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Abstract

Bacteriophages infect Gram-negative bacteria by attaching to molecules present on the bacterial outer membrane, often lipopolysaccharides (LPS). Modification of the LPS can lead to phage resistance. LPS modifications also impact antibiotic susceptibility, allowing for phage-antibiotic synergism. The mechanism for these synergistic interactions is unclear. Here, we show that antibiotics affect the evolution of phage resistance using phage ΦX174 and Escherichia coli C wildtype. We use a collection of E. coli C LPS mutants, each of which is resistant to ΦX174, and has either a “rough” or “deep-rough” LPS phenotype. Growth of deep rough mutants is inhibited at subinhibitory chloramphenicol concentrations. In contrast, gentamicin has no major effect on growth. Hypothesis testing shows that treating E. coli C wildtype with ΦX174 and chloramphenicol eliminates deep rough mutants, and reduces phage resistance evolution. Our results show that differential survival of phage resistant mutants with antibiotics explains phage-antibiotic synergism in our model system.