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Pathogenicity is, in a sense, a highly skilled trade, and only a tiny minority of all the numberless tons of microbes on the earth has ever involved itself in it; most bacteria are busy with their own business, browsing and recycling the rest of life. Indeed, pathogenicity often seems to me a sort of biological accident in which signals are misdirected by the microbe or misinterpreted by the host.

—Lewis Thomas, The Medusa and the Snail


These words refer to all microorganisms and infectious diseases but are particularly appropriate for those caused by bacteria. In the previous chapter, we learned of their astounding diversity and adaptability made possible by simplicity, speed, and robust genetic exchange mechanisms. When antibiotics came into use in the middle of the last century, it was supposed to be the end for the bacteria. How wrong we were! Except for those prevented by immunization, the bacterial pathogens occupy as prominent position as any time since the widespread implementation of public health measures a century ago. The emergence of new pathogens and the resistance of familiar ones to the antimicrobial agents developed in the “arms race” against them are primarily responsible. Staphylococcus aureus, the “all-time champion” of pathogens is just as prominent and just as confounding a cause of disease today as when Sir Alexander Ogston observed it in the wounds of his surgical patients in the 1880s.


This chapter lays out the basic mechanisms that bacteria use to produce disease. The purpose is to provide a foundation for explaining how these mechanisms are used by the bacterial pathogens in Chapters 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, and 41. Before beginning, a few definitions are in order:


  • Pathogenicity—The ability of any bacterial species to cause disease in a susceptible human host.

  • Pathogen—A bacterial species able to cause such disease when presented with favorable circumstances (for the organism).

  • Virulence—A term which presumes pathogenicity, but allows expression of degrees from low to extremely high, for example:


  • Low virulenceStreptococcus salivarius is universally present in the oropharyngeal flora of humans. On its own, it seems incapable of disease production, but if during a transient bacteremia it lands on a damaged heart valve, it can stick and cause slow but steady destruction.

  • Moderate virulenceEscherichia coli is universally found in the colon, but if displacement to other sites such as adjacent tissues or the urinary bladder regularly causes acute infection.

  • High virulenceBordetella pertussis, the cause of whooping cough, is not found in the resident flora, but if encountered it is highly infectious and causes disease in almost every nonimmune person it contacts.

  • Extremely high virulenceYersinia pestis, the cause of plague, ...

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