Bactericidal synergism between daptomycin and the phage lysin Cpl-1 in a mouse model of pneumococcal bacteraemia

Combination therapy may improve the outcome of Streptococcus pneumoniae-induced bacteraemia. Here we tested the combination of two antipneumococcal agents, daptomycin and Cpl-1 (the pneumococcal Cp-1 bacteriophage lysin), in a mouse model of pneumococcal bacteraemia. Mice were challenged intraperitoneally (i.p.) with 106 CFU of the extremely virulent serotype 2 S. pneumoniae D39 isolate. Subtherapeutic doses of daptomycin (0.4 mg/kg) and Cpl-1 (0.4 mg/kg and 1 mg/kg) were administrated i.p. either alone or in combination by a single bolus injection 1 h after bacterial challenge. Survival rates of animals were followed over a period of 7 days. Daptomycin (0.4 mg/kg) in combination with Cpl-1 (0.4 mg/kg) significantly increased the percentage of surviving mice at Day 7 (80%) compared with the untreated control (0%) and daptomycin or Cpl-1 monotherapy (35% and 0%, respectively). Whilst increasing the concentration of Cpl-1 to 1.0 mg/kg did not improve survival when injected alone, its combination with 0.4 mg/kg daptomycin further increased the survival rate to 95%. Thus, it was found that the combination of daptomycin with Cpl-1 was synergistic and bactericidal against S. pneumoniae in a mouse model of pneumococcal bacteraemia. To our knowledge, this is the first report of synergism between daptomycin and a phage lysin demonstrated in vivo. Such a combination could represent an interesting alternative therapy for the treatment of pneumococcal bacteraemia/sepsis and possibly other severe pneumococcal infections.