Objectives: DC-159a and sitafloxacin show greater bactericidal activity against Streptococcus pneumoniae than garenoxacin and other quinolones. We investigated whether the autolysis induced by these quinolones contributes to their rapid bactericidal activity. Methods: Time-kill studies were conducted against a S. pneumoniae clinical isolate in broth with choline chloride, which is known to inhibit autolytic amidases, and lytA mutants. Western blot analysis was performed to examine LytA production. Scanning electron microscopy (SEM) was used to investigate morphological differences after exposure to quinolone. Results: Bactericidal activity of DC-159a and sitafloxacin against S. pneumoniae at 2 h of exposure to twice the minimum inhibitory concentration (MIC) was found to decrease by approximately 1 log CFU/ml when autolytic amidases were blocked. Time-kill studies using lytA mutants showed that DC-159a exhibited slower killing than that against the lytA-positive strains. On exposure to the MIC and twice the MIC of DC-159a and sitafloxacin, R6 and a clinical isolate overexpressed LytA, while garenoxacin caused a less significant increase in LytA than DC-159a and sitafloxacin. Scanning electron microscopy images revealed that R6 treated with DC-159a underwent distinct morphological changes, while the lytA mutant did not. Conclusions: The present study demonstrated that quinolone-induced autolysis may provide quinolones more powerful bactericidal activity against S. pneumoniae.

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