Spatial constraint drives negative frequency dependent selection of phage weaponization

Abstract

Biofilm growth and phage exposure are common to diverse bacterial species. Studying phage-host interaction and population dynamics in biofilms with cellular resolution remains a significant challenge, especially when attempting to recapitulate the natural environments that microbes occupy. Here we study the population dynamics of phage K139 lysogenized and non-lysogenized Vibrio cholerae when growing in biofilms on the surface of chitin particles in seawater, replicating key features of V. cholerae ecology in the marine environment. We find that lysogenized V. cholerae, via spontaneous lytic induction and phage release, kill and displace non-lysogenized bacteria on chitin surfaces. After lysogens become common, however, they can no longer displace remaining non-lysogenized cells. Using a combination of modeling approaches and microscopy experiments, we show that the lysogens capacity to displace non-lysogens depends on the ability of phages released by spontaneous induction to reach susceptible non-lysogens. Phage access to non-lysogenized bacterial hosts declines once lysogens become common, and this occurs due to the spatial constraints inherent to biofilm growth as well as to superinfection immunity, which neutralizes phage particles adsorbed to lysogens. Once lysogens comprise the majority of the host population, they can be selected against, because they still incur the cost of spontaneous induction without gaining the benefit of killing non-lysogen cells via phage release. The cost of lytic induction, phage-mediated killing of non-lysogens, and constraints on phage mobility within host bacterial biofilms together yield population dynamics that are consistent with negative frequency dependent selection for lysogenized cells under physiologically realistic growth conditions. Significance StatementBacteria often produce and live within biofilm communities in natural environments, where they also encounter many threats including bacteriophages. Here we show how temperate phages can confer a competitive advantage to their hosts via lytic induction and phage release within biofilms of V. cholerae on lab-grown marine snow particles. However, the spatial constraints of biofilm architecture and phage-neutralizing superinfection immunity place limits on the extent to which lysogens can competitively displace non-lysogens via phage release.