a Institute for Medical Microbiology and Virology, Heinrich-Heine University Düsseldorf, Universitätsstrasse 1, Geb. 22.21, 40225 Düsseldorf; c Institut für Tierzucht und Tierverhalten der Bundesforschungsanstalt für Landwirtschaft (FAL), Celle, Germany; b Eijkman-Winkler Institute for Clinical Microbiology, Utrecht University, The Netherlands
Sir,
The ketolides, e.g. telithromycin, represent a new class of antimicrobial agents, and differ from erythromycin in that a 3-keto group is substituted for l-cladinose on the erythromycin A ring. In addition to the ketone group at position 3, the novel ketolide ABT-773 is modified by an O-allyl-3 quinoline at the 6 position and a cyclized carbamate group between the 11 and 12 positions. In the present study, we determined the in vitro activities of two ketolides against erythromycin-susceptible and -resistant Staphylococcus aureus strains with defined macrolide resistance gene status.
The predominant form of macrolide resistance in S. aureus is target modification mediated by erm genes of various classes, all of which code for 23S rRNA methylases. The two genes erm(A) and erm(C) proved to be the dominant erm genes in European S. aureus isolates from the SENTRY study.1 Expression of these genes can be inducible or constitutive. When expression is constitutive, the staphylococci are resistant to all macrolides, lincosamides and streptogramin B antibiotics (M1416LSB resistance). When expression is inducible, the strains are primarily resistant to 14- and 15-membered macrolides, and only when the system is induced is resistance to 16-membered macrolides, lincosamides and streptogramin B antibiotics also observed. In addition to erm-mediated target site modification, efflux of macrolides via MsrA has also been documented in staphylococci. The msrA gene confers the MS phenotype, resistance to 14-membered macrolides and streptogramin B antibiotics. Macrolide inactivating enzymes such as the esterase EreB have been detected only rarely in staphylococci.1
Recently, we analysed the prevalence of the macrolide resistance genes erm(A), erm(C), msr(A) and ere(B) in erythromycin-resistant S. aureus isolates from European university hospitals.1 These genetically characterized isolates (n = 772) served for the determination of MICs of erythromycin, clindamycin, telithromycin and ABT-773 by a microdilution broth method according to NCCLS criteria.2 For comparative reasons, another 100 macrolide- and lincosamide-susceptible S. aureus isolates were also included in this study.
The results of the MIC determination for the S. aureus strains that harbour the different resistance genes are shown in the Table. The two ketolides were highly active against S. aureus isolates that are either susceptible to erythromycin and clindamycin or are erythromycin resistant due to efflux or enzymic degradation of macrolides. S. aureus isolates showing inducible resistance to M1416LSB antibiotics exhibited slightly elevated MIC90 values of 0.25 mg/L of telithromycin and 0.120.25 mg/L of ABT-773, irrespective of the erm gene involved. However, when M1416LSB resistance is based on the constitutive expression of the erm(A) and/or erm(C) gene, the respective S. aureus isolates were resistant against the new ketolides.
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The data from the present study showed that the type of erm gene did not have an influence on the MIC50 and MIC90 values for the S. aureus isolates. However, the expression of the erm gene proved to be critical for the in vitro activity of the ketolides against S. aureus. Whenever erm(A), erm(C) or erm(A) + erm(C) was expressed constitutively, neither telithromycin nor ABT-773 was active. In a previous study we analysed constitutively expressed erm(A) genes of S. aureus isolates for the types of mutation that caused the switch from inducible to constitutive erm(A) gene expression.4 Analysis of these strains for their MICs of telithromycin and ABT-773 confirmed that the same high MIC of >128 mg/L was seen irrespective of whether different sized deletions or tandem duplications in the erm(A) translational attenuator accounted for constitutive erm(A) gene expression.
Judging from the MICs (Table), it seems that S. aureus isolates that carry inducibly expressed erm(A) and/or erm(C) genes would be susceptible to telithromycin and ABT-773. A recent study, however, showed that both ketolides are able to select for constitutively expressed mutants when S. aureus strains that harbour an inducibly expressed erm(A) gene are cultivated in the presence of either of the two ketolides.5 Such mutants also display high level resistance to both ketolides and could be obtained in plate selection5 as well as broth selection experiments. A similar observation was made by Hamilton-Miller & Shah,3 who selected constitutive mutants of Staphylococcus haemolyticus by growth in the presence of telithromycin.6
In summary, the new ketolides proved to be active against erythromycin-susceptible S. aureus isolates as well as against erythromycin-resistant isolates that could export or enzymically degrade macrolides. In general, ABT-773 displayed a two- to four-fold higher in vitro activity as compared with telithromycin. However, both ketolides were inactive against M1416LSB-resistant S. aureus that carried constitutively expressed erm(A) and/or erm(C) genes. The observation that cultivation in the presence of ketolides converted the type of erm gene expression from inducible to constitutive expression strongly indicated not to recommend the use of ketolides for the control of staphylococci that display inducible resistance to M1416LSB antibiotics.
Notes
* Corresponding author. Tel/Fax: +49-2132-72040; E-mail: schmitfj{at}uni-duesseldorf.de
References
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Schmitz, F.-J., Sadurski, R., Kray, A., Boos, M., Geisel, R., Köhrer, K. et al. (2000). Prevalence of macrolide-resistance genes in Staphylococcus aureus and Enterococcus faecium isolates from 24 European university hospitals. Journal of Antimicrobial Chemotherapy 45, 8914.
2 . National Committee for Clinical Laboratory Standards. (2000). Methods for Dilution Antimicrobial Susceptibility Tests for Bacteria that Grow AerobicallyFifth Edition: Approved Standard M7-A5. NCCLS, Villanova, PA.
3 . Hamilton-Miller, J. M. T. & Shah, S. (1998). Comparative in-vitro activity of ketolide HMR 3647 and four macrolides against Gram-positive cocci of known erythromycin susceptibility status. Journal of Antimicrobial Chemotherapy 41, 64953.[Abstract]
4
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Schmitz, F.-J., Petridou, J., Astfalk, N., Scheuring, S., Köhrer, K., Verhoef, J. et al. (2001). Analysis of structural alterations in the translational attenuator of constitutively expressed erm(A) genes in Staphylococcus aureus. Antimicrobial Agents and Chemotherapy 45, 16034.
5 . Schmitz, F.-J., Petridou, J., Jagusch, H., Astfalk, N., Scheuring, S. & Schwarz, S. (2002). Molecular characterization of ketolide resistant erm(A)-carrying Staphylococcus aureus isolates selected in-vitro by telithromycin, ABT-773, quinupristin and clindamycin. Journal of Antimicrobial Chemotherapy, in press.
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Hamilton-Miller, J. M. T. & Shah, S. (2000). Patterns of phenotypic resistance to the macrolide-lincosamide-ketolide-streptogramin group of antibiotics in staphylococci. Journal of Antimicrobial Chemotherapy 46, 9419.