In vitro analysis of colistin and ciprofloxacin antagonism of Pseudomonas aeruginosa phage PEV2 infection activities

Phage therapy is a century-old technique employing viruses (phages) to treat bacterial infections. In the clinic, phage therapy often is used in combination with antibiotics. Antibiotics, however, interfere with critical bacterial activities, such as DNA and protein synthesis, which also are required for phage infection processes. Resulting antagonistic impacts of antibiotics on phages nevertheless are not commonly determined in association with phage therapy studies using standard, planktonic approaches. Here we assess the antagonistic impact of two antibiotics, colistin and ciprofloxacin, on the bactericidal, bacteriolytic, and new virion production activities of Pseudomonas aeruginosa podovirus PEV2, using a broth culture, optical density-based lysis profile assay. Though phage-antibiotic combinations were more potent in reducing cell viability than phages or antibiotics alone, colistin substantially interfered with phage PEV2 bacteriolytic and virion-production activities at minimum inhibitory concentration (MIC). Ciprofloxacin, by contrast, had no such impact at 1x MIC or 3x MIC. At higher but still clinically relevant concentrations (9x MIC) burst sizes were still significant (~30 phages/infected bacterium). We corroborated these lysis profile results by more traditional measurements (colony forming units, plaque forming units, one-step growth experiments) and two other P. aeruginosa phages. To our knowledge this is the first study in which detailed antibiotic impact on P. aeruginosa phage infection activities has been determined under conditions similar to those used to determine antibiotic MICs and could point especially to ciprofloxacin as a minimally antagonistic phage therapy co-treatment of P. aeruginosa infections.