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What is a pathogen? Medical textbooks usually define a pathogen as any microorganism that causes disease. However, this widespread definition is problematic on a number of counts. Moreover, a generally accepted definition is not forthcoming among medical microbiologists, immunologists, and physicians. Here it is argued that there is another, and more pressing question to be asked, namely: what makes some organisms pathogenic and others not? Asking these questions instead allows for distinguishing pathogens from non-pathogens in a more flexible way, while at the same time emphasizing the roles of ecological and evolutionary processes in determining pathogenicity in infectious diseases. Historically, the difference between pathogenic and non-pathogenic variants was of great significance to medical bacteriologists who produced sophisticated classifications such as the Atlas of Bacteria Pathogenic in Man. The concept of “pathogenic germ” was also embedded in Robert Koch’s postulates and is partly reflected in today’s concepts of “virulence genes” and “pathogenicity islands”. From the birth of classical bacteriology until the present, several pathogen-like organisms were identified and classified based on morphological, cellular, or genetic criteria. As most pathogenic organisms known to nineteenth-century bacteriologists exhibited polysaccharide capsules that prevented phagocytosis, it was sensible to consider that pathogenic microbes markedly differed from those that did not cause disease. More broadly, within the Koch-Pasteur tradition, “microbes” were long regarded as being potentially dangerous and healthy tissues were presumed to be germ-free.

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2012-09-26   |   568 visitas   |   Evalua este artículo 0 valoraciones

Vol. 6 Núm.9. Septiembre 2012 Pags. 689-691 J Infect Developing Countries 2012; 6(9)