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One has only to peruse the table of contents of this book to appreciate the diversity of medical pathogens that are associated with infectious diseases. It has been estimated that we currently have the capacity to identify fewer than 10% of the pathogens responsible for causing human disease because of our inability to culture or target these organisms using molecular probes. Yet the diversity of even these identifiable pathogens is so great that it is important to understand the subtle differences associated with each infectious agent. The reason for understanding these subtleties is significant because each infectious agent has specifically adapted to a particular mode(s) of transmission, a mechanism(s) to grow in human hosts (colonization), and a mechanism(s) to cause disease (pathology). As such, a vocabulary that consistently communicates the unique characteristics of infectious organisms to students, microbiologists, and health care workers is critical to avoid the chaos that would ensue without the organizational restraints of bacterial taxonomy (Gk. taxon = arrangement; eg, the classification of organisms in an ordered system that indicates a natural relationship).

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Classification, nomenclature, and identification are three separate but interrelated areas of bacterial taxonomy. Classification is the categorization of organisms into taxonomic groups. Classification of bacteria requires experimental and observational techniques; this is because biochemical, physiologic, genetic, and morphologic properties are often necessary for an adequate description of a taxon. Nomenclature refers to the naming of an organism by international rules (established by a recognized group of medical professionals) according to its characteristics. Identification is practical use of a classification scheme to (1) isolate and distinguish desirable organisms from undesirable ones, (2) verify the authenticity or special properties of a culture in a clinical setting, and (3) isolate and identify the causative agent of a disease. The latter may lead to the selection of specific pharmacologic treatments directed toward their eradication, a vaccine mitigating their pathology, or a public health measure (eg, handwashing or use of a condom) that prevents their transmission.

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Identification schemes are not classification schemes, although there may be some superficial similarity. An identification scheme for a group of organisms can be devised only after that group has first been classified (ie, recognized as being different from other organisms). For example, the popular literature has reported Escherichia coli as being a cause of hemolytic uremic syndrome (HUS) in infants. There are hundreds of different strains that are classified as E coli but only a few that are associated with HUS. These strains can be distinguished from the many other E coli strains by antibody reactivity with their O- and H-antigens, as described in Chapter 2 (eg, E coli O157:H7). Taxonomy, and the nomenclature that accompanies it, is an imprecise and evolving science. Just as our societal vocabulary evolves, so does the vocabulary of medical microbiology. Any professional associated with infectious disease should be aware of the evolving taxonomy of infectious microorganisms.

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Taxonomic ranks form the basis ...

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