Community-associated methicillin-resistant Staphylococcus aureus, Singapore.

To the Editor: Community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) is an emerging phenomenon that has been reported from almost every continent in the world (1-4). Such strains are usually characterized by multisusceptibility to non-[beta]-1actam antimicrobial drugs, production of Panton-Valentine leukocidin (PVL), and presence of staphylococcal chromosome cassette mec (SCCmec) IVa, a novel smaller variant of the methicillin-resistance locus (5). The genetic backgrounds of CA-MRSA strains from different parts of the world are distinct and specific for each geographic region (1-5).

We conducted a study at our institution, a 1,600-bed adult acute-care, tertiary-level public hospital, to determine evidence and the clinical and molecular profile of CA-MRSA in Singapore. We reviewed the microbiology laboratory records at our institution for multidrug-susceptible MRSA strains isolated from January 1, 2001, to April 15, 2004. S. aureus was identified by colony morphologic features, coagulation of citrated rabbit plasma with EDTA (BBL Becton Dickinson and Co., Cockeysville, MD, USA), and production of clumping factor and protein A (BactiStaph, Remel, Lenexa, KS, USA). Methicillin resistance was determined by susceptibility testing and confirmed by latex agglutination for penicillin binding protein-2a (6). Multidrug-susceptible strains were defined by susceptibility to cotrimoxazole and gentamicin as determined by the Kirby-Bauer disk diffusion method following NCCLS guidelines (7).

The medical records of patients infected by these MRSA were reviewed, and strains were labeled community-associated if they had been isolated within 48 hours of hospitalization from patients who had not been in any hospital for >1 year. Community-associated strains were tested for PVL genes (8), and the SCCmec was typed by following a previously described method (9). Molecular typing was done by pulsed-field gel electrophoresis (PFGE) with restriction endonuclease Sinai and multilocus sequence typing (10). These strains were sent to the French Reference Center for Staphylococci, where genetic sequences encoding accessory gene regulator (agr) subtypes, enterotoxins, exfoliative toxins, toxic-shock syndrome toxin-1, hemolysins, and LukE-LukD leukotoxin were detected by polymerase chain reaction (PCR)-based methods. Comparisons with CA-MRSA strains worldwide in terms of toxin profile and PFGE patterns were also performed in France; the latter was achieved by using Taxotron software (Institut Pasteur, Paris, France) to digitize and analyze Sinai macrorestriction patterns.

Eight of 266 multidrug-susceptible strains fulfilled the criteria for community acquisition, but only 5 of these strains (corresponding to patients l, 3, and 6-8) had been archived. The demographic and clinical data of the patients are shown in the Table. Most were young, healthy adults with cutaneous abscesses. Patient 1 had diabetes mellitus but had never been hospitalized; he was the only patient with severe bacteremic pneumonia. Patient 6 had early-stage endometrial cancer resected in 2000 but had not attended her follow-up appointments for >1 year before her hospitalization. Patient 8 had traveled to Taipei, Taiwan, for a month; the abscess developed 3 days after her return home. Travel history was not documented in the other patients' records. Patients 2-8 received [beta]-1actam antimicrobial drugs in addition to surgical drainage of their abscesses and recovered without any complications.

All 5 archived strains had different molecular and toxin profiles, and the only consistent feature was the presence of PVL genes. Isolates 3 and 7 possessed SCCmec IV. Isolates 1, 6, and 8 were mecA positive, but their SCCmec belonged to none of the 4 major structural types. Comparisons with published data on CA-MRSA strains showed that isolate 7 was identical to the European strain of CAMRSA in terms of PFGE pattern, toxin profile, and sequence type (ST 80) (2,5). Isolate 3 had an identical PFGE pattern and sequence type (ST 30) compared to the Oceanian Southwest Pacific strain but differed slightly in toxin profile, as the LukD-LukE leukocidin genes were absent (3,5). Isolate 8 was similar to the Taiwanese strains: it was ST 59 and had non-typable SCCmec (4). It belonged to agr 1 and tested positive for enterotoxin sek, [gamma]2-hemolysin, and [beta]-hemolysin genes.

Isolate 6 had a PFGE pattern that may be distantly related to U.S. strains; the similar sequence type (ST 1) served to emphasize this, although the presence of nontypable SCCmec rather than SCCmec IV implied that methicillin resistance was acquired differently. It belonged to agr 3 and tested positive for LukD-LukE leukocidin, enterotoxins seb and seh, and [gamma]2-hemolysin genes. Isolate 1 is unique to Singapore in that it had a novel sequence type (ST524: 7-6-1-5-71-5-6 and SCCmec. It belonged to agr 1 and tested positive for [gamma]-hemolysin gene as well as for the enterotoxin gene cluster.

Widely diversified CA-MRSA strains exist in Singapore. The demographic profile and clinical symptoms of local patients infected with these strains were consistent with published literature (2-4). The lack of a pediatric unit at our institution prevented a more complete epidemiologic description.

