In vitro interactions of LytA, the major pneumococcal autolysin, with two bacteriophage lytic enzymes (Cpl-1 and Pal), cefotaxime and moxifloxacin against antibiotic-susceptible and -resistant Streptococcus pneumoniae strains

Objectives: In an innovative therapeutic exploitation against antibiotic-resistant Streptococcus pneumoniae, here we have evaluated the in vitro activity of a purified bacterially-encoded cell wall lytic enzyme, LytA (the major pneumococcal autolysin), and compared it with those of Cpl-1 and Pal (pneumococcal phage lytic enzymes) and two antibiotics versus four pneumococcal strains. Methods: Two serotype 3, penicillin-susceptible strains and two penicillin-resistant (serotypes 19F and 19A, respectively) S. pneumoniae clinical isolates were used. The effect of several combinations of lytic enzymes and antibiotics (cefotaxime and moxifloxacin) was studied by chequerboard and time-kill assays, the latter at concentrations of 0.25x MIC. Results: LytA was more active than Cpl-1 and Pal. By the chequerboard technique, the combination of LytA and cefotaxime was synergistic for one of the two cefotaxime-resistant strains studied. The combined use of Cpl-1 and Pal was synergistic for three of the four strains, as was Cpl-1 with antibiotics for two of the three strains studied. In the time-kill assays, after 5 h of exposure to LytA, Cpl-1 or Pal, the mean differences in colony counts versus controls were -3.55, -2.66 and -2.71 log(10) cfu/mL, respectively. The combination of LytA/Pal reduced the bacterial inoculum > 2 log units for three of the four strains. LytA combined with cefotaxime or moxifloxacin achieved > 3 log units decrease for the strains tested. Particularly, a strong synergism was observed with LytA/cefotaxime for one cefotaxime-resistant meningeal strain. LytA/moxifloxacin was synergistic for the quinolone-resistant strain when tested by time-kill methodology, and just close to synergistic (fractional inhibitory concentration index of 0.58) by the chequerboard technique. Antagonism was not observed for any combination when assayed by either method. Conclusions: LytA, Cpl-1 or Pal, alone or in combination, might prove to be effective in combination therapy, as well as in monotherapy against S. pneumoniae. These results suggest avenues of research to study the cell wall lytic enzymes as anti-pneumococcal therapeutic agents.