Host-Pathogen Interactions: Basic Concepts of Microbial Commensalism, Colonization, Infection, and Disease

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   Fig. 1.

   Outcome of the host-microbe interaction in the context of the damage framework proposed in reference 7. The various states shown in this figure are defined in Table 2. Double-headed arrows indicate conditions where there may be variable amounts of damage. Single-headed arrows indicate the following. A, acquisition of a microbe can be followed by elimination through physical defenses or immune mechanisms. B, acquisition of certain microbes results in damage and disease in certain hosts. C, certain commensal microbes can cause disease if the state of commensalism is disturbed by immune impairment or alterations of the host microbial flora (e.g., C. albicans can cause pharyngeal candidiasis and vaginal candidiasis in the settings of immune suppression and antibiotic use, respectively). D, the state of colonization may be terminated by an immune response (e.g., transient nasopharyngeal carriage of N. meningiditis or S. pneumoniae). The trigger for the immune response is not well understood, but may occur after the damage threshold is reached. E, the state of colonization may lead to disease if sufficient damage ensues from the interaction. The damage may be host-mediated, pathogen-mediated, or both. F, the state of colonization may lead to a state of persistence (chronicity and latency), whereby the immune response is unable to eradicate the infection despite continued damage (e.g., latent M. tuberculosis or Histoplasma capsulatum infection in tissue granuloma). G, an immune response or therapy may eradicate the infection but this does not always terminate disease, because the damage may be irreversible (e.g., poliomyelitis) or continue through immunological mechanisms (e.g., reactive arthropathies). H, if sufficient damage is incurred as a result of the host-microbe interaction, death ensues. I, persistent infections may reactivate and cause overt disease (e.g., reactivation tuberculosis).

• Copyright © 2000 American Society for Microbiology