In contrast to previous reports (1-5), our findings are unique in that most of our strains do not have a distinctive molecular profile and may be related to strains from different parts of the world. Epidemiologic and molecular data strongly suggest that isolate 8 was imported from Taiwan. Some of the other strains (especially isolates 3 and 7) may have been imported from other countries too, as Singapore is an international travel hub with >6 million visitors annually.

CA-MRSA has only been isolated sporadically in Singapore, and no dominant clone was seen among our isolates. Singapore may be in an early phase of CA-MRSA emergence, and healthcare workers should remain vigilant for future outbreaks.

Table. Demographic and clinical data of patients
with community-associated methicillin-resistant
Staphylococcus aureus (MRSA)

Patient                1              2           3            4

Date of MRSA       Mar 2001       Nov 2002     Jan 2003     Feb 2003
isolation
Ethnicity           Indian        Filipino     Chinese      Chinese
Age                   52             20           38           37
Sex                    M              F           M            M
Coexisting         Diabetes          --           --           --
conditions         mellitus
Infection type    Pneumonia,        Hand         Hand         Hand
                  bacteremia       abscess     abscess      abscess
Therapy *        IV vancomycin       I&D         I&D          I&D
Appropriate           Yes            No           No           No
antimicrobial
drug
usage

Patient                5              6           7            8

Date of MRSA       Mar 2003       May 2003     Oct 2003     Apr 2004
isolation
Ethnicity          Filipino        Chinese     Filipino     Chinese
Age                   31             56           21           33
Sex                    F              F           F            F
Coexisting            --         Endometrial     --           --
conditions                         cancer
Infection type       Hand           Hand         Chin      Abdominal
                    abscess        abscess     abscess    wall abscess
Therapy *             I&D            I&D         I&D          I&D
Appropriate           No             No           No           No
antimicrobial
drug
usage

* Therapy: IV, intravenous;
--, not applicable; I&D,
incision and drainage of abscess.

Acknowledgment

We thank the staff of the Microbiology Laboratory at the Singapore General Hospital, especially Lan-Huay Ong and Grace Wang, for their assistance in this study.

This study was funded by a grant from Singapore General Hospital's Department of Clinical Research.

References

(1.) Naimi TS, LeDell KH, Boxrud DJ, Groom AV, Steward CD, Johnson SK, et al. Epidemiology and clonality of community-acquired methicillin-resistant Staphylococcus aureus in Minnesota, 1996-98. Clin Infect Dis. 2001;33:990-6.

(2.) Dufour P, Gillet Y, Bes M, Lina G, Vandenesch F, Floret D, et al. Community-acquired methicillin-resistant Staphylococcus aureus infections in France: emergence of a single clone that produces Panton-Valentine leukocidin. Clin Infect Dis. 2002;35:819-24.

(3.) O'Brien FG, Lim TT, Chong FN, Coombs GW, Enright MC, Robinson DA, et al. Diversity among community isolates of methicillin-resistant Staphylococcus aureus in Australia. J Clin Microbiol. 2004;42: 3185-90.

(4.) Wang CC, Lo WT, Chu ML, Siu LK. Epidemiological typing of community-acquired methicillin-resistant Staphylococcus aureus from children in Taiwan. Clin Infect Dis. 2004;39:481-7.

(5.) Vandenesch F, Naimi T, Enright MC, Lina G, Nimmo GR, Hefferman H, et al. Community-acquired methicillin-resistant Staphylococcus aureus carrying Panton-Valentine leukocidin genes: worldwide emergence. Emerg Infect Dis. 2003;9:978-84.

(6.) Nakatomi Y, Sugiyama J. A rapid latex agglutination assay for the detection of penicillin-binding protein 2'. microbiol Immunol. 1998;42:739-43.

(7.) National Committee for Clinical Laboratory Standards. Performance standards for antimicrobial susceptibility testing. Fourteenth informational supplement. NCCLS document M100-S14. Wayne (PA): The Committee: 2004.

(8.) Lina G. Piemont Y, Godail-Gamot F, Bes M, Peter MO, Gauduchon V, et al. Involvement of Panton-Valentine leukocidin-producing Staphylococcus aureus in primary skin infections and pneumonia. Clin Infect Dis. 1999;29:1128-32.

(9.) Oliveira DC, de Lencastre H. Multiplex PCR strategy for rapid identification of structural types and variants of the mec element in methicillin-resistant Staphylococcus aureus. Antimicrob Agents Chemother. 2002;46:2155-61.

(10.) Enright MC, Day NP, Davies CE, Peacock SJ, Spratt BG. Multilocus sequence typing for characterization of methicillin-resistant and methicillin-sensitive clones of Staphylococcus aureus. J Clin Microbiol. 2000;38:1008-15.

Address for correspondence: Li-Yang Hsu, Department of Internal Medicine, Singapore General Hospital, Outram Rd, S169608, Singapore; fax: [phone]; email: liyang_ [e-mail]

Li-Yang Hsu *, Anne Tristan, ([dagger]) Tse-Hsien Koh *, Michele Bes, ([dagger]) Jerome Etienne, ([dagger]) Asok Kurup, * Thuan-Tong Tan, * and Ban-Hock Tan *

* Singapore General Hospital, Singapore; and ([dagger]) Faculte de Medecine Laennec, Lyon, France