The SmaI macrorestriction fragment profiles of 957 S. aureus isolates were determined by PFGE. A dendrogram of percent similarity, calculated with Dice coefficients from the PFGE data using a cutoff of 80%, revealed eight major clusters of isolates, designated as PFTs USA100 through USA800 (Fig. ). Of the 667 U.S. ORSA isolates, 622 (93%) clustered within these eight PFTs. In addition, 134 of the 235 OSSA isolates (57%) had PFGE patterns that fell within the same eight PFTs. The results of MLST, spa typing, SCCmec typing, antimicrobial resistance profiles, and other relevant properties of ORSA strains are summarized in Table . All but four PFTs (USA300 and -500, and -100 and -800) had a unique MLST sequence type and spa type motif. Five of the eight PFTs (USA100, -200, -500, -600, and -800) contained isolates that were predominantly obtained from health care-associated infections, while the isolates from two PFTs (USA300 and -400) were from community infections. USA700 isolates were obtained from patients in both community and health care settings.
FIG. 1. Dendrogram of PFTs with type strain (most frequent pattern) and a variant strain. Also shown is the corresponding MLST for each PFT (18, 19, 20).
Of isolates from health care-associated infections, USA100 was the largest and most diverse of the PFTs, containing 292 ORSA isolates from throughout the United States. For the seven housekeeping genes, these isolates shared a common MLST allelic profile (1- 4- 1- 4- 12- 1- 10), which is designated as ST 5, and a common spa
motif (MDMGMK). USA100 isolates were usually spectinomycin resistant (consistent with SCCmec
II) and multiresistant to commonly used therapeutic agents. This group included seven of eight U.S. VISA isolates and VISA isolates from Japan (Mu50) and Korea. USA100 also included the two U.S. VRSA isolates from Michigan and Pennsylvania. All isolates were resistant to erythromycin: 72% were constitutively clindamycin resistant, and 28% showed inducible clindamycin resistance. S. aureus
isolates BK2464, PA237, and JA48, from the New York/Japan clone (1
), belonged to this PFT.
USA800 isolates shared the same MLST sequence type and spa
motif as the USA100 isolates. These isolates were primarily from community surveillance studies, were spectinomycin susceptible (suggesting the lack of Tn554
[consistent with SCCmec
type I or IV]) and were generally resistant only to β-lactam drugs. Twenty-three percent were erythromycin resistant. When we tested the isolates, SCCmec
typing showed that most of them were SCCmec
type IV. S. aureus
isolates HDE1, HDE288, and COB94 from the Pediatric clone (24
) belonged to this group. Other isolates, such as EMRSA3, which carried SCCmec
type I and fell just outside the USA100 and -800 clusters, shared the same MLST and spa
USA200, the second most common health care-associated PFT among U.S. isolates, contained ORSA isolates that were spectinomycin resistant (consistent with SCCmec II) and were multiresistant to therapeutic agents. All ORSA isolates were erythromycin resistant, with 98% showing constitutive resistance to clindamycin. The isolates had the same MLST profile, i.e., ST 36 (2- 2- 2- 2- 3- 3- 2), and spa type motif (WGKAKAOMQQQ) as isolates from the EMRSA16 epidemic clone.
Isolates from community onset infections belonging to PFT USA400 had the MLST ST 1 profile (1- 1- 1- 1- 1- 1- 1) and spa
type motif (UJJJFE). These ORSA isolates were spectinomycin susceptible, carried SCCmec
IV, and were not multiresistant. S. aureus
MW2, an isolate from a rapid fatal infection in a child from Minnesota (6
), was included in this group.
Although representative isolates from USA300 and USA500 had the same MLST allelic profile (ST 8, 3- 3- 1- 1- 4- 4- 3) and spa
type motif (MBQBLO), these isolates clustered into separate, but contiguous, groups by Sma
I PFGE. USA300 isolates carried SCCmec
IV, were resistant to β-lactam drugs, were frequently resistant to erythromycin, and were predominantly from community onset skin infections. Eighty-five percent of the erythromycin-resistant isolates were susceptible to clindamycin and were not inducible with erythromycin (probably due to msrA
). On the other hand, USA500 isolates were generally from health care-related infections. The majority of USA500 isolates were spectinomycin susceptible, indicating the absence of Tn554
(consistent with SCCmec
type I or IV). Most of the isolates were resistant to clindamycin, erythromycin, gentamicin, levofloxacin, tetracycline, trimethoprim-sulfamethoxazole, and the β-lactams. The remaining U.S. VISA isolate and the three isolates for which the vancomycin MICs were 4 μg/ml had PFGE profiles belonging to this PFT. PFGE patterns of S. aureus
isolates PER34, E2125, and HPV107 from the Archaic/Iberian clones (13
); EMRSA isolates 2, 5, 6, 10, 12, 13, and 14; VISA isolates from Hong Kong and France; and the Iberian clonal-type isolate from Scotland clustered near the USA500 isolates. EMRSA isolates 2, 6, 12, 13, and 14 shared the same MLST profile (3- 3- 1- 1- 4- 4- 3) as the USA300 and USA500 isolates, which differed at a single locus from ST 250 (3- 3- 1-1- 4-4 -16) of isolate Per34. S. aureus
E2125 and HPV107 (Archaic/Iberian clonal-type isolates) and the S. aureus
EMRSA5 isolate have the MLST profile ST 247 (3- 3- 1- 12- 4- 4- 16), which differed at one locus from ST250 and two loci from ST 8. All of these isolates shared a similar spa
type motif (MBQBLO).
USA600 contained 23 isolates. Representative strains were ST 45 (10- 14- 8- 6- 10- 3- 2) and spa type A2AKEEMBKB. All but four of the ORSA isolates in this PFT were spectinomycin resistant (SCCmec II) and multiresistant; the remaining four were spectinomycin susceptible. Isolates from USA700 shared a unique MLST type, i.e., ST 72 (1- 4- 1- 8- 4- 4- 3), and spa motif (UJGFMGGM) from community surveillance and a hospital-acquired outbreak.
There were two clusters within the remaining 45 U.S. ORSA isolates. The first cluster (14 isolates) included surveillance isolates, mainly from Alaska, and isolates from an outbreak of soft-skin infections among Vermont wrestlers (37
). The second cluster included three isolates that clustered with S. aureus
isolates HU25, HSJ216, and HUSA304 (Brazilian and Hungarian type strains [13
]); EMRSA isolates 1, 4, and 11; and MRSA and VISA isolates from Scotland. These isolates had the same MLST profile (ST 239) and spa
type motif (WGKAOMQ). Twenty-eight isolates had either miscellaneous PFGE patterns or patterns that could not be identified within an 80% PFT cutoff.
Of the 235 U.S. OSSA isolates, 134 (57%) clustered within the eight PFTs. All of the PFTs, with the exception of USA500, contained OSSA. Twenty OSSA isolates were USA800, 1 isolate was USA100, 78 isolates were USA200, 9 isolates were USA300, 1 isolate was USA400, 22 isolates were USA600, and 3 isolates were USA700. Twenty-eight OSSA isolates clustered in a unique PFT. There were 10 OSSA clusters with 3 to 13 isolates, and 23 isolates with unique SmaI PFGE profiles.