Comparative in vitro study of the activity of moxifloxacin and other antibiotics against 150 strains of penicillin non-susceptible Streptococcus pneumoniae and against 110 strains of ampicillin-resistant Haemophilus influenzae isolated in 1999-2000 in Spain

Sir, Antimicrobial resistance amongst strains of Haemophilus influenzae and Streptococcus pneumoniae has limited the usefulness of first-line agents in some clinical settings. There is thus a clear need for new antibiotics, in particular those active against resistant strains. Moxifloxacin is an 8-methoxyquinolone with high activity against Grampositive bacteria, including penicillin-intermediate and -resistant pneumococci. As with earlier fluoroquinolones, moxifloxacin shows excellent activity against H. influenzae. Moxifloxacin has a bactericidal effect against both pathogens, and its post-antibiotic effect was similar to that of other fluoroquinolones and increased with increasing concentrations. The aim of this study was to assess the antimicrobial susceptibility to moxifloxacin and 14 other antimicrobial agents of 150 pneumococci of intermediate resistance or resistant to penicillin isolated from clinical samples in Madrid in 1999 and 2000, and the antimicrobial susceptibility to moxifloxacin and 17 other antibiotics of 110 ampicillin-resistant strains of H. influenzae isolated from clinical samples in Spain in 1999. We have also carried out time–kill studies using nine penicillin-resistant pneumococci (MICs 2–4 mg/L) using 4 MIC of moxifloxacin for each strain. A total of 150 clinical isolates (one per patient) of S. pneumoniae were used, collected in 11 laboratories from 10 of the 11 Health Authority Areas of Madrid (c. 5 000 000 inhabitants) in 1999 and 2000. The sample size was proportionally stratified according to the number of inhabitants of each Health Authority Area. There were 57 strains of intermediate penicillin resistance (MICs 0.12–1 mg/L) and 93 penicillin-resistant strains (MIC 1 mg/L). A total of 110 clinical isolates (one per patient) of ampicillin-resistant H. influenzae were used, collected in 21 laboratories throughout Spain (c. 40 000 000 inhabitants) in 1999 during a nationwide Spanish surveillance study. The country was arbitrary divided into 21 geographical areas. The sample size was proportionally stratified according to the number of inhabitants of each area. The great majority, 105 strains, were -lactamase producers, but five were non-lactamase producers. Antimicrobial susceptibility testing was performed by the agar dilution method following the guidelines of the National Committee for Clinical Laboratory Standards (NCCLS). Antibiotics were obtained as standard reference powders of known potency from Sigma Chemical Co., St Louis, MO, USA (penicillin G, ampicillin, cefaclor, cefotaxime, tetracycline, chloramphenicol, rifampicin, nalidixic acid, erythromycin and clindamycin), Abbott, Chicago, IL, USA (clarithromycin), SmithKline Beecham, Toledo, Spain (amoxicillin and clavulanate), Bristol-Myers Squibb, Barcelona, Spain (cefprozil), Eli Lilly, Indianapolis, IN, USA (loracarbef), Hoechst-Marion-Roussel, Romainville, France (cefpodoxime and levofloxacin), Schering Plough, Kenilworth, NJ, USA (ceftibuten), Merck, Barcelona, Spain (cefixime), Glaxo Wellcome, Madrid, Spain (cefuroxime), Pfizer Inc, New York, NY, USA (azithromycin and trovafloxacin), Bayer Q.F., Barcelona, Spain (ciprofloxacin and moxifloxacin) and Menarini, Barcelona, Spain (miocamycin). Mueller–Hinton agar medium with 5% sheep blood was used for S. pneumoniae. Haemophilus Test Medium (HTM) was used for H. influenzae. The range of interpretative categories for each antibiotic were those recommended by the NCCLS. The MIC breakpoints for miocamycin were 1 mg/L for susceptibility and 4 mg/L for resistance, as defined by the Comité de l’Antibiogramme de la Societé Française de Microbiologie. For moxifloxacin, we used the breakpoints of grepafloxacin: 0.5 mg/L susceptible and 2 mg/L resistant. Killing curves were obtained for nine penicillin-resistant pneumococci by adding moxifloxacin at a concentration corresponding to 4 MIC to log-phase bacterial cultures. Cultures were exposed to moxifloxacin when the OD620 reached 0.25–0.3 (corresponding to 5 10 cfu/mL). Colony counts were determined at 3, 6 and 24 h by removing samples at each time point and, after serial 10-fold dilution in sterile 0.85% NaCl, and plating on Columbia blood agar plates. Results were charted graphically by plotting log10 cfu against time. Time–kill kinetics were carried out