MRSA USA300 at Alaska Native Medical Center, Anchorage, Alaska, USA, 2000–2006

To determine whether methicillin-resistant Staphylococcus aureus (MRSA) USA300 commonly caused infections among Alaska Natives, we examined clinical MRSA isolates from the Alaska Native Medical Center, Anchorage, during 2000–2006. Among Anchorage-region residents, USA300 was a minor constituent among MRSA isolates in 2000–2003 (11/68, 16%); by 2006, USA300 was the exclusive genotype identified (10/10).


The Study
During 2000-2003, 695 clinical MRSA isolates were obtained by the Clinical Microbiology Laboratory of ANMC. A convenience sample of 567 isolates was collected and termed the retrospective collection. This collection was stratifi ed by year of isolation and by 3 geographic regions of Alaska in which patients resided: 1) the Anchorage region (Anchorage, the Mat-Su region, and the Aleutian Islands); 2) the region of southwestern Alaska previously studied (2); and 3) all other regions. A randomly selected sample of 163 (28.7%) of the 567 isolates, stratifi ed by year of isolation, was chosen for genotyping, including 20% of the isolates from Anchorage-region residents, 20% from residents of southwestern Alaska, and all isolates from residents of other regions ( Table 1).
The prospective collection collected in 2004-2006 consisted of the fi rst 5 clinical MRSA isolates obtained each month by the ANMC Clinical Microbiology Laboratory from different patients. Although 177 MRSA isolates had been collected, 2 were not available, and 2 lacked the mecA gene by PCR, leaving 173 isolates for further study. Genotyping was carried out on a random sample of 20% of isolates from this collection, stratifi ed by year of isolation from the Anchorage-region patients, and on samples from all patients from all other regions (Table 1).
Clinical and demographic information was collected about the patients comprising the retrospective and prospective isolate groups. Site of care was recorded only for the prospective collection. Active surveillance for MRSA was not performed at ANMC during 2000-2006. Isolates were genotyped by multilocus sequence typing (MLST), and clonal complexes (CCs) were assigned to closely related sequence types as described (6,7). Staphylococcal cassette chromosome mec (SCCmec) typing was performed (8), and the presence of Panton-Valentine leukocidin (PVL) genetic determinants was assessed as described (9). Additionally, to clarify the relationship between ST and typing by pulsed-fi eld gel electrophoresis (PFGE), a random sample of strains that were ST8 and ST1 were tested by PFGE as described (10). Control strains were USA300-LAC for USA300 and strain 649, a clinical strain identical to MW2 by PFGE, for USA400. Antimicrobial drug susceptibilities were determined by using automated testing (bioMérieux Vitek, Durham, NC, USA). The D-zone test for inducible clindamycin resistance was performed for isolates resistant to erythromycin and susceptible to clindamycin by singleagent testing (11). Results were compared by χ 2 or Fisher exact tests using Stata version 11 (StataCorp LP, College Station, TX, USA).
When our sample was adjusted to account for the sampling strategy, we were able to estimate the genotypic spectrum of all isolates. In the retrospective collection, an estimated 12% (67/567) of MRSA isolates were ST8, and 42% (236/567) were ST1. Similarly, we estimated in the prospective collection that 61% (105/173) were ST8, and 25% (44/173) were ST1.

Conclusions
We documented the emergence and rapid dominance of USA300 among clinical MRSA isolates from Anchorageregion patients who received treatment at the ANMC in 2002-2006. This complete strain replacement by 2006 suggests that Anchorage-region patients were exposed to a growing reservoir of USA300 during this era. In contrast, in 2006, USA300 still remained a less common cause of MRSA infections among ANMC patients drawn from other regions of Alaska, perhaps because patients from other regions were less likely to have been referred to ANMC for uncomplicated SSTIs.
Of the tested PVL-positive ST8 MRSA isolates bearing SCCmec IV, we confi rmed that 100% (30/30) were USA300 by PFGE. This fi nding has useful implications for comparing MLST and PFGE typing methods. ST1 isolates frequently had inducible clindamycin resistance, whereas strains with PVL and SCCmec IV of other genetic backgrounds rarely did. This may explain the seemingly anomalous data from the USA400 era in Chicago that 31/33 (94%) of clindamycin-susceptible, erythromycin-resistant strains of MRSA were D-zone test positive (12).
Documented introduction of USA300 has not resulted in strain replacement in Europe (13), Asia (14), or Australia (15). That USA300 emerged and came to predominate among Anchorage-region residents, mirroring the process that occurred earlier in several cities in North America (4,5), suggests that some characteristic of USA300 provides a survival advantage, enhanced virulence, or both, relative to other MRSA pulsotypes in those regions. The rapid appearance and emergence of USA300 at ANMC in 2002-2006 were remarkable, but its limited global spread remains unexplained.   ‡CC1 has recently been changed to CC15 by the administrators of the multilocus sequence typing system. A randomly selected sample of 14 ST1 isolates that carried the SCCmec IV element and were Panton-Valentine leukocidin positive underwent PFGE, and 93% (13/14) were USA400.
§42/43 ST8 strains identified in the study carried the SCCmec type IV element and were Panton-Valentine leukocidin positive. They are all considered to be USA300 because 30 of the 42 were randomly selected to undergo PFGE, and 100% (30/30) had the USA300 pulsotype.