USA300 is the dominant strain responsible for community-associated (CA) methicillin-resistant Staphylococcus aureus (MRSA) infections in most of the United States. As a result, the epidemiology of CA-MRSA has become complex ( 5 ) Novel MRSA isolates that are less likely to be resistant to antimicrobial drugs other than β-lactams have been identified in association with epidemic CA-MRSA infections.

Volume 14, Number 11–November 2008. Research. Molecular Epidemiology of Methicillin-Resistant Staphylococcus aureus, Rural Southwestern Alaska 1. . Michael Z. David, .

USA300 is the dominant strain responsible for community-associated (CA) methicillin-resistant Staphylococcus aureus (MRSA) infections in most of the United States. We examined isolates from outbreaks of MRSA skin infections in rural southwestern Alaska in 1996 and 2000 (retrospective collection) and from the hospital serving this region in 2004–2006 (prospective collection).

Among 36 retrospective collection isolates, 92% carried Panton-Valentine leukocidin (PVL) genes; all carried staphylococcal chromosomal cassette mec (SCCmec) type IV. None belonged to clonal complex (CC) 8, the CC associated with USA300; 57% were sequence type (ST) 1, and 26% were ST30; 61% were clindamycin resistant. In the prospective collection, 42 isolates were PVL+ and carried SCCmectype IV; 83.3% were ST1, 9.5% were ST30, and 7.1% were ST8.

In the United States, epidemic infection with community-associated (CA) methicillin-resistant Staphylococcus aureus (MRSA) is occurring, with many reports of MRSA infections among persons without traditional healthcare-associated MRSA risk factors ( 2–4 ). As a result, the epidemiology of CA-MRSA has become complex ( 5 ) Novel MRSA isolates that are less likely to be resistant to antimicrobial drugs other than β-lactams have been identified in association with epidemic CA-MRSA infections.

These CA-MRSA strains are commonly susceptible to drugs such as clindamycin, gentamicin, tetracyclines, and rifampin. Moreover, the genes encoding the pore-forming, bicomponent cytotoxin, Panton-Valentine leukocidin (PVL), are nearly universally present in novel CA-MRSA strains. However, evidence from animal studies has been contradictory in assessing the importance of PVL in the virulence of these isolates ( 6,7 )

In addition to the PVL genes, strains that cause CA-MRSA infections typically carry staphylococcal chromosomal cassette mec (SCCmec) types IV and V, small genetic resistance elements that are presumably mobile. The reason for the dominance of USA300 is not clear In an area of rural Alaska, SSTIs caused by CA-MRSA isolates have been a public health concern since 1996 ( 12–14 ).

Two collections of MRSA isolates were available for study. They were obtained from the same region of southwestern Alaska. Southwestern Alaska is a roadless area populated principally by Alaska Natives, the descendants of the indigenous people of Alaska. The regional commercial center is Bethel (population 5,700), which is served by the 50-bed Yukon Kuskokwim Delta Regional Hospital (YKDRH) ( 13,14 ).

This hospital is the only one serving the 25,000 persons in the YKHC, which is a tribal health corporation that operates a comprehensive system of care in the region for Alaska Natives, including hospital and village-based clinics.

Retrospective Collection.

Prospective Collection.

Results. Thirty-six isolates were available from the retrospective collection. Six were from the 1996 investigation outbreak collected from January 6, 1997, through January 6, 1998, and 30 from the 2000 outbreak collected from April 17, 2000, through September 20, 2000. Of the 2000 outbreak isolates, 3 were collected from steam bath bench surfaces, 12 from nasal survey cultures, and 15 from material obtained from SSTIs.

Of 36 isolates in the retrospective collection, 33 (92%) were PVL+ ( 14 ) . SCCmec and MLST typing were performed on the retrospective collection isolates. All carried SCCmec type IV. ST1 was the most common MLST genotype, accounting for 20 (57%) of 36 isolates; ST30 accounted for 9 (26%) of 36 and ST59 for 1 (2.8%) of 36. When isolates were grouped in clonal complexes, 22 (63%) belonged to CC1, 11 (32%) to CC30, and 2 (6%) to CC59.

None were ST8 or belonged to CC8, the CC most closely associated with USA300 ( Table 1 ) Only 14 (39%) of 36 MRSA retrospective collection isolates were susceptible to clindamycin when clindamycin single-agent testing and D-test results were taken into account; 31% were susceptible to erythromycin. Susceptibility to gentamicin, ciprofloxacin, rifampin, vancomycin, and trimethoprim-sulfamethoxazole was nearly universal ( Table 1 )

. A random sample of one third of the prospective collection isolates (42/120) spanning the entire collection interval were assayed for PVL toxin genes, SCCmec, and MLST type. All 42 isolates tested were PVL+, and all carried the SCCmec type IV element. Most MRSA isolates in the prospective collection were ST1 (35/42, 83.3%), which likely represent USA400. Other types included ST30 (4/42, 9.5%) and ST8 (3/42, 7.1%).

