Benchmarking the in vitro activities of moxifloxacin and comparator agentsagainst recent respiratory isolates from 377 medical centers throughout the United States

To benchmark the activity of moxifloxacin (a newer fluoroquinolone), a U.S. study comprising 16,141 contemporary isolates of Streptococcus pneumoniae (5,640), Haemophilus influenzae (6,583), and Moraxella catarrhalis (3,648) referred from 377 institutions during 1998 is described. For S. pneumoniae the modal MIC and MIC at which 90% of the isolates were inhibited (MIC90) f or moxifloxacin were 0.12 and 0.25 mu g/ml, respectively, independent of su sceptibility to other drug classes, geography, or site of infection. Eleven isolates were intermediate or resistant to levofloxacin and grepafloxacin; of these isolates, 1 remained susceptible to sparfloxacin, 2 remained susc eptible to moxifloxacin, and 4 remained susceptible to trovafloxacin. All 1 1 isolates possessed classic mutations in gyrA and/or parC known to confer reduced susceptibility to fluoroquinolones. Four isolates (originating from four separate states) belonging to a multidrug-resistant, fluoroquinolone- resistant clone were identified by pulsed-field gel electrophoresis. For mo xifloxacin and trovafloxacin, at least 87% of isolates demonstrated MICs gr eater than or equal to 3 twofold concentrations below the susceptibility br eakpoints, in contrast to no more than 15% for levofloxacin, grepafloxacin, and sparfloxacin. Of the isolates that were multidrug resistant (7.4%), >9 8% remained susceptible to moxifloxacin. The modal MIC and MIC90 for M. cat arrhalis (both 0.06 mu g/ml) and for H. influenzae (both 0.03 mu g/ml) were independent of beta-lactamase production. These data demonstrate the in vi tro activity of moxifloxacin and establish a baseline for future studies.