Comparative molecular analysis of community- or hospital-acquired methicillin-resistant Staphylococcus aureus

Abstract

Community-acquired methicillin-resistant Staphylococcus aureus (CA-MRSA) is a growing public health concern that has been associated with pediatric fatalities. It is hypothesized that the evolution of CA-MRSA is a recent event due to the acquisition of mec DNA by previously methicillin-susceptible strains that circulated in the community. This study investigated the genetic relatedness between CA-MRSA, hospital-associated MRSA (HA-MRSA), and nonmenstrual toxic shock syndrome (nmTSS) isolates. Thirty-one of 32 CA-MRSA isolates were highly related as determined by pulsed-field gel electrophoresis and spa typing yet were distinguishable from 32 HA-MRSA strains. The 31 related CA-MRSA isolates produced either staphylococcal enterotoxin B (n = 5) or C (n = 26), and none made TSS toxin 1. All CA-MRSA isolates tested contained a type IV staphylococcal cassette chromosome mec (SCCmec) element. In comparison, none of the HA-MRSA isolates (n = 32) expressed the three superantigens. Antibiotic susceptibility patterns were different between the CA-MRSA and HA-MRSA isolates; CA-MRSA was typically resistant only to beta-lactam antibiotics. Six of twenty-one nmTSS isolates were indistinguishable or highly related to the CA-MRSA isolates. MnCop, an nmTSS isolate obtained in Alabama in 1986, was highly related to the CA-MRSA isolates except that it did not contain an SCCmec element. These data suggest that CA-MRSA strains may represent a new acquisition of SCCmec DNA in a previously susceptible genetic background that was capable of causing nmTSS. CA-MRSA poses a serious health risk not only because it is resistant to the antibiotics of choice for community-acquired staphylococcal infections but also because of its ability to cause nmTSS via superantigen production.

FIG. 1.

PFGE subtyping and dendrogram of CA-MRSA, HA-MRSA, and nmTSS isolates. A PFGE pattern seen in more than one isolate was given a letter-and-number designation (e.g., A1). PFGE groups are noted on the far right end of the tree. The spa type, spa profile, and SCCmec type, if testing was performed, are shown on the right end of the figure.

FIG. 2.

Results of PFGE of a representative isolate from each CA-MRSA PFGE group (A1 to A8). Lanes: 1, REF607 (group A1); 2, REF571 (group A2); 3, C98-370 (group A3); 4, REF559 (group A4); 5, MnCop (group A5); 6, C99-193 (group A6); 7, REF553 (group A7); 8, REF1239 (group A8); 9, REF1247 (group A8); and 10, COL.

FIG. 3.

Results of PFGE and subsequent hybridization with mecA and gyrA DNA probes. (A) Lanes: 1, NCTC8325; 2, C99-459; and 3, MnCop. White arrows show bands in both C99-459 and MnCop that differentiate these strains by PFGE. (B) Hybridization with a mecA probe. Lanes: 1 and 1A, NCTC8325; 2 and 2A, MnCop; and 3 and 3A, C99-459. White arrows show bands that hybridize with a mecA probe in C99-459. (C) Hybridization with a gyrA probe. Lanes: 1 and 1A, NCTC8325; 2 and 2A, MnCop; and 3 and 3A, C99-459. White arrows show bands that hybridize with a gyrA probe in all three strains. Black arrows in panel A show the size of the SmaI bands in NCTC8325, which approximates the size of the bands that hybridize to gyrA in MnCop and both mecA and gyrA in C99-459.