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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


Chapter 21 describes the astounding diversity and adaptability of bacteria 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, bacterial pathogens occupy as prominent a position as at 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. This chapter lays out the basic mechanisms that bacteria use to produce disease and the genetic mechanisms involved in their deployment. The purpose is to provide a foundation for explaining how these mechanisms are used by specific bacterial pathogens described in Chapters 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41.


  • 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, is also highly infectious, but in addition leads to death in a few days in over 70% of cases.


As discussed in Chapter 1, humans have a rich microbiota, and the composition of that flora is mostly bacterial. Long-term survival for a primary pathogen ...

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