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Methicillin-resistant Staphylococcus aureus (MRSA) is an important public health threat, with significant disease burden. Of particular concern are the growing number of cases that have origins in the community (CA-MRSA); these infections often affect individuals without underlying risk factors for disease. Because MRSA colonizes a large number of people, can be easily spread by fomites, and survives on surfaces for months, the environment is an important element in MRSA transmission. Public transportation vehicles, which have high levels of crowding and significant hand-to-fomite contact, may represent an important site for MRSA transmission. This study determined the background prevalence of MRSA on buses from a large transportation agency. Prior to vehicle sampling, a laboratory-based study was conducted to evaluate the performance of four sampling methods for recovery of S. aureus from a stainless steel surface. Results indicated that overall, the electrostatic wipe resulted in the greatest performance across all inoculating concentrations ([relative sampling efficiency=SE; analytical sensitivity=Sn] SE48h=