Plasmid dependent phages: helping bacteria make the most out of plasmids

Bacterial communities use conjugative plasmids to store and transmit genes, such as those for antibiotic resistance or auxiliary metabolism, that address specific environmental challenges. When these challenges subside, the plasmid can be lost from the population, thus avoiding any costs associated with the now irrelevant genes. But how can these genes persist for the long term, to be used when the environmental challenge appears again? We propose that plasmid-dependent phages can solve this problem by, counterintuitively, promoting plasmid persistence. By targeting the conjugative pili that enables a plasmid to spread among bacteria, plasmid-dependent phages suppress the plasmid's spread but do not necessarily drive it to extinction. We investigate this hypothesis using population modeling, and show that for realistic parameter regimes, the effective population-level costs of carrying a plasmid and of phage predation can both be negligible during periods between environmental challenges. Thus, in these models, plasmid-dependent phages render even costly plasmids effectively neutral.