ORIGINAL ARTICLE Molecular epidemiology of predominant clones and sporadic strains of methicillin resistant Staphylococcus aureus in Switzerland and comparison with European epidemic clones D. S. Blanc 1, D. Pittet 2, C. Ruef 3, A. F. Widmer 4,K.Mühlemann 5, C. Petignat 1, S. Harbarth 2, R. Auckenthaler 2, J. Bille 1, R. Frei 4, R. Zbinden 6, P. Moreillon 1, P. Sudre 1 and P. Francioli 1 1 Centre Hospitalier Universitaire Vaudois, Lausanne, 2 Hôpitaux Universitaires de Genève, 3 Universitäts-Spital, Zürich, 4 Universitäts-Spital, Basel, 5 Institut für Medizinische Mikrobiologie, Bern and 6 Institut für Mikrobiologie, Zürich, Switzerland Objective To assess the molecular epidemiology and risk factors of predominant clones and sporadic strains of methicillin-resistant Staphylococcus aureus (MRSA) in Swiss hospitals and to compare them with European strains of epidemic clones. Material and methods One-year national survey of MRSA cases. Analysis of epidemiological and molecular typing data (PFGE) of MRSA strains. Results In 1997, 385 cases of MRSA were recorded in the five Swiss university hospitals and in 47 community hospitals. Half of the cases were found in Geneva hospitals where MRSA was already known to be endemic. Molecular typing of 288 isolates (one per case) showed that 186 (65%) belong to four predominant clones, three of which were mostly present in Geneva hospitals. In contrast, the fourth clone (85 cases) was found in 23 hospitals (in one to 16 cases per hospital). The remaining 35% of the strains were clustered into 62 pulsed field gel electrophoresis types. They accounted for one to five patients per hospital and were defined as sporadic. Multivariate analysis revealed no independent risk factors for harboring a predominant versus a sporadic strain, except that transfer from a foreign hospital increases the risk of harboring a sporadic strain (OR, 42; 95% CI, 5 360). Conclusion While cases with predominant clones were due to the local spread of these clones, most sporadic cases appear to be due to the continuous introduction of new strains into the country. With the exception of a transfer from a hospital outside Switzerland, no difference in the clinical or epidemiological characteristics was observed between patients harboring a predominant clone and those with a sporadic strain. Keywords Epidemic, molecular typing, MRSA, sporadic Accepted 11 January 2002 Clin Microbiol Infect 2002; 8: 419 426 INTRODUCTION Staphylococcus aureus strains that are resistant to methicillin (MRSA), are frequently resistant to Corresponding author and reprint requests: D. Blanc, Division Autonome de Médecine Préventive Hospitalière, Centre Hospitalier Universitaire Vaudois, 1011 Lausanne, Switzerland Tel: þ41 21 314 02 59 Fax: þ41 21 314 02 62 E-mail: Dominique.Blanc@chuv.hospvd.ch many other classes of antibiotics and are one of the greatest challenges for modern antimicrobial chemotherapy, especially with the emergence of S. aureus with intermediate susceptibility to glycopeptides in Japan [1]. In the early 1990s, a European survey indicated that MRSA tended to be more frequent in southern than in northern Europe. In countries such as France, Spain and Italy more than 30% of hospital isolates of S. aureus were resistant to methicillin [2]. However, within a country, this proportion may vary substantially from one hospital to another [3]. ß 2002 Copyright by the European Society of Clinical Microbiology and Infectious Diseases
420 Clinical Microbiology and Infection, Volume 8 Number 7, July 2002 The epidemiology of MRSA has been investigated using molecular typing methods in many surveys at the hospital level and in some studies at a regional or national level [4 10]. Most of these studies have shown that predominant or epidemic clones accounted for the majority of MRSA cases. Many studies were focused on the investigation of the spread of predominant clones, but relatively little information is available on those strains which are considered sporadic. Although the reported proportion of S. aureus being resistant to methicillin in countries surrounding Switzerland is high, low proportions (0 4%) have been reported in Swiss hospitals, except in Geneva hospitals where rates in the range of 20% over several years have been reported [11,12]. The objective of the present study was twofold: first, to establish the frequency and the diversity of MRSA strains isolated in