Methods of quantitative microbiological analyses that support the diagnosis, treatment, and prognosis of human infection.

The microorganisms responsible for the production of an infection may be considered to be in two classes: classical microbes and host-defined microbes. Classical microbes are those pathogens which fulfill the Koch-Henle postulates, and their isolation from a host indicates infection. They are not normally part of the body's normal flora, although they may be acquired by the host and enter into a passive relationship known as the carrier state. Examples of this type of microbe are Bacillus anthracis (anthrax), Yersinia pestis (plague), and Bordetella pertussis (whooping cough). Pathogens that require specific hosts have largely replaced the classical pathogen as a cause of infection in hospitalized patients. Especially in recent years, with the advent of new modes of anticancer treatment and the general ability of the medical community to extent a patient's life span by chemotherapy and innovative surgery, the contribution to morbidity and mortality by microbes has substantially increased. These host-specific pathogens are largely part of the body's normal flora. It is incumbent upon the clinical microbiologist to be able to distinguish the patient's normal microbial load, an increased load due to physiological factors, but not representing infection, and a significant change from normal which should be considered infection. The ability to distinguish infection from noninfection is one of the prime responsibilities of the clinical microbiology laboratory and has contributed to the development of the infectious disease subspecialty of internal medicine. This article will examine a critical question: Is there a relationship between the numbers of microorganisms isolated from a specimen and the production of infection, and, if so, does this relationship vary for the different anatomical sites of the body?