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Figure. Four mechanisms by which antibiotic treatment can create
selection for resistance in the population, showing direct effects—increased
resistance in treated (yellow) vs. untreated (white) hosts, and indirect
effects—increased resistance in others (turquoise) due to treatment
of specific hosts. (A) Subpopulations (usually mutants) of resistant
(red) bacteria are present in a host infected with a predominantly susceptible
(green) strain; treatment fails, resulting in outgrowth of the resistant
subpopulation, which can then be transmitted to other, susceptible hosts
(turquoise). (B) Successful treatment of an individual infected
with a susceptible strain reduces the ability of that host to transmit
the infection to other susceptible hosts, making those hosts more likely
to be infected by resistant pathogens than they would otherwise have been,
and shifting the competitive balance toward resistant infections. (C)
Treatment of an infection eradicates a population of susceptible bacteria
carried (often commensally) by the host, making that host more susceptible
to acquisition of a new strain. If the newly acquired strain has a high
probability of being resistant (as in the context of an outbreak of a
resistant strain), this can significantly increase the treated individual’s
risk of carrying a resistant strain, relative to an untreated one. (D)
Treatment of an infection in an individual who is already colonized (commensally)
with resistant organisms may result in increased load of those organisms
if competing flora (perhaps of another species) are inhibited—leading
to increased shedding of the resistant organism and possibly to increased
individual risk of infection with the resistant organism.
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