a Institute for Medical Microbiology and Virology, Heinrich-Heine University Düsseldorf, Düsseldorf, Germany; b Eijkman-Winkler Institute for Medical Microbiology, University Medical Center Utrecht, Utrecht, The Netherlands
Sir,
Methicillin-resistant Staphylococcus aureus (MRSA) is considered a major bacterial pathogen, because it is resistant to a variety of antibiotics and causes nosocomial infections worldwide. In clinical practice, one of the important hygienic measures used to prevent its spread is the decontamination of potentially contaminated rooms, utensils and colonized patients. It is alarming, therefore, that S. aureus isolates carrying antiseptic resistance genes have been isolated from clinical samples.13
Three determinants, the qacA, qacB and qacC genes, have been identified that confer resistance to organic cations by means of proton motive force-dependent multidrug efflux.13
The qacA gene confers resistance to a range of structurally disparate organic cations, including monovalent cations such as ethidium, benzalkonium and cetrimide, and divalent cations like chlorhexidine and pentamidine. It is located on a variety of plasmids, including the pSK1 family of plasmids, and on chromosomes.1,2
The qacB gene confers resistance primarily to monovalent organic cations and at low levels to some divalent compounds. It is located on several plasmids, including heavy metal-resistance plasmids like pSK23. The qacB gene is closely related to the qacA gene. In fact, sequence analysis has shown that there are only seven base pair differences between qacA and qacB, and that they are distributed over the entire gene.1,3 These differences result in only one amino acid change at residue 323: Asp in QacA and Ala in QacB. This mutation, however, has a strong impact on the substrate specificity of the efflux proteins, since QacB confers only low or no resistance to divalent organic cations.1,3
The qacC gene, which is identical to the qacD, ebr and smr genes, confers resistance to quaternary ammonium compounds and to ethidium bromide. It is usually located on both conjugative and nonconjugative plasmids (class III) in clinical isolates of S. aureus and other staphylococci.1,4,5
Noguchi et al.5 were the first to analyse the distribution of the three antiseptic resistance genes in parallel. They reported the distribution of these genes during their screening study of 71 MRSA isolates collected in Japan in 1992.5
The purpose of the present study was to update our knowledge of the prevalence and distribution of antiseptic resistance genes in European clinical isolates of S. aureus and to determine whether there is a difference between MRSA and methicillin-susceptible S. aureus (MSSA) isolates in Europe. We screened 297 MRSA and 200 MSSA isolates for the qacA/B (simultaneously) and qacC genes. The qacA and qacB genes were considered together, because simple PCRs cannot discriminate between them. This survey of the prevalence and distribution of antiseptic resistance genes in European MRSA and MSSA strains constitutes the largest and broadest collection of recent clinical S. aureus isolates studied so far.
The S. aureus strains were isolated between 1997 and 1999 in 24 different European university hospitals in 14 countries as part of the European SENTRY study.6 The randomly selected isolates originated from either blood cultures or skin and soft tissue infections. Only one isolate per patient was permitted.
The following primers were used for the amplification of the qacA/B and qacC genes. qacA/B: forward primer 5'-CTATGGCAATAGGAGATATGGTGT, reverse primer 5'-CCACTACAGATTCTTCAGCTACATG, and size of the amplification product 321 bp; qacC: forward primer 5'-AAACAATGCAACACCTACCACT, reverse primer 5'-AACGAAACTACGCCGACTATG, and size of the amplification product 157 bp. PCR was performed according to Noguchi et al.5 Strains TS77 and L20, kindly provided by Noguchi and associates, served as controls for qacA/B and qacC, respectively.
The qacA/B genes were found in 210/497 S. aureus isolates (42%): 186/297 MRSA (63%) and 24/200 MSSA (12%) isolates. The rather high prevalence of these genes might be the result of selective pressure imposed by the disinfection agents (e.g. acriflavine, cetyltrimethylammonium bromide and benzalkonium chloride) used in hospitals. Furthermore, it is noteworthy that significantly more MRSA than MSSA isolates possessed the qacA/B genes. One reason for this may be the location of the qacA/B genes on widespread multi-resistance plasmids like the pSK1 (qacA) and the ß-lactamase/heavy metal-resistance (qacA, qacB) families. These plasmids frequently carry a number of determinants that specify resistance to a range of antimicrobial agents.1 Since MRSA multi-resistance is also partly due to such multi-resistance plasmids, the high incidence of qacA/B in MRSA is comprehensible. The qacC gene was detected in 29/497 S. aureus isolates (5.8%): 19/297 MRSA (6.4%) and 10/200 MSSA (5%) isolates. Thus, qacA/B occurred more frequently in European S. aureus isolates than qacC. These results differ in part from those of Noguchi et al.,5 who detected the qacA/B genes in 10/71 (14%) and the qacC gene in 20/71 (28%) MRSA isolates. We were also not able to detect a significant difference between MRSA and MSSA isolates with regard to the prevalence of qacC. This, however, might be due to the low prevalence of qacC (5.8%) in our S. aureus population.
The qacA/B and qacC genes were detected in S. aureus isolates from all 24 participating centres, regardless of the specimen source or the age of the person. This means that the prevalence of antiseptic resistance genes is a widespread, not a local, problem in European hospitals. Furthermore, the qacA/B and qacC genes were detected concomitantly in five isolates (1%), namely in one MSSA and four MRSA isolates. Up to now, this had only been described for coagulase-negative staphylococci.
In summary, the qacA/B genes were found in 210/497 S. aureus isolates (42%), while the qacC gene could only be detected in 29/497 (5.8%). There was no statistically significant difference between MRSA (6.4%) and MSSA (5%) isolates with regard to the qacC gene. In contrast, significantly more MRSA (63% compared with 12% for MSSA) isolates carried the qacA/B genes. Furthermore, the qacA/B and qacC genes were detected concomitantly in five isolates (1%). This is the first time that this phenomenon has been reported for S. aureus. Since the antiseptic-resistance genes were found in S. aureus isolates from all 24 participating centres, the prevalence of antiseptic-resistance genes is a widespread, not a local, problem in European hospitals.
Acknowledgments
This work was funded in part by the European SENTRY Antimicrobial Surveillance Programme, which is funded by an educational grant from Bristol-Myers Squibb Pharmaceutical Company, and by the European Network for Antimicrobial Resistance and Epidemiology (ENARE) (European Union grant ERBCHRCT940554).
We thank the following members of the SENTRY participants group for referring isolates and epidemiological data from their institutes for use in this study: Helmut Mittermayer, Marc Struelens, Jacques Acar, Vincent Jarlier, Jerome Etienne, Rene Courcol, Franz Daschner, Ulrich Hadding, Nikos Legakis, Gian-Carlo Schito, Carlo Mancini, Piotr Heczko, Waleria Hyrniewicz, Dario Costa, Evilio Perea, Fernando Baquero, Rogelio Martin Alvarez, Jacques Bille, Gary French, Nathan Keller, Volkan Korten, Deniz Gür and Serhat Unal.
Notes
J Antimicrob Chemother 2001; 47: 896897
* Correspondence address. Institute for Medical Microbiology and Virology, Heinrich-Heine University Düsseldorf, Universitätsstraße 1, Geb. 22.21, 40225 Düsseldorf, Germany. Tel/Fax: +49-2132-72040; E-mail: schmitfj{at}uni-duesseldorf.de
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