How are antibiotic-resistant variants selected
in the laboratory versus in natural
environments?
Traditionally, agar-plate based bacterial genetics has studied
mutations that confer large phenotypic effects. In
the laboratory genetic selections are generally set-up with
strong selective pressures to allow identification of preexisting
mutants with clear-cut phenotypes. As a result,
mutants are obtained that have distinct phenotypes (e.g.
growth or no growth under specific conditions or highlevel
antibiotic resistance). The reason for this bias is
practical since evaluation of phenotypes is classically
based on ocular inspection of bacterial behavior on agar
plates. Even though this approach has been extremely
successful it only allows observation of the fraction of all
potential mutations that confers large phenotypes (with
regard to selection coefficients most classically isolated
mutants have Djsj > 0.1). As a result, mutants with small
effects (for example mutants with a small reduction in
growth rate or survival), are often not detectable in the
laboratory using classical genetic methods, and the major
part of the mutational space is not identified, that is,
mutations with Djsj-values ranging from 0.1 down to 108
for a bacterial species like E. coli with an effective
population size estimated from 105 [13,14] to 108 [15].
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