Bactericidal activity of gemifloxacin and other quinolones against Streptococcus pneumoniae

This study compared the bactericidal activity of gemifloxacin (SB-265805) and a panel of test quinolones against two ciprofloxacin-resistant pneumococcal strains (Streptococcus pneumoniae 502226 and 503244) and one ciprofloxacin-sensitive strain (S. pneumoniae C3LN4). Activities were compared by calculating the bactericidal index of these agents. Gemifloxacin was found to be the most bactericidal quinolone tested against these strains. This finding confirms previous data indicating the superior in vitro activity of gemifloxacin against pneumococci, including ciprofloxacin-resistant strains. Although both ciprofloxacin-resistant strains tested had similar quinolone MICs, they differed considerably in their susceptibility to the bactericidal action of these agents. S. pneumoniae 502226 was more readily killed by quinolones than S. pneumoniae 503244 but, as would be expected, both were less susceptible than the ciprofloxacin-sensitive strain. Of the quinolones tested, trovafloxacin showed disproportionally poor activity against the ciprofloxacin-resistant strains even though potent activity was present against the ciprofloxacin-sensitive strain. These data highlight the importance of assessing quinolone bactericidal activity in addition to the MIC when evaluating new members of this antimicrobial class.