Phage therapy potentiates second-line antibiotic treatment against pneumonic plague
Plague pandemics and outbreaks have killed millions of people during the history of humankind. The disease, caused by Yersinia pestis bacteria, can currently be treated efficiently with antibiotics. However, in the case of multidrug-resistant (MDR) bacteria, alternative treatments are required. Bacteriophage (phage) therapy has shown efficient antibacterial activity in various experimental animal models and in human patients infected with different MDR pathogens. Herein, we evaluated the efficiency of {phi}A1122 and PST phage therapy, alone or in combination with second-line antibiotics, using a well-established mouse model of pneumonic plague. Phage treatment significantly delayed mortality and limited bacterial proliferation in the lungs. However, the treatment did not prevent bacteremia, suggesting that phage efficiency may decrease in circulation. Indeed, in vitro phage proliferation assays indicated that blood has inhibitory effects on lytic activity, which may be the major cause of treatment inefficiency. Combining phage therapy and second-line ceftriaxone treatment, which are individually insufficient, provided protection that led to survival of all infected animals, presenting a synergistic protective effect that represents a proof of concept for efficient combinatorial therapy in an emergency event of a plague outbreak involving MDR Y. pestis strains. Author summaryPlague, caused by Yersinia pestis bacteria, can be efficiently treated with antibiotics. However, alternative therapies for the case of natively evolved or maliciously generated antibiotic-resistant Y. pestis must be developed. Due to the global increase in antibiotic resistance, there is renewed interest in examining the effectiveness of bacteriophage-based alternative therapies. Here, using a mouse model of pneumonic plague, we demonstrate that phage treatment significantly delayed mortality. By monitoring bioluminescence of engineered Y. pestis strain and live bacterial counts, we show that phage therapy effectively inhibited bacterial proliferation in the lung but not in blood. In vitro analyses showed decreased phage activity in the presence of blood, which probably explains the low efficacy of phage treatment alone. Because combination therapies will be used in an emergency situation, we tested the efficacy of Y. pestis-lysing phages as adjunctive therapy with a second-line antibiotic, ceftriaxone. Whereas each individual treatment was insufficient, the combination provided effective protection and rescued all infected animals. These results clearly demonstrated the synergistic effect of combined phage and antibiotic therapy and represent a proof of concept for this alternative therapy against multidrug-resistant Y. pestis strains.