Institute of Medical Microbiology, Immunology and Hygiene, University of Cologne, Goldenfelsstrasse 1921, 50935 Cologne, Germany
Keywords: Acinetobacter, fluoroquinolones, resistance
Sir,
Acinetobacter baumannii has emerged as a significant nosocomial pathogen. Major epidemiological features of these organisms are their multidrug resistance, propensity for clonal spread and involvement in hospital outbreaks. Epidemic spread may be related to long-term survival on environmental surfaces and antimicrobial resistance, in particular, resistance to fluoroquinolones.1 In A. baumannii, mutations associated with decreased susceptibility to ciprofloxacin have been described in both gyrA and parC.24 In recent years, several new fluoroquinolones have been introduced with increased activity against both Gram-positive and -negative pathogens. The purpose of this study was to investigate the genetic background and epidemiological implication of resistance to fluoroquinolones using a well-defined collection of clinical A. baumannii isolates.
Isolates were collected between 1991 and 2000 from various hospitals in Europe and the United States.5 A total of 147 strains of A. baumannii from Europe (n = 79) and the United States (n = 68) were included. They represented outbreak-related (n = 39), i.e. strains isolated from more than three patients, as well as sporadic (n = 108) strains, and each had a unique macrorestriction digest pattern by pulsed-field gel electrophoresis. MICs were determined by an agar dilution method according to the NCCLS recommendations.6 The overall respective MIC50s and MIC90s were: ciprofloxacin, 0.5 and 64 mg/L; clinafloxacin, 0.12 and 4 mg/L; gatifloxacin, 0.12 and 8 mg/L; gemifloxacin, 0.12 and 16 mg/L; levofloxacin, 0.25 and 8 mg/L; moxifloxacin, 0.12 and 16 mg/L; and trovafloxacin, 0.06 and 16 mg/L. PCR-amplified DNA of the gyrA and parC portions homologous to the quinolone resistance-determining region (QRDR) of Escherichia coli was sequenced in both directions. Sequences that were identical to the published sequences of the gyrA2 and parC3 genes of A. baumannii were considered to have no mutations.
Mutations in the gyrA gene were present in 49 of 147 strains (33%); double mutations in both gyrA and parC genes were detected in 25 strains (17%). The spectrum of mutations and the corresponding MIC distributions of the various compounds are shown in Table 1. Mutations were significantly more frequent in outbreak-related strains compared with sporadic strains (62% versus 23%; P = 0.005; OR 2.6; 95% CI 1.35.5). Double mutations leading to high-level ciprofloxacin resistance (MICs 32 mg/L) and decreased susceptibility to newer quinolones were found in 26% of outbreak-related strains and 14% of sporadic strains. This is consistent with the suggestion that resistance to fluoroquinolones could be a risk factor for epidemic behaviour in A. baumannii.1
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Vila et al.2,3 described mutations in gyrA and parC of 15 A. baumannii strains that correlated with resistance to ciprofloxacin. Among 25 Acinetobacter isolates obtained from various locations worldwide, Seward & Towner7 confirmed mutations in gyrA and parC of 11 ciprofloxacin-resistant isolates and showed that 10 of these isolates were epidemiologically unrelated, as determined by randomly amplified polymorphic DNA analysis. Our findings confirmed that substitution of Ser-83 in GyrA is sufficient to confer ciprofloxacin MICs of 4 mg/L. In agreement with other studies,3,4,7 mutations in parC were only found in strains that also had mutations in gyrA. Double mutants with substitutions in ParC of Ser-80 to either Leu or Phe, or of Gly-84 to Lys had an elevated ciprofloxacin MIC of
32 mg/L. Other mutations in the gyrA gene of A. baumannii that were described in previous reports, Gly-81
Val and Ala-84
Pro,2,3,7 could not be detected in our study.
Newer fluoroquinolones were generally less affected by mutations in gyrA and parC genes, i.e. the highest increase in the median MIC was 10 log2 steps for ciprofloxacin compared with five log2 steps for clinafloxacin. There were no significant differences in the MIC distributions of the newer compounds tested.
In conclusion, mutations in gyrA and parC were significantly more frequent in outbreak-related strains than in sporadic strains. MICs varied widely for strains with the same mutations in gyrA and/or parC, and specific mutations had a different impact on quinolone compounds with different chemical properties. It is likely that other mutations or other mechanisms of resistance also contribute to fluoroquinolone resistance in A. baumannii. The role of efflux mechanisms in particular remains to be determined.
Acknowledgements
We wish to thank D. Stefanik for excellent technical assistance, F. J. Schmitz for valuable discussions of the project and R. P. Wenzel for providing some of the isolates investigated. This work was supported by the Koeln Fortune Program, Faculty of Medicine, University of Cologne, and was presented in part at the Twelfth European Congress of Clinical Microbiology and Infectious Diseases, Milan, Italy, 2427 April 2002 (Abstract P828).
Footnotes
* Corresponding author. Tel: +49-221-478-3009; Fax: +49-221-478-3067; E-mail: harald.seifert{at}uni-koeln.de
References
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