Bactericidal activity, morphological alterations, and synergistic interactions of rufloxacin, a new fluoroquinolone, alone and in combination with its N-desmethylate D derivative (MF 922).

The in vitro activity of rufloxacin, alone and in combination with its metabolite (MF 922) against common respiratory and urinary tract pathogens and anaerobes was assessed. No synergistic interaction between rufloxacin and MF 922 was observed by the checkerboard technique against aerobic bacteria. When the time-kill system was employed, 24 synergistic interactions were noted out of 30 tests performed (80%), of which 14 (100%) were with Enterobacteriaceae isolates, 2 with Moraxella catarrhalis (50%), 4 with Haemophilus influenzae (100%), 2 with Staphylococcus aureus (50%) and 2 with Streptococcus pneumoniae and Streptococcus pyogenes (50%). Synergism was found with all Bacteroides fragilis irrespectively of the method used. Rufloxacin alone or in combination with MF 922, at concentrations achieved in vivo, induced morphological alterations in all the pathogens analyzed with the exception of M. catarrhalis and H. influenzae. Towards S. pyogenes and S. pneumoniae, the same levels of rufloxacin and MF 922 were capable of inducing only bacteriostatic rather than bactericidal effect, followed by reversible morphological modifications. The presence of 50% human serum in the test media did not affect the results.