Experimental Examination of BacteriophageLatent-Period Evolution as a Response to BacterialAvailability

Abstract

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 (>ϳ10 7 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 ϳ70 to 80% of that of the wild type-which is nearly as short as the RB69 eclipse period-and which has a corresponding burst size that is ϳ30% of that of the wild type. We show that at higher host densities (>ϳ10 7 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 (<ϳ10 6 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.Phage serve as important viral predators of bacteria (17,38,39), and the rate that bacteria are phage adsorbed is proportional to phage density (2, 24). Here we consider two factors affecting lytic-phage population growth to higher densities: fecundity and generation time. The fecundity of an individual phage is its per-bacterium burst size. Generation time is less straightforward, consisting of a diffusion-limited extracellular search for bacterial hosts followed by a latent period. The latent period encompasses the active phage infection of an individual bacterium. The extracellular search is the period of inert phage dissemination. A higher host densities-e.g., 10 7 versus 10 5 bacteria/ml-phage populations can grow more rapidly simply because phage can find bacteria faster (7).As originally demonstrated in the classic work of Doermann (15), the truncation of an active phage infection, e.g., as occurs following artificial lysis, reduces the duration of intracellular phage progeny maturation. This truncation results in a decline in a phage's burst size. Various authors nonetheless have theorized that individual phages can achieve faster population growth through latent-period reduction despite the associated reduction in burst size (1,8,11,27,37). This advantage occurs only at higher ...

Genomewide Patterns of Substitution in Adaptively Evolving Populations of the RNA Bacteriophage MS2

“…The expression level of both the lysis and the replicase genes is ordinarily likely to be under stabilizing selection, with optimal expression at an intermediate level. In the case of the lysis gene, an optimal lysis time allows phage to balance the definite benefits of linear growth inside the host with the possibility of exponential growth through infection of new hosts (Bull et al 2000;Abedon et al 2003). Similarly, phage growth is maximized when replicase is expressed at an intermediate level, probably because excess replicase protein inhibits coat gene translation and may poison host cells (van Himbergen et al 1993).…”

Optimal Foraging by Bacteriophages through Host Avoidance

“…Selection was measured directly in evolutionary competition between a broad and a narrow host range genotype, avoiding the usual difficulties of inferring fitness from a proxy such as energy intake. Few studies have actually measured evolution in response to optimal foraging predictions, although there are some precedents (Abedon et al 2003;Heineman and Bull 2007;K. L. Hillesland, personal communication).…”

“…Optimal diet notions are also typically intended for animals, although they have been applied to bacterial dispersal (K. L. Hillesland, personal communication), and trade-off models incorporating diminishing returns have been used to explain bacterial metabolic specialization (Pfeiffer et al 2005). Also, a somewhat related model of optimal phage lysis time has been interpreted from the perspective of optimal foraging in a patchy environment (Abedon et al 2003;Wang 2006;Heineman and Bull 2007).…”

Bacteriophage Adsorption Rate and Optimal Lysis Time

“…As the analysis showed, the optimal lysis time is shorter when the host density is high and vice versa (Wang et al 1996). Several empirical studies using different phage systems have given some support to the utility of this optimality model in describing the effect of host density on the evolution of phage lysis time (Abedon et al 2003;Wang 2006;Heineman and Bull 2007); although the original model tends to break down under conditions of low host density (Abedon et al 2001;Heineman and Bull 2007). This is because when the host density (N) is high, the average search time for a phage to be adsorbed onto the host cell surface can be approximated by t S ¼ 1/rN, again assuming the mass-action kinetics.…”

“…The wealth of phage biology also allows us to hypothesize trade-off relationships among various phage life-history traits (De Paepe and Taddei 2006;Caraco and Wang 2008). So far, among these traits, the problem of optimal lysis time has received the most theoretical and experimental attention (Abedon 1989;Wang et al 1996;Abedon et al 2001Abedon et al , 2003Bull 2006;Wang 2006;Heineman and Bull 2007). The existence of an optimal lysis time can be understood as the consequence of the trade-off between generation time and burst size.…”