Infection is another term used for colonization, which does not imply disease, but rather the association of the microbe with the human hosts for a time. The transient and prolonged colonization imply a distinction based on the duration of the interaction, which may extend to weeks, months, or years.