Temperate Phage-Antibiotic Synergy Eradicates Bacteria Through Depletion of Lysogens

Mounting stress from multidrug-resistant bacteria has renewed interest in bacteriophage (bacterial virus, phage) therapy as an alternative to antibiotics. While virulent phages quickly infect and kill their host through a lytic lifecycle, temperate phages can stably integrate into the host’s genome until exposed to stressors that can cause a switch to a lytic cycle. Dormant phages can protect their host, known as lysogen, from further phage infections. In vitro this results in rapid host regrowth, which would likely complicate treatment. All case studies and clinical trials to date use virulent phages. However, temperate phages are far easier to find and more abundant in (and on) humans, playing a vital role in vivo. To make use of these abundant entities, we propose a synergy using stressors such as antibiotics, which are known to awaken dormant phages. Here we demonstrate that co-administration of the temperate phage HK97 with sub-inhibitory concentrations of a fluoroquinolone antibiotic, ciprofloxacin, results in bacterial eradication (≥8 log reduction) in vitro. This synergy is up to 4.5 x105 fold greater than the multiplicative effects of the phage and antibiotic alone. Furthermore, there is no large increase in final phage titre, latency period or burst size – as such, this synergy is distinct from traditional phage-antibiotic-synergy described with virulent phages. We demonstrate the presence of the antibiotic prevents lysogen colony formation over time, the biggest hurdle in the current use of temperate phages in therapy. As the interaction between temperate phages and stressors such as ciprofloxacin are known to be widespread, the findings from this work may allow for the use of these phages in combination with antibiotics for the treatment of antimicrobial-resistant infections.