Community-acquired methicillin-resistant Staphylococcus aureus arrives in Hong Kong

P. L. Ho1,*, Cindy W. S. Tse2, Gannon C. Mak1, K. H. Chow1 and T. K. Ng2

1 Division of Infectious Diseases, Department of Microbiology, The University of Hong Kong, Queen Mary Hospital, Pokfulam Road, Pokfulam, Hong Kong SAR; 2 Department of Clinical Pathology, Princess Margaret Hospital, Hong Kong SAR, China

Keywords: community-acquired infections , methicillin resistance , S. aureus; molecular diagnostic techniques

Sir,

Methicillin-resistant Staphylococcus aureus (MRSA) is a worldwide, firmly established healthcare-associated pathogen. Risk factors for MRSA include recent hospitalization or surgery, nursing home residence, renal dialysis and exposure to invasive medical devices. Recently, cases of MRSA have been identified in healthy community-dwelling persons without risk factors for MRSA acquisition. Such community-acquired (CA) MRSA infections have been reported in Australia, New Zealand, Canada, the USA and Europe. Here, we report the first case of CA-MRSA in Hong Kong.

In May 2004, a 50-year-old man presented to the Accident and Emergency department of a regional hospital with a 1 week history of a carbuncle at the back of his neck. It measured 4 cm in diameter and there was purulent discharge from the lesion. At presentation, he had a tympanic temperature of 38°C. The attending doctor performed an incision and drainage. A deep wound swab of the pus was sent for bacterial culture. The patient was discharged with a 7 day course of oral ampicillin and cloxacillin. Aerobic and anaerobic culture of the pus yielded heavy and pure growth of a Gram-positive coccus, identifying as S. aureus by positive results for catalase, slide and tube coagulase, acid from D-mannitol and heat-stable nuclease. A negative result was obtained for ornithine decarboxylase. Investigation using a Vitek system (GPI, bioMérieux Vitek, Hazelwood, MO, USA) yielded the same identification. A ß-lactamase test by nitrocefin disc was positive. Antibiotic susceptibility testing was performed by the disc diffusion method in accordance with the NCCLS recommendations. There was no inhibition zone around a 1 µg oxacillin disc. The isolate was susceptible to the following antibiotics: gentamicin, erythromycin, clindamycin, fusidic acid, ciprofloxacin, co-trimoxazole, tetracycline, chloramphenicol, rifampicin and vancomycin. Following identification of the non-multiresistant MRSA, the patient was contacted by phone for further information. Direct questioning confirmed that risk factors for healthcare-associated MRSA were absent. The patient enjoyed good past health. There was no history of hospitalization, surgery, dialysis or residence in a nursing home in the previous 18 months. He had no history of exposure to persons at risk for MRSA. None of his family members were healthcare workers. Following discharge, the neck wound gradually healed without the use of other antibiotics.

The presence of the mecA gene in the isolate was investigated by PCR. The staphylococcal chromosomal cassette type (SCCmec) including mecR1, mecI and the chromosomal cassette recombinase ccr genes were evaluated using specific primers described by Lim et al.1 PCR for mecA was positive. PCR with primers specific for the membrane-spanning domain of mecR1 was positive, whereas those for the penicillin-binding domain of mecR1 and mecI were negative. The isolate amplified with type 2 ccr gene complex primers but not with primers for type 1, 3 and 4 ccr genes. The result indicates the presence of SCCmectype IV in this CA-MRSA strain. In order to identify the genetic background of this strain, the organism was characterized by multilocus sequence typing (MLST). Specific primers described for arc, aro, glp, gmk, pta, tpi and yqi were used to amplify internal fragments (size 402–516 bp) of seven housekeeping genes. The amplified products were purified and sequenced in both directions. Conditions for amplification of the seven loci and sequence interpretation were those described on the MLST website.2 This strain has an allelic profile 2-2-2-2-6-3-2, corresponding to ST30. According to Vandenesch et al.,3 ST30 represents the most frequent ST of CA-MRSA in the Southwest Pacific. In Hong Kong, typing studies have not been performed on healthcare-associated MRSA isolates from the whole territory. In an analysis of strains from one hospital,4 it was reported that the predominant PFGE types belong to one group and fall in the same cluster as EMRSA-1, -4, -7, -9, and -11 isolates. Preliminary MLST and SCCmec analysis by the same group suggest that representative isolates are related to ST239-MRSA-III (allelic profile for ST239 is 2-3-1-1-4-4-3).

The present case has several features common to CA-MRSA reported elsewhere.1,5 First, established risk factors for healthcare-associated infections are absent. Second, the strain is not multiresistant. Third, this strain carries SCCmec type IV, which is the smallest methicillin-resistance locus among the four known types. Unlike other SCCmec elements, SCCmec IV does not encode additional resistance determinants other than the mecA gene, which may explain the non-multiresistance characteristic of CA-MRSA. Fourth, the MLST analysis showed that the genetic background of our strain did not correspond to that of the major pandemic healthcare-associated MRSA clones.6

In conclusion, this report adds to the expanding distribution of CA-MRSA infections. As yet, the origin and the factors that contribute to their emergence are poorly understood. If community strains continue to spread, this will add to the pressing public health threat from this pathogen.

Acknowledgements

This work was supported by a research grant from the UDF Project-Research Centre of Emerging Infectious Diseases.

Footnotes

* Corresponding author. Tel: +852-2855-4897; Fax: +852-2855-1241; Email: plho{at}hkucc.hku.hk

References

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2 . Chan, M. S., & Aanensen, D. (2004). Multilocus sequence typing home page [Online.] http://www.mlst.net (3 August 2004, date last accessed).

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