Sustained coevolution of phage Lambda and Escherichia coli involves inner as well as outer membrane defenses and counter-defenses

Bacteria often evolve resistance to phage through the loss or modification of cell-surface receptors. In Escherichia coli and phage {lambda}, such resistance can catalyze a coevolutionary arms race focused on host and phage structures that interact at the outer membrane. Here, we analyze another facet of this arms race involving interactions at the inner membrane, whereby E. coli evolves mutations in mannose permease-encoding genes manY and manZ that impair {lambda}s ability to eject its DNA into the cytoplasm. We show that these man mutants arose concurrently with the arms race at the outer membrane. We tested the hypothesis that {lambda} evolved an additional counter-defense that allowed them to infect bacteria with deleted man genes. The deletions severely impaired the ancestral {lambda}, but some evolved phage grew well on the deletion mutants, indicating they regained infectivity by evolving the ability to infect hosts independently of the mannose permease. This coevolutionary arms race fulfills the model of an inverse-gene-for-gene infection network. Taken together, the interactions at both the outer and inner membranes reveal that coevolutionary arms races can be richer and more complex than is often appreciated. IMPACT STATEMENTLaboratory studies of coevolution help us understand how host defenses and pathogen counter-defenses change over time, which is often essential for predicting the future dynamics of host-pathogen interactions. One particular model, termed "inverse-gene-for-gene" coevolution, predicts that coevolution proceeds through alternating steps, whereby hosts lose the features exploited by pathogens, and pathogens evolve to exploit alternative features. Using a classic model system in molecular biology, we describe the nature and timing of a previously overlooked step in the coevolution of E. coli and bacteriophage lambda. Our work demonstrates that this mode of coevolution can profoundly re-shape the interactions between bacteria and phage.