Abstract
Coevolution with bacteriophages is a major selective force shaping bacterial populations and communities. A variety of both environmental and genetic factors has been shown to influence the mode and tempo of bacteria–phage coevolution. Here, we test the effects that carriage of a large conjugative plasmid, pQBR103, had on antagonistic coevolution between the bacterium Pseudomonas fluorescens and its phage, SBW25ϕ2. Plasmid carriage limited bacteria–phage coevolution; bacteria evolved lower phage-resistance and phages evolved lower infectivity in plasmid-carrying compared with plasmid-free populations. These differences were not explained by effects of plasmid carriage on the costs of phage resistance mutations. Surprisingly, in the presence of phages, plasmid carriage resulted in the evolution of high frequencies of mucoid bacterial colonies. Mucoidy can provide weak partial resistance against SBW25ϕ2, which may have limited selection for qualitative resistance mutations in our experiments. Taken together, our results suggest that plasmids can have evolutionary consequences for bacteria that go beyond the direct phenotypic effects of their accessory gene cargo.
Highlights
Lytic phages are abundant in natural environments and a major cause of bacterial mortality [1]
It is increasingly recognized that bacteria–phage coevolution, the reciprocal evolution of bacterial resistance and phage infectivity, is an important evolutionary process shaping microbial communities [2,3]
Bacterial genetic background can affect the outcome of bacteria–phage coevolution: for example, epistatic interactions between the costs of deleterious mutations and phage resistance mutations can constrain the rate of bacterial resistance evolution and thereby limit the rate of coevolution [12]
Summary
Lytic phages are abundant in natural environments and a major cause of bacterial mortality [1]. All populations were founded with approximately 108 bacterial cells plus approximately 106 SBW25f2 particles in phage-containing treatments, and cultured in 30 ml microcosms containing 6 ml of King’s broth (KB) supplemented with 8 mM HgCl2 to ensure retention of the plasmid [19]. Preliminary experiments showed that at 8 mM HgCl2 there was no significant difference in growth between plasmidcontaining and plasmid-free cultures (electronic supplementary material, figure S1).
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