Journal of Antimicrobial Chemotherapy (2000) 46, 1037-1038
© 2000 The British Society for Antimicrobial Chemotherapy
Correspondence |
In vitro activity of gemifloxacin and five other fluoroquinolones against defined isogenic mutants of Escherichia coli, Pseudomonas aeruginosa and Staphylococcus aureus
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 topoisomerases—DNA 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 Mueller–Hinton broth, so standard broth no. I (Merck, Darmstadt, Germany) was used instead for these strains. Mutation rates were determined by plating c. 1010–1011 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 Mueller–Hinton 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 10–10 for gemifloxacin and 5 x 10–8 for ciprofloxacin, those with S. aureus ATCC 6835 were 1.2 x 10–8 for gemifloxacin and 9.2 x 10–8 for ciprofloxacin and those with P. aeruginosa ML 5087 were 1.6 x 10–8 for gemifloxacin and 8.2 x 10–8 for ciprofloxacin.
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MICa) for wild-type strains and mutants of E. coli, S. aureus and P. aeruginosa
Gemifloxacin had the highest activity of all the fluoroquinolones tested against the S. aureus wild-type strain. It was superior to pefloxacin, norfloxacin and ofloxacin for all three wild-type strains. It had comparable activity to sparfloxacin and ciprofloxacin for E. coli and was at least as effective as sparfloxacin but less active than ciprofloxacin against P. aeruginosa. Mutations affecting the respective primary target, such as the S83L mutation of E. coli gyrA and the S80F or E84K mutation of S. aureus grlA, affected the MIC of all six fluoroquinolones, but did not result in clinical resistance to gemifloxacin and sparfloxacin (MIC < 1 mg/L). Mutations affecting the secondary quinolone target, such as a parC S80I mutation in a gyrA+ background of E. coli and a gyrA S84L or E88K mutation in a grlAR background in S. aureus, resulted in an identical or lower MIC increase for gemifloxacin compared with the five other quinolones. The impact on the susceptibility of any of the three mutations in nalB, nfxB and nfxC associated with quinolone permeability in P. aeruginosa was identical (ciprofloxacin, sparfloxacin, pefloxacin) or lower (ofloxacin, norfloxacin) for gemifloxacin compared with the other agents tested. However, the intrinsic susceptibility of P. aeruginosa to gemifloxacin was lower than that to ciprofloxacin. The activity of gemifloxacin was superior against two-step mutants of E. coli and S. aureus, all retaining at least intermediate susceptibility (MIC
2mg/L). Like the other fluoroquinolones tested, susceptibility to gemifloxacin is affected by a mutation altering the mar- controlled drug extrusion system in E. coli. However, two additional mutations are required for clinical resistance. Gemifloxacin is c. 400 times less prone to select quinolone-resistant mutants of E. coli than ciprofloxacin and about five times less prone to select quinolone-resistant mutants of S. aureus and P. aeruginosa.
Notes
J Antimicrob Chemother 2000; 46: 1037–1038
* Corresponding author. Tel: +49-228-73-5247; Fax: +49-228-73-5267; E-mail: a.schulte{at}uni-bonn.de 
References
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