Pharmaceutical Microbiology, University of Bonn, Meckenheimer Allee 168, 53115 Bonn, Germany
Sir,
Gemifloxacin is a novel antibacterial drug belonging to the quinolone class.1 Quinolones inhibit bacterial type II topoisomerasesDNA gyrase and topoisomerase IV. Fluoroquinolones containing 6-fluoro- and 7-piperazinyl-substituents, such as norfloxacin, ofloxacin and ciprofloxacin, preferentially inhibit Gram-negative bacteria. Derivatives such as sparfloxacin, grepafloxacin and trovafloxacin, containing in addition a 5-amino, 5-methyl or 7- (6-amino-3-azabicyclo[3.1.0]hex-3-yl) substituent, respect ively, show enhanced activity against Gram-positive bacteria. These preferences could be reflected in a higher affinity of the different drugs for the respective primary target of quinolone action, which is DNA gyrase in Gram-negative organisms and topoisomerase IV in Grampositive bacteria. As has been shown for Escherichia coli, a high level of fluoroquinolone resistance requires the stepwise acquisition of mutations in the respective genes of the A subunits of DNA gyrase (gyrA) and topoisomerase IV (parC).2 Thus, to prevent development of resistance, the ideal quinolone would either act equally well against both targets or preferentially inhibit one target with high efficiency. Gemifloxacin carries a novel 3-aminomethyl-4-syn-methoxyimino-1-pyrrolidinyl substituent at the C7 position of the 6-fluoro-1,8-naphthyridone core, and has a broad spectrum of antibacterial activity against both Gramnegative and Gram-positive bacteria.3 To evaluate the antibacterial potency further, we determined the MICs of gemifloxacin, ciprofloxacin, sparfloxacin, pefloxacin, norfloxacin and ofloxacin for parental wild-type strains of E. coli, Staphylococcus aureus, Pseudomonas aeruginosa and corresponding single-step and multiple-step mutants carrying known resistance mutations in the target genes gyrA and parC/grlA and in loci associated with reduced quinolone accumulation (marR, nalB, nfxB and nfxC).
Mutants of the E. coli wild-type strain were obtained and characterized as described previously.4 Mutants of S. aureus strain ATCC 6835 were kindly provided by Dr H.-G. Wetzstein, Bayer AG, Leverkusen, Germany. The P. aeruginosa parental strain, ML 5087, and its nalB and nfxB mutants were kindly provided by Dr K. Poole, Queen's University, Kingston, Canada. An nfxC mutant of ML 5087 was selected in vitro on agar plates containing chloramphenicol 1000 mg/L and characterized by its resistance profile according to Köhler et al.5 Drug susceptibilities were determined by the broth microdilution method according to NCCLS6 using Micronaut-S microtitre plates kindly provided by Merlin-Diagnostik, Bornheim, Germany. P. aeruginosa nfxB and nalB mutants did not grow adequately in MuellerHinton broth, so standard broth no. I (Merck, Darmstadt, Germany) was used instead for these strains. Mutation rates were determined by plating c. 10101011 cfu on agar plates containing the respective fluoroquinolone at a concentration of 4 x MIC.
The results of the MIC determinations are summarized in the Table. For P. aeruginosa wild-type strain ML 5087, MICs obtained with MuellerHinton and standard broth no. I were comparable (within one serial dilution step). The mutation rates determined with E. coli wild-type were 1.2 x 1010 for gemifloxacin and 5 x 108 for ciprofloxacin, those with S. aureus ATCC 6835 were 1.2 x 108 for gemifloxacin and 9.2 x 108 for ciprofloxacin and those with P. aeruginosa ML 5087 were 1.6 x 108 for gemifloxacin and 8.2 x 108 for ciprofloxacin.
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Notes
J Antimicrob Chemother 2000; 46: 10371038
* Corresponding author. Tel: +49-228-73-5247; Fax: +49-228-73-5267; E-mail: a.schulte{at}uni-bonn.de
References
1 . Kim, Y. F., Choi, H., Kim, S. H., Chang, J. H., Nam, N. H., Kim, Y. Z. et al. (1995). Synthesis and antibacterial activities of LB20304a, a new fluoronaphthyridone antibiotic containing novel oxime functionalized pyrrolidone. In Program and Abstracts of the Thirty-Fifth Interscience Conference on Antimicrobial Agents and Chemotherapy, San Francisco, CA, 1995. Abstract F204, p.148. American Society for Microbiology, Washington, DC.
2 . Heisig, P. (1996). Genetic evidence for a role of parC mutations in development of high-level fluoroquinolone resistance in Escherichia coli. Antimicrobial Agents and Chemotherapy 40, 87985.[Abstract]
3 . Cormican, M. G. & Jones, R. N. (1997). Antimicrobial activity and spectrum of LB20304, a novel fluoronaphthyridone. Antimicrobial Agents and Chemotherapy 41, 204211.[Abstract]
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Bagel, S., Hüllen, V., Wiedemann, B. & Heisig, P. (1999). Impact of gyrA and parC mutations on quinolone resistance, doubling time, and supercoiling degree of Escherichia coli. Antimicrobial Agents and Chemotherapy 43, 86875.
5 . Köhler, T., Michea-Hamzehpour, M., Plesiat, P., Kahr, A. L. & Pechere, J. C. (1997). Differential selection of multidrug efflux systems by quinolones in Pseudomonas aeruginosa. Antimicrobial Agents and Chemotherapy 41, 25403.[Abstract]
6 . National Committee for Clinical Laboratory Standards. (1997). Methods for Dilution Antimicrobial Susceptibility Tests for Bacteria that Grow AerobicallyFourth Edition: Approved Standard M7-A4. NCCLS, Wayne, PA.