Longitudinal analysis of methicillin-resistant and methicillin-susceptible Staphylococcus aureus carriage in healthy adolescents

To determine the long-term carriage patterns, strain relatedness, and incidence of subsequent infections among methicillinresistant Staphylococcus aureus (MRSA) and methicillin-susceptible S. aureus (MSSA) carriers, we screened 154 high school students for nasal carriage of S. aureus on 8 occasions over 11 months. Persistent carriage was defined as a positive culture on ≥ 7 occasions. Two consecutive isolates from the same subject comprised a pair, and strain relatedness was determined for each pair by molecular typing. Of 1,232 nasal swab cultures obtained on 8 occasions, 323 (26.2%) were positive for S. aureus. Forty-five isolates (3.7%) were MRSA and 278 isolates (22.6%) were MSSA from 12 and 63 subjects, respectively. Thirty-five (77.8%) MRSA isolates harbored a type IV or V_T staphylococcal chromosomal cassette mec element. Among the 154 subjects, 52 (33.8%) were intermittent (1 to 6 positive swabs) carriers. Persistent carriage was identified in 23 (14.9%) subjects, and the incidence was not significantly different for MRSA and MSSA carriers (3/12 [25%] versus 20/63 [31.7%]; P = 0.7449). The MRSA and MSSA isolates were composed of 33 and 215 strain pairs, respectively. Of them, an indistinguishable genotype was identified in 33 (100%) MRSA pairs and 173 (80.5%) MSSA pairs (P = 0.0053). Five subjects developed cellulitis, and the incidence of this was higher for MRSA carriers (2/12 [16.7%]) than for MSSA carriers (1/63 [1.58%]; P = 0.0632) and noncarriers (2/79 [2.56%]; P = 0.0828). In conclusion, the longterm carriage patterns for MRSA and MSSA in healthy individuals were similar. MRSA carriers were more likely to carry a single strain, with a trend toward a higher chance of developing cellulitis than for MSSA carriers.