Servicio de Microbiología, Hospital de Móstoles, 28935 Móstoles, Madrid, 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.1,2 As with earlier fluoroquinolones, moxifloxacin shows excellent activity against H. influenzae.1,2 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.3
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 timekill studies using nine penicillin-resistant pneumococci (MICs 24 mg/L) using 4 x 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 0.121 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 ß-lactamase producers, but five were non-ß-lactamase producers.
Antimicrobial susceptibility testing was performed by the agar dilution method following the guidelines of the National Committee for Clinical Laboratory Standards (NCCLS).4 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). MuellerHinton 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.5 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 x MIC to log-phase bacterial cultures. Cultures were exposed to moxifloxacin when the OD620 reached 0.250.3 (corresponding to 5 x 107 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. Timekill kinetics were carried out twice for each strain in order to ensure reproducibility. Moxifloxacin was considered bactericidal if it reduced the original inoculum by 3 log10 cfu/mL at each of the time periods.
The MIC ranges, MIC50s and MIC90s for the isolates and the percentage of susceptible, intermediate and resistant strains are shown in the Table.
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The timekill kinetic data showed that in the strains of pneumococci tested moxifloxacin at 4 x MIC exhibited a pronounced bactericidal activity (>3 log10 decrease in cfu/mL) at 24 h.
Against ampicillin-resistant strains of H. influenzae moxifloxacin was very active (MIC50 0.004 mg/L and MIC90 0.03 mg/L) though slightly less active than ciprofloxacin. These two fluoroquinolones were the most active antibiotics tested, with the same activity against ß-lactamase-producing and non-ß-lactamase-producing strains.
The excellent activity of moxifloxacin in a large and recent collection of resistant strains of S. pneumoniae and H. influenzae suggests that it may be useful in the treatment of infections caused by these microorganisms, mainly respiratory tract infections. Our results are in agreement with the findings of other authors.1,2,6
In our study moxifloxacin was equally active against penicillin-intermediate or -resistant strains and macrolide-resistant or -susceptible strains of pneumococci. Others have found the same.1,2,6 Moxifloxacin activity was independent of the clinical source or geographical origin of the isolates.
In addition to the MIC, the bactericidal activity of an antibiotic is also important. With the high potency of this quinolone against pneumococci, concentrations of 4 x MIC90 are achieved with 1 mg/L. In our study moxifloxacin was bactericidal after 24 h against the nine penicillin-resistant strains tested.
In conclusion, our findings indicate that moxifloxacin could be a very good alternative in the management of infections caused by resistant strains of pneumococci and H. influenzae.
Acknowledgments
Members of The Spanish Group for the Study of Infection in the Primary Health Care Setting sent H. influenzae and S. pneumoniae clinical isolates used in this study. This work was supported in part by a grant from Q.F. Bayer Spain.
Notes
* Corresponding author. Tel: +34-91-6648695; Fax: +34-91-6471917; E-mail: nachoalos{at}microb.net
References
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2 . Woodcock, J. M., Andrews, J. M., Boswell, F. J., Brenwald, N. P. & Wise, R. (1997). In vitro activity of BAY 12-8039, a new fluoroquinolone. Antimicrobial Agents and Chemotherapy 41, 1016.[Abstract]
3 . Boswell, F. J., Andrews, J. M., Wise, R. & Dalhoff, A. (1999). Bactericidal properties of moxifloxacin and post-antibiotic effect. Journal of Antimicrobial Chemotherapy 43, Suppl. B, 439.[ISI][Medline]
4 . National Committee for Clinical Laboratory Standards. (2000). Methods for Dilution Antimicrobial Susceptibility Tests for Bacteria that Grow AerobicallyFifth Edition: Approved Standard M7-A5. NCCLS, Wayne, PA.
5 . National Committee for Clinical Laboratory Standards. (2000). Performance Standards for Antimicrobial Susceptibility TestingTenth Informational Supplement (Aerobic Dilution): Approved Standard M100-S10 (M7). NCCLS, Wayne, PA.
6 . Buxbaum, A., Straschil, U., Moser, C., Graninger, W., Georgopoulos, A. & The Austrian Bacterial Surveillance Network. (1999). Comparative susceptibility to penicillin and quinolones of 1385 Streptococcus pneumoniae isolates. Journal of Antimicrobial Chemotherapy 44, Suppl. B, 138.