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 фA1122 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.