Switzerland, in order to identify predominant clones and to compare them with predominant clones of MRSA from surrounding countries; secondly, to investigate possible epidemiological differences between cases harboring predominant or sporadic MRSA strains, in order to plan prevention and education. MATERIALS AND METHODS General setting and organization of the survey In Switzerland, five university hospitals serve as tertiary-care centers for a country of 7 million inhabitants. In 1997, these centers participated in a national survey of MRSA. Each center was responsible for gathering data and collecting isolates from its own hospital and from community hospitals in its vicinity that agreed to participate. MRSA detection and control measures at the hospital level Surveillance and control measures used in the participating institutions were standardized for the purpose of the study [13]. In short, patients with MRSA were identified by surveillance of microbiological laboratory data from clinical specimens as well as by cultures obtained from patients who were room-mates of patients infected or colonized with MRSA. When a cluster was suspected, patients on the ward as well as staff were screened. In addition, surveillance cultures were performed on readmitted of patients previously known to have been positive for MRSA and on patients transferred from foreign hospitals. Cultured sites included at least the anterior nares, perineum and infected sites such as open wounds. Bacterial isolates MRSA isolates were sent by the microbiology laboratories of the participating hospitals to the university centers. For all isolates, identification of S. aureus was confirmed by standard methods and susceptibility testing was performed by disk diffusion on Mueller Hinton agar with 24-h incubation at 35 8C. Interpretation criteria were those of the National Committee for Clinical Laboratory Standards (NCCLS) [14]. Resistance to oxacillin was confirmed by the screen agar test [15] and on a subset of isolates by amplification of the meca gene. In addition, 40 European MRSA strains, representing 20 epidemic clones, were obtained from different countries (see ref. [16] for more details) in order to compare Swiss isolates with European epidemic clones. Molecular typing Isolates were typed by pulsed field gel electrophoresis (PFGE) after SmaI DNA digestion using a CHEF III apparatus (BioRad, Hercules, CA) as already described [17]. The banding pattern of the different gels was analyzed with the software GELCOMPAR (Applied Maths, Ghent, Belgium). Considering the stability of the PFGE patterns of MRSA over a short period of time [17], isolates showing an indistinguishable PFGE pattern were assigned to a type and numbered. When more than ten isolates from the same hospital showed an indistinguishable pattern, they were considered as belonging to a predominant clone. Isolates showing patterns related (one to six band differences) to the pattern of the predominant strain were considered as subtypes and labelled with a letter. The predominant type and its subtypes were considered as belonging to the same clone [18,19]. Epidemiological data, definitions and statistics A case was defined as a hospitalized patient found to be infected or colonized with MRSA during
Blanc et al Predominant and sporadic MRSA strains 421 the study period. For each case, the following characteristics and risk factors for MRSA colonization/infection were recorded: demographics, geographic origin, admission and discharge dates, prior hospitalization during the preceding 3 years, past history of MRSA colonization or infection, comorbidities (Charlson score [20]), functional status (Karnofsky index [21]), underlying diagnosis, infection versus colonization with MRSA, indwelling catheter, wound, antibiotic treatment, drains, intubation and operation. Nosocomial clusters were defined as two or more patients who were hospitalized in the same ward within a 6-month period and who harbored MRSA isolates which were indistinguishable by molecular typing. Standard definitions of the Centers for Diseases Control and Prevention for the diagnosis of nosocomial infections were used [22]. Variables with a significant Odd Ratio (OR) in the univariate analysis were candidates for multivariate analysis. Multivariate modelling was performed by using stepwise logistic regression analysis. All statistical tests were two-tailed. OR differences of unity