The body fluids which include blood, cerebrospinal fluid, syno-vial, pleural, pericardial, peritoneal, and other exudates are nor-mally sterile or transiently infected by microbes.
Some microbes from the mouth and gastrointestinal tract can invade the blood stream in healthy individuals (during tooth brushing or a bowel movement). These organisms are rap-idly removed and are of little significance. Thus, the isolation of an organism from body fluids should be considered significant unless the specimen is contaminated during its collection.
Organ tissues are also generally sterile unless they are infected following systemic spread. Some bacteria like Mycobacterium tuberculosis may be disseminated to lungs, liver,and kidney during initial infection and remain dormant. In this situation, the organism can be recovered from tissue samples. Long-term colonization with organisms like Pneumocystis carinii or latent viruses, such as herpes simplex virus or cytomegalovirus may commonly occur. The external auditory meatus, being an extension of the skin, is colonized primarily by S. epidermidis and diphtheroids.
Human body serves as a home for numerous microbes qual-itatively and quantitatively. They cover the skin and mucosal surfaces; occasionally, they invade into sterile sites to produce disease. These microorganisms act as a barrier to more viru-lent microorganisms, provide vitamins and required growth factors, or exist as commensal inhabitants. The complexity of the microbiota is influenced over time by environmental, host, and microbial factors. Thus, the knowledge of the human microbiota forms a fundamental building block of the normal physiological processes in the human body.