Theory suggests that phage displaying SLPs can possess both larger plaques (25
) and—in broth at higher host densities—a population growth rate advantage over phage displaying LLPs (1
). Here we describe a phage RB69 SLP, large-plaque variant, dubbed sta5, that we isolated following repeated serial transfers of phage RB69 WT to bacterium-containing broth cultures. Compared to RB69 WT we find that the sta5 mutant displays an SLP (~70 to 80% of WT), a commensurately smaller burst size (~30% of WT), and essentially identical eclipse period and adsorption kinetics (Fig. ). The phage T4 holin (t
) gene is thought to control the timing of phage-induced bacterial lysis (31
), and upon sequencing we have found a single missense difference between the gene t
of phage RB69 WT and that of the sta5 mutant (Fig. and ).
In cultures initiated with similar densities of both RB69 sta5 and WT (i.e., same-culture competition) the sta5 mutant appears to display a significant growth advantage over WT (Fig. ). This sta5 advantage is present given relatively high bacterial densities (≥~107
; e.g., as may be observed for E. coli
within mammalian colons prior to the formation of feces [1
]) but is lost if bacterial densities, under the conditions employed here, are reduced to less than ~106
bacteria/ml (Fig. ). These results are qualitatively consistent with hypotheses that phage with SLPs, despite displaying smaller burst sizes, can exhibit a within-culture, broth growth advantage so long as bacterial densities are sufficiently high (1
). It is difficult to extend this consistency to a more quantitative corroboration between theory and experimentation, however, since differences in population growth rates between phage RB69 WT and sta5, as we have observed (Fig. ), are smaller than differences between actual and predicted growth rates as presented by Abedon et al. (8
Though providing a selective benefit at higher bacterial densities (Fig. and ), shorter generation times still come at a fecundity cost (Fig. ). Indeed, during stock preparation the sta5 mutant is quite “sick,” with WT stocks typically displaying titers that are fivefold or greater than sta5 stock titers. This fecundity cost should be felt not only when bacterial densities are low (Fig. ) but also when free-phage decay rates are high (22
). For example, a 0.01 survival rate (0.99 prereproduction rate of decay) would reduce a burst size of 100 to just 1, which represents a population growth rate of zero. The same decay rate would reduce a burst size of 300 to 3, implying instead a threefold population increase per phage generation. Of perhaps greater relevance, high phage decay rates as well as significant phage dilution could reduce the likelihood of greater-than-one phage multiplicities of transmission between bacterial cultures. LLP phage, upon repeated low-multiplicity dispersal to unexploited (i.e., phage-free) bacterial cultures, therefore—given the greater LLP-phage productivity when grown absent within-culture competition—could come to dominate extended phage populations, even if bacterial densities are habitually high within individual cultures (Fig. ). Similarly, the marginal value theorem from optimal foraging theory (12
), as has been applied to phages elsewhere to derive within-culture optimal latent periods (37
), suggests that greater distances or costs between exploitable environments should select for more complete, e.g., LLP-like (Fig. ) exploitation of resources within individual environments.
Selection for SLP phage can also be viewed from the perspective of later-offspring discounting (22
): offspring produced sooner can be more valuable but only if they themselves can quickly contribute to phage population growth. A quick contribution of phage offspring to population growth between environments, however, would be the case only if environments are sufficiently close together. As exploitable resource-containing environments become ever closer, then a well-mixed total environment is increasingly approximated, which is just the situation in which we would expect SLP phage to outcompete LLP phage (Fig. and and reference 8
). In other words, if phage habitually initiate growth as significant-sized populations within bacterium-containing environments and if within-culture bacterial densities also are sufficiently high, then we may expect SLP phage to maintain a mixed-culture advantage both within and between cultures. Just such conditions were approximated during our original Hershey-type (21
) serial transfers that enriched our RB69 stock for SLP phage.
The possibility of selection between cultures for LLP phage helps explain why, by and large, lytic phage do not display latent periods that are nearly as short as their eclipse periods, i.e., as we observe here with phage RB69 sta5 (Fig. ). That is, any selective advantage displayed by the very short sta5 latent period probably should be interpreted as a consequence during serial passage of a relaxed selection for more effective phage transmission between cultures (26
). Thus, on the one hand SLP phage appear to be specialists for within-culture competition and then only when bacterial densities are sufficiently high. On the other hand, we suggest that LLP phage may be the more effective strategists, regardless of within-culture bacterial density, in terms of low-multiplicity transmission between cultures. We expect, therefore, that actual phage latent periods will represent an adaptive compromise between conflicting selection for SLPs when bacteria within cultures are increasingly available and for LLPs if new cultures are necessary for continued phage propagation and challenging to acquire.