were considered significant when the 95% confidence interval did not overlap with unity. Statistics were run on SPSS 8.0 (SPSS Inc, Chicago, IL, USA). RESULTS Between January and December 1997, 359 cases of MRSA were identified in the five university hospitals and in 39 community hospitals throughout the country. Half the cases were reported from the university hospitals of Geneva (4.8 prevalent cases/1000 admissions). The remaining cases were distributed among 43 hospitals (0.16 0.97 prevalent cases/1000 admissions for university hospitals and a mean of 0.68 prevalent cases/1000 admissions for community hospitals) throughout the rest of Switzerland. Epidemiological data and MRSA isolates were available for 225 (63%) of the cases, whereas only epidemiological data or isolates were available for 71 and 63 cases, respectively. Molecular typing (PFGE) of 288 isolates (one isolate per case) indicated that 65% belonged to four genetically different clones (Figure 1). Three clones (GE1, GE2 and GE3) were found to be predominant in Geneva university hospitals where they accounted for 73% (108/148) of the cases. Among 51 patients harboring clone GE1, 41 were hospitalized in Geneva University hospitals and 10 in six other hospitals. Clone GE2 was present in 20 patients in Geneva hospitals and in six patients from five other hospitals. Finally, clone GE3 was recovered from 26 patients hospitalized only in Geneva university hospitals. Thus, the majority of the isolates of these three clones (83%) were identified in Geneva hospitals. In contrast, the fourth predominant clone (85 cases) was identified in 23 different hospitals (one to 16 cases per hospital) from western Switzerland (clone WCH) (Figure 2). The remaining 35% of the strains represented 66 PFGE types and accounted for clusters of only one to five patients per hospital, and thus were considered sporadic. Risk factors for MRSA colonization/infection with a sporadic strain vs. a predominant clone (GE1, GE2, GE3, WCH) were investigated by univariate analysis (Table 1). Transfer from a foreign hospital, previous acquisition of MRSA, hospitalization in a community hospital and the presence of a drain or intubation were all factors increasing the risk of harboring a sporadic strain. In contrast, a higher co-morbidity index (Charlson score 4) and hospitalization in a long-term care or rehabilitation center increased the risk of harboring a predominant clone. These significant factors were all included in the multivariable logistic regression model (Table 2). Only the transfer from a foreign hospital was independently associated with the carriage of a sporadic strain (OR, 42; 95% CI, 5 360). Comparison of Swiss MRSA isolates and European epidemic clones Forty strains from 20 epidemic clones of MRSA, most of them from Europe, were analysed by PFGE and compared with the 288 Swiss isolates (Figure 3). The results confirmed that the clone WCH was indistinguishable from the Belgian clone 2 [16,23] and was related to the Ontario epidemic clone (Canada) [24]. The clone GE1 was genetically related to the Belgian clone 1 (four bands different) [23], to the North Germany phage group III clone [4], and the Portuguese clone I::E::A [25]. The clone GE2 was probably related to the UK EMRSA 16 (two bands different), and the clone GE3 has never been isolated elsewhere in Switzerland and was found to be related to none of the European clones analyzed.
422 Clinical Microbiology and Infection, Volume 8 Number 7, July 2002 Figure 1 Distribution of PFGE types in community and university hospitals (Basel, Bern, Geneva, Lausanne and Zurich) (one strain per case). The four predominant clones (WCH, GE1, GE2 and GE3) are represented by their major type (1, 9, 40 and 57, respectively) and their subtype (letters). All the other types are unique PFGE patterns.
