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Applied and Environmental Microbiology, December 2003, p. 7499-7506, Vol. 69, No. 12
0099-2240/03/$08.00+0 DOI: 10.1128/AEM.69.12.7499-7506.2003
Copyright © 2003, American
Society for
Microbiology. All Rights Reserved.
Department of Microbiology, Ohio State University, Mansfield, Ohio,1 NanoFrames LLC, Boston, Massachusetts2
Received 21 July 2003/ Accepted 22 September 2003
For
obligately lytic bacteriophage (phage) a trade-off exists between
fecundity (burst size) and latent period (a component of generation
time). This trade-off occurs because release of phage progeny from
infected bacteria coincides with destruction of the machinery necessary
to produce more phage progeny. Here we employ phage mutants to explore
issues of phage latent-period evolution as a function of the density of
phage-susceptible bacteria. Theory suggests that higher bacterial
densities should select for shorter phage latent periods. Consistently,
we have found that higher host densities
(
107 bacteria/ml) can enrich stocks of
phage RB69 for variants that display shorter latent periods than the
wild type. One such variant, dubbed sta5, displays a latent period that
is
70 to 80% of that of the wild typewhich is
nearly as short as the RB69 eclipse periodand which has a
corresponding burst size that is
30% of that of the
wild type. We show that at higher host densities
(
107 bacteria/ml) the sta5 phage can
outcompete the RB69 wild type, though only under conditions of direct
(same-culture) competition. We interpret this advantage as
corresponding to slightly faster sta5 population growth, resulting in
multifold increases in mutant frequency during same-culture growth. The
sta5 advantage is lost, however, given indirect (different-culture)
competition between the wild type and mutant or given same-culture
competition but at lower densities of phage-susceptible bacteria
(
106 bacteria/ml). From these observations
we suggest that phage displaying very short latent periods may be
viewed as specialists for propagation when bacteria within cultures are
highly prevalent and transmission between cultures is easily
accomplished.
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