Do variable environments promote virulence? A bacteriophage model.
Temperate bacteriophages account for a substantial fraction of known viruses. This class may undergo two possible modes of replication: a lytic life cycle or a lysogenic life cycle. When in a lytic life cycle, the virus infects and kills host cells, releasing a large number of progeny virus into the environment. In contrast, when in a lysogenic life cycle, the virus integrates its genome into the host cell DNA, relying on cell division to further propagate its lineage. Given that a lytic cycle seemingly confers an immediate benefit to the virus, the conditions under which the lysogenic cycle is favoured have been debated for many years [2]. A recent study of over 2000 bacterial genomes identified a correlation between the host's minimal doubling time and the probability of lysogeny [3]. From other work [1], it is known that bacteria with decreased minimal doubling times are often found in variable environments. Here, we develop a system of ordinary differential equations to explore the effects that a variable environment will have on the optimal probability of lysogeny. This chemostat-based model includes a population of bacterial hosts (both slow- and fast-growing bacteria), and viral populations in both the environment and integrated into the bacterial genomes, as well as a concentration of resource available to the bacteria. By manipulating the variability in the environment, we delineate parameter regimes in which lysogeny is favoured. This occurs when virus in the environment is washed out or loses infectivity faster than viral genomes that are integrated in host genomes.
This is joint work with L.M. Wahl.
References
[1] Koch, A.L. (2001). Oligotrophs versus copiotrophs. Bioessays, 27(3), 657-661.
[2] Stewart, F.M., \& Levin, B.R. (1984). The population biology of bacterial viruses: why be temperate. Theoretical Population Biology, 26, 93-117.
[3] Touchon, M., Bernheim, A., \& Rocha, E.P.C. (2016). Genetic and life-history traits associated with the distribution of prophages in bacteria. The ISME Journal, 10(11), 2744-2754.