Blanc et al Predominant and sporadic MRSA strains 423 Table 1 Risk factors for MRSA colonization/infection with a sporadic strain of MRSA versus a predominant clone (GE1, GE2, GE3, or WCH): univariate analysis Sporadic a (%) Predominant a b (%) Odds ratio CI 95 Sex (F/M) 40/53 74/112 0.86 0.53 1.45 Origin (%): home 36/79 (46) 75/114 (66) Reference nursing home 6/79 (8) 12/114 (11) 1.03 0.38 2.77 transfer from Swiss hospital 12/79 (15) 21/114 (18) 1.19 0.51 2.76 transfer from foreign hospital 25/79 (32) 6/114 (5) 8.56 3.23 22.73 Prior hospitalization 73/80 (91) 123/136 (90) 1.10 0.42 2.89 Previous acquisition of MRSA 81/101 (80) 99/165 (60) 2.70 1.51 4.82 Infections with MRSA 51/98 (52) 64/126 (51) 1.20 0.69 2.07 Blood catheter 44/68 (65) 80/131 (61) 1.04 0.57 1.89 Indwelling urinary catheter 30/68 (44) 57/131 (44) 1.03 0.57 1.85 Wound c and ulcer 44/61 (72) 62/101 (61) 1.59 0.80 3.16 Antibiotic treatment d 44/61 (72) 89/110 (81) 0.90 0.45 1.80 Drain 14/62 (23) 14/131 (11) 2.44 1.08 5.50 Intubation 22/65 (34) 19/131 (15) 3.02 1.49 6.12 Operation 21/63 (33) 38/131 (29) 1.22 0.64 2.33 Charlson score: (n ¼ 182) 0 1 23 41 Reference 2 3 27 42 1.15 0.57 2.32 4 10 7 42 0.30 0.12 0.77 Karnovsky score: (n ¼ 122) 10 40 16 19 Reference 50 60 14 30 0.55 0.22 1.39 65 90 19 24 0.94 0.38 2.31 Institution type: (n ¼ 258) University hospitals 62 102 Reference Community hospitals 21 11 3.14 1.42 6.95 Long-term care and rehabilitation centers 7 55 0.21 0.09 0.49 Values shown in bold are significant. a Data were not available for all cases. b 95% confidence interval. c Operative and non-operative wound. d During this hospitalization and 3 weeks before. Table 2 Independent risk factors for colonization/infection with a sporadic strain of MRSA versus a predominant clone: stepwise logistic regression model Sporadic Predominant Odds ratio CI 95 a Origin (%): Home 36/79 (46%) 75/114 (66%) Reference Nursing home 6/79 (8%) 12/114 (11%) 0.65 0.14 3.09 Transfer from Swiss hospital 12/79 (15%) 21/114 (18%) 0.66 0.15 2.91 Transfer from foreign hospital 25/79 (32%) 46/114 (5%) 42.49 5.02 359.6 Previous acquisition of MRSA 81/101 99/165 1.67 0.64 4.41 Drain 14/62 14/131 0.78 0.19 3.23 Intubation 22/65 19/131 1.98 0.60 6.60 Charlson score: (n ¼ 182) 0 1 23 41 Reference 2 3 27 42 2.03 0.81 5.05 4 10 7 42 0.45 0.13 1.52 Institution type: (n ¼ 258) University hospitals 62 102 Reference Community hospitals 21 11 1.12 0.19 6.83 Long-term care and rehabilitation centers 7 55 1.29 0.38 4.43 Values shown in bold are significant. a 95% confidence interval.
424 Clinical Microbiology and Infection, Volume 8 Number 7, July 2002 and strains related to the South German clone [4] were recovered in 12 patients from eight hospitals. DISCUSSION Figure 2 Geographical dissemination of the three predominant clones of MRSA in Switzerland in 1997 (one symbol per institution and per clone; the number in the symbol indicates the number of cases harboring the clone in the institution). Some sporadic isolates showing related patterns to other European epidemic clones were recovered in Switzerland (Figure 3). One strain, related to UK EMRSA 15, was isolated in a community hospital, The results of molecular typing of Swiss MRSA isolates disclosed that the majority of isolates belonged to a few predominant clones, whereas the remaining isolates showed a great genetic diversity, a situation that has been already described in other hospitals or countries [7,8,26,27]. In addition, the present study shows that there was no difference between patients with predominant or sporadic strains regarding underlying diseases and other demographic or co-morbidity factors, except for their location prior to admission. Based on our study, we postulate that cases with sporadic strains became colonized either in foreign hospitals or during a previous hospitalization in Switzerland, whereas cases with predominant clones acquired their strains mainly in relation to the local spread of these clones within a given Figure 3 PFGE patterns (analyzed with the program GELCOMPAR) of European epidemic MRSA related to the four predominant Swiss clones and to some sporadic Swiss strains.
Blanc et al Predominant and sporadic MRSA strains 425 hospital. Indeed, given the relatively low frequency of MRSA observed in the present study, and the fact that most cases admitted with MRSA had been hospitalized previously, it seems unlikely that there is an important circulation of MRSA in the community. The genetic relatedness of the predominant clones (with the exception of clone GE3) and of some sporadic isolates to European epidemic MRSA clones strongly suggests that they may have been imported from surrounding countries. The high diversity of sporadic strains suggests that new strains are continuously imported from outside the country. At least one-third of these strains were definitely imported during the transfer of the patients from a foreign hospital. The origin of the other two-thirds remains speculative. Previous hospitalization in a foreign country and the immigration of staff members from highly endemic areas probably account for a substantial number of cases, as already suggested [5,11,28]. One can speculate that during the dissemination of a strain, the accumulation of mutations allows the diversification of the genome. This genomic microevolution is probably detected by the highly discriminant typing method used in this study (PFGE) [29]. Thus, most of these highly diverse strains might be descended from a few ancestors. Using a typing method that is able to keep track of ancestors (multiprimer random amplification polymorphism DNA), we have shown that all of the Swiss isolates were related to other European epidemic MRSA clones [30]. Thus, even in the central and eastern parts of Switzerland where no predominant clone was present, most of the MRSA isolates probably originated from surrounding countries, and were presumably imported either through patients or through staff migration. The three clones from Geneva (GE1, GE2 and GE3) were mainly restricted to the Geneva university hospitals. In contrast, the predominance of the clone WCH could not be explained only by the lack of MRSA control measures since they were implemented before the first detection of this clone. This clone was first isolated in Switzerland in 1995, was responsible for several outbreaks in 1995 6 [16], and its spread continued during the present study in 23 hospitals in which control measures were implemented. It was found to be identical to the Belgium epidemic clone 2 [23] and related to an Ontario epidemic clone [24]. Some intrinsic factors might be responsible for this higher capacity for spreading. Interestingly, this widespread clone was only present with a few cases in Geneva hospitals. In conclusion, a great diversity of MRSA strains was found in Switzerland, suggesting that they were imported either through patients or through staff migration. However, transmission within and between Swiss hospitals also occurred, as suggested by the fact that 65% of MRSA cases were due to four predominant clones. Although there was no epidemiological difference between patients with a predominant or a sporadic MRSA strain, the use of a typing method, preferably molecular, and contributes to a better understanding of the intraand interhospital epidemiology, and thus to base prevention and education on precise data. ACKNOWLEDGMENTS Special thanks are given to all Swiss hospitals and laboratories who participated in the study. We are grateful to B. Willey, H. de Lencaste, I. Sanches, J. Etienne, M. Struelens, T. Berendt, I. Bowler, W. Hryniewicz, K. Trzinski and W. Witte for providing the European epidemic MRSA clones. We are also grateful to M. Bernard for technical assistance. This work was supported by a grant from the Swiss National Foundation for Research no. 32 45820.95. REFERENCES 1. Hiramatsu K, Aritaka N, Hanaki H et al. Dissemination in Japanese hospitals of strains of Staphylococcus aureus heterogeneously resistant to vancomycin. Lancet 1997; 350: 1670 3. 2. Voss A, Milatovic D, Wallrauch-Schwarz C et al. Methicillin-Resistant Staphylococcus aureus in Europe. Eur J Clin Microbiol Infect Dis 1994; 13: 50 5. 3. Struelens MJ, Mertens R. and Groupement pour le Dépistage lelpdih. National Survey of Methicillin- Resistant Staphylococcus aureus in Belgian Hospitals. Detection methods, prevalence trends and infection control measures. Eur J Clin Microbiol Infect Dis 1994; 13: 56 63. 4. Witte W, Kresken M, Braulke C et al. Increasing incidence and widespread dissemination of methicillin-resistant Staphylococcus aureus in hospitals in central Europe, with special reference to German hospitals. Clin Microbiol Infect 1997; 3(4): 414 22. 5. Deplano A, Witte W, van Leeuwen WJ et al. Clonal dissemination of epidemic methicillin-resistant Staphylococcus aureus in Belgium and neighbouring countries. Clin Microbiol Infect 2000; 6: 239 45.
