Measurement of the bactericidal activity of fluoroquinolones against Streptococcus pneumoniae using the bactericidal index method

Kill curve analysis alone showed trovafloxacin to be more bactericidal than levofloxacin, grepafloxacin and moxifloxacin against four isolates of Streptococcus pneumoniae. However, using the bactericidal index (BI) method, levofloxacin was the most bactericidal fluoroquinolone using serum or lung biopsy concentration levels against the ofloxacin-susceptible strains and trovafloxacin was the most bactericidal against the ofloxacin-intermediate strain. None of the fluoroquinolones was bactericidal against the ofloxacin-resistant strain. With BIs using epithelial lining fluid or alveolar macrophage concentration levels, trovafloxacin or grepafloxacin was most bactericidal, respectively. These data illustrate that simple analysis of traditional kill curves may not be adequate in the evaluation of fluoroquinolone bactericidal activity. The results of this study suggest a need for further investigation to assess the role of tissue concentration and bactericidal activity in antimicrobial efficacy.