ST8 isolates likely represent USA300 ( Table 1 ) Antimicrobial drug susceptibility data were available for all 120 isolates in the prospective collection; 51 (42.5%) of 120 were susceptible to erythromycin and clindamycin. There was no discordance in erythromycin and clindamycin susceptibility; therefore, no isolate underwent the D-zone test.

ST59 strains were also isolated at a decreasing frequency in 1997–2001 from patients in a California jail ( 15 ), and in Western Europe and Singapore ( 11 ). We found 1 ST59 isolate and 1 single-locus variant of ST59 in the retrospective collection from the 1996 and 2000 outbreaks but found none in the prospective collection. . The evolutionary history of ST30 MRSA strains is complex; the acquisition of the SCCmec element and the PVL genes has likely occurred in this genetic background on several occasions.

Phage type 80/81 strains of S|aureus, virulent nosocomial pathogens in the 1950s and 1960s, shared this ST background ( 26 ). By examining the pattern of resistance-gene carriage in various MRSA genetic backgrounds, Diep et al. proposed an evolutionary relationship among ST30 strains, suggesting that an MSSA ST30 strain sequentially added to its genome phage-encoded PVL toxin genes and the SCCmec IV element ( 27 ).

However, a strain of ST30 MRSA isolated in 1991 from Wisconsin lacked the PVL genes but carried SCCmec IV ( 25 ), which suggested that the sequence of events hypothesized by Diep et al. does not universally describe the evolution of these strains ( 27 ). Among 5 ST30 MRSA isolates collected in Japan in 1979–1985, 3 were PVL+ and all carried the SCCmec type I element ( 28 ).

ST30 isolates reported from various regions commonly carry PVL genes and the SCCmec IV element but can differ in spa type, which suggests a continued and complex evolutionary trajectory for this prevalent sequence type ( 8 , 11,18 , 24,29–32 ). . The difference in PVL+, SCCmec IV strain types of MRSA in rural Alaska compared with those in the lower 48 states suggests that Alaska may represent an earlier part of the epidemic curve of CA-MRSA.

For example, there was a shift from USA400 to USA300 as the predominant clindamycin-susceptible, PVL+, SCCmec IV-containing MRSA strain in Chicago after 2000 ( 10 ). The predominance of ST1 strains in southwestern Alaska may reflect geographic isolation of this region or improved fitness of the strain in the rural Alaskan environment. The clinical spectrum of these community-onset cases is similar to MRSA disease elsewhere with a predominance of SSTIs and few associated instances of bacteremia or other invasive illnesses.

This disease spectrum is also similar to that of earlier reports of infections caused by MRSA from this region ( 13,14 ). O'Hara et al., using phylogenetic analyses of the lukSF-PV sequences coding PVL toxin in a sample of international clinical MRSA isolates, recently hypothesized that USA300 emerged after a CC8 MRSA strain acquired the PVL genes from the preexisting, virulent MW2 strain ( 33 ).

If this event was the genesis of USA300, this event may have occurred in the lower 48 states, and USA300 had not spread to southwestern Alaska, where USA400 strains still predominated in early 2006. . All MRSA isolates we tested carried SCCmec type IV. So-called healthcare-associated MRSA isolates typically carry SCCmec types II or III, lack PVL genes, tend to be resistant to a greater number of non–β-lactam antimicrobial drugs, and were predominant among strains isolated from cases of hospital infections in the United States before 2000 ( 5 ).

Such healthcare-associated MRSA isolates were absent from our isolate collections, even from the prospective collection, which was a random sample of MRSA isolates that included inpatients in the region served by the hospital laboratory for 2 years. In contrast, at the University of Chicago in 2004–2005, 8.6% of MRSA isolates from pediatric infections and 51.7% from adult infections carried SCCmec II ( 5 ).

The PVL+, SCCmec IV–bearing strains of MRSA from Alaska that we studied showed a high percentage of clindamycin resistance (57.5%). In contrast, strains of MRSA that cause community-onset skin infections elsewhere in the United States are commonly susceptible to clindamycin ( 2–4,8 ), although exceptions have been documented ( 34 ), most recently in San Francisco among men who have sex with men infected by USA300 strains ( 35 ).

Taneike I, Otsuka T, Dohmae S, Saito K, Ozaki K, Takano M, et al. Molecular nature of methicillin-resistant Staphylococcus aureusderived from explosive nosocomial outbreaks of the 1980s in Japan.

Selected characteristics of MRSA isolates from southwestern Alaska, 1997, 2000, and 2004–2006 . Demographic and clinical characteristics for 120 patients infected with MRSA isolates in the prospective collection, southwestern Alaska, 2004–2006