426 Clinical Microbiology and Infection, Volume 8 Number 7, July 2002 6. de Lencastre H, Severina EP, Milch H et al. Wide geographic distribution of a unique methicillinresistant Staphylococcus aureus clone in Hungarian hospitals. Clin Microbiol Infect 1997; 3(3): 289 96. 7. Melter O, Santos SI, Schindler J et al. Methicillinresistant Staphylococcus aureus clonal types in the Czech Republic. J Clin Microbiol 1999; 37(9): 2798 803. 8. Leski T, Oliveira D, Trzcinski K et al. Clonal distribution of methicillin-resistant Staphylococcus aureus in Poland. J Clin Microbiol 1998; 36(12): 3532 9. 9. van Belkum A, van Leeuwen W, Verkooyen R et al. Dissemination of a single clone of methicillinresistant Staphylococcus aureus among turkish hospitals. J Clin Microbiol 1997; 35(4): 978 81. 10. Roberts RB, Lencastre A, Eisner W et al. Molecular epidemiology of methicillin-resistant Staphylococcus aureus in 12 New York hospitals. MRSA Collaborative Study Group. J Infect Dis 1998; 178(1): 164 71. 11. Harbarth S, Romand J, Frei R et al. Transmission inter- et intrahospitalière de staphylocoques dorés résistants à la méticilline. Schweiz Med Wochenschr 1997; 127: 471 8. 12. Harbarth S, Martin Y, Rohner P et al. Effect of delayed infection control measures on a hospital outbreak of methicillin-resistant Staphylococcus aureus. J Hosp Infect 2000; 46: 43 9. 13. Francioli P, Furrer H, Pittet D et al. Staphylocoques dorésrésistants à la méthicilline: situation et enjeux. Swiss-Noso 1995; 4(4): 25 9. 14. Jorgensen JH, Cleeland R, Craig W et al. Performance Standards for Antimicrobial Disk Susceptibility Testing. Approved Standard M2 A4. National Committee for Clinical Laboratory Standards. 1991. 15. Isenberg HD, ed. Essential Procedures for Clinical Microbiology. Washington DC: ASM 1998. 16. Blanc DS, Petignat C, Moreillon P et al. Unusual spread of a penicillin-susceptible methicillin-resistant Staphylococcus aureus clone in a geographic area of low incidence. Clin Inf Dis 1999; 29(6): 1512 18. 17. Blanc DS, Struelens MJ, Deplano A et al. Epidemiological validation of pulsed-field gel electrophoresis patterns for methicillin-resistant Staphylococcus aureus. J Clin Microbiol 2001; 39(10): 3442 5. 18. Struelens MJ. and Members of the ESGEM. Consensus guidelines for appropriate use and evaluation of microbial epidemiologic typing systems. Clin Microbiol Infect 1996; 2(1): 2 11. 19. Tenover FC, Arbeit RD, Goering RV et al. Interpreting chromosomal DNA restriction patterns produced by pulsed-field gel electrophoresis: criteria for bacterial strain typing. J Clin Microbiol 1995; 33(9): 2233 9. 20. Wilson WR, Karchmer AW, Dajani AS et al. Antibiotic Treatment of Adults With Infective Endocarditis Due to Streptococci, Enterococci, Staphylococci, and HACEK Microorganisms. JAm Med Assoc 1995; 274(21): 1706 13. 21. Karnofsky DA, Burchenal JH. The clinical evaluation of chemotherapeutic agents in cancer. In: MacLeod CM, ed. Evaluation of Chemotherapeutic Agents. New York: Columbia University Press, 1949, 191 205. 22. Pittet D, Harbarth S, Ruef C et al. Prevalence and risk factors for nosocomial infections in four University hospitals in Switzerland. Infect Control Hosp Epidemiol 1999; 20(1): 37 42. 23. Hoefnagels-Schuermans A, Peetermans WE, Struelens MJ et al. Clonal analysis and identification of epidemic strains of methicillin-resistant Staphylococcus aureus by antibiotyping and determinant of protein A gene and coagulase gene polymorphisms. J Clin Microbiol 1997; 35(10): 2514 20. 24. Preston M, Borczyk A, Jamieson F. Epidemic methicillin-resistant Staphylococcus aureus strain- Ontario. Can Communicable Dis Report 1998; 24(6): 47 9. 25. de Lencastre H, Couto I, Santos I et al. Methicillin- Resistant Staphylococcus aureus Disease in a Portuguese Hospital: Characterization of Clonal Types by a Combination of DNA Typing Methods. Eur J Clin Microbiol Infect Dis 1994; 13: 64 73. 26. Kerr S, Kerr GE, Mackintosh CA et al. A survey of methicillin-resistant Staphylococcus aureus affecting patients in England and Wales. J Hosp Infect 1990; 16: 35 48. 27. Schneider C, Weindel M, Brade V. Frequency, clonal heterogeneity and antibiotic resistance of methicillin-resistant Staphylococcus aureus (MRSA) isolated in 1992 1994. Zbl Bakt 1996; 283: 529 42. 28. Witte W, Cuny C, Braulke C et al. Clonal dissemination of two MRSA strains in Germany. Epidemiol Infect 1994; 113: 67 73. 29. van Leeuwen W, van Belkum A, Kreiswirth B et al. Genetic diversification of methicillin-resistant Staphylococcus aureus as a function of prolonged geographic dissemination and as measured by binary typing and other genotyping methods [published erratum appears in Res Microbiol 1998; 149(10): 775]. Res Microbiol 1998; 149(7): 497 507. 30. Blanc DS, Banuls AL, Hauser PM et al. Methicillin resistant Staphylococcus aureus: phylogenetic relatedness between European epidemic clones and Swiss sporadic strains. Microbial Drugs Resistance 2000; 6(3): 231 8.