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Correspondence |
Microbiology Division, Public Health Laboratory Services Branch, Centre for Health Protection, Department of Health, Hong Kong SAR
Correspondence
Janice Yee Chi Lo
(janicelo{at}dh.gov.hk)
Meticillin-resistant Staphylococcus aureus (MRSA) strains producing the potent tissue necrotizing toxin Panton–Valentine leukocidin (PVL) encoded by the pvl gene, and harbouring SCCmec type IV or V elements, have been implicated as being associated with MRSA infection acquired in the community setting (Vandenesch et al., 2003; Boyle-Vavra & Daum, 2007). Since the 1990s, community-associated (CA)-MRSA infections have shown a steady increase, and are becoming a significant public-health concern. PVL-positive MRSA isolates have been shown to have distinct genetic backgrounds in different areas, for instance, multilocus sequence typing (MLST) types ST80, ST8 and ST59 were found in the major clones in Europe, USA and Taiwan, respectively (Vandenesch et al., 2003; Tristan et al., 2007; Chen et al., 2005).
In Hong Kong, CA-MRSA infection is a notifiable disease, defined by clinical and epidemiological criteria (Millar et al., 2007), together with molecular characterization of the strain, as belonging to SCCmec type IV or V, and being pvl-positive (CHP, 2008). In an attempt to understand the characteristics and epidemiology of pvl-positive MRSA strains in the community in Hong Kong, we characterized 140 CA-MRSA strains isolated in 2006. Our laboratory is a diagnostic and public health laboratory processing clinical specimens from outpatients and hospital inpatients from both the public and private sector. We also serve as a referral centre for characterization of isolates from other laboratories. For any isolate suspected to be CA-MRSA based on clinical and epidemiological grounds, investigations undertaken included antibiogram determination by the disc diffusion method (CLSI, 2005), SCCmec typing (Okuma et al., 2002; Ito et al., 2004) and pvl PCR (Lina et al., 1999). Isolates found to be pvl-positive were further subjected to molecular typing studies, including spa typing (Shopsin et al., 1999) using the Ridom SpaServer, MLST (Enright et al., 2000) and PFGE (McDougal et al., 2003).
One hundred and forty putative CA-MRSA isolates from unique patients were examined during 2006. Forty-two isolates (30 %) were found to carry pvl. Among these, SCCmec IV and V elements were found in 30 (71.4 %) and 12 (28.6 %) isolates, respectively; no other SCCmec element was found in this collection (Table 1
). This is in accordance with other reports (Tristan et al., 2007; Boyle-Vavra & Daum, 2007) that pvl carriage is much more frequently associated with SCCmec types IV and V than other types (I, II or III) in S. aureus. In Hong Kong, pvl-positive SCCmec type V isolates have been reported in five MRSA strains (Ho et al., 2007). Our results confirmed the previous findings at an extended scale.
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All 42 pvl-positive S. aureus were confirmed to be resistant to β-lactam antibiotics. Resistance to erythromycin and clindamycin was present in 42.9 % (18/42). In addition, 13 isolates were resistant to tetracycline (31 %) and 9 to chloramphenicol (21.4 %). Resistance to ciprofloxacin (3/42), co-trimoxazole (2/42), rifampicin (1/42) and gentamicin (1/42) were less common. Eighteen isolates (42.9 %) were resistant to two or more non-β-lactam agents and seven (16.7 %) were resistant to four or more, whilst one isolate was resistant to five non-β-lactam agents (Table 1
). Some studies have suggested that antibiotic resistance in SCCmec IV or V isolates was often limited to resistance to β-lactams (Takano et al., 2007). However, multiple non-β-lactam antimicrobial resistance phenotypes were found in our SCCmec type V isolates, and all of these strains were resistant to at least two, to a maximum of four, non-β-lactam drugs. These data are in line with reports from other localities (Chen et al., 2005; Coombs et al., 2006). In addition, three of our strains were resistant to ciprofloxacin, and susceptibility to fluoroquinolones as a marker for recognizing putative CA-pvl-positive MRSA strains may be unreliable (Millar et al., 2007). These findings further highlight the importance to public health and the clinical importance of pvl-positive MRSA.
Among the 42 strains, 5 MLST types, 5 PFGE types (with 80 % similarity cut-off) and 11 spa types were resolved (Table 1, Fig. 1). The most prevalent MLST types were ST30 and ST59, shared by 22 (52.4 %) and 15 (35.7 %) isolates, respectively. Further expanding on a published report by Ho et al. (2007), our study showed that the predominant CA-MRSA clones in Hong Kong possessed the genetic background ST30-IV. Our pvl-positive MRSA isolates were segregated by PFGE into 5 clusters, with 2 predominant clusters, A and E, comprising 21 and 17 strains, respectively (Fig. 1). The majority of the isolates were thus shown to be genetically related. Within PFGE group A, all of the strains had an ST30-IV background, while PFGE group E strains were mainly of ST59.
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). Overall, spa typing was more discriminatory than MLST, a finding that is in agreement with published studies (Strommenger et al., 2006). Among the molecular methods described for typing, MRSA, spa typing and MLST are increasingly favoured over PFGE because their results can easily be transferred, exchanged and compared between different laboratories. In general, genetically closely related spa types were grouped together to form a ‘lineage’, and the result of spa typing at lineage level appeared concordant with MLST and PFGE, implying that this typing method is suitable in both outbreak investigations and long-term population studies for MRSA strains. In conclusion, pvl-positive putative CA-MRSA strains in Hong Kong belonged to SCCmec types IV and V, and were mainly associated with SSTIs. Antimicrobial resistance to various groups of agents, including fluoroquinolones, was not uncommon. Most strains belonged to ST30 and ST59, and exhibited spa type t019 and t0437, respectively. Continued monitoring of the epidemiology of such strains is important to provide a basis for control measures and an evaluation of their effectiveness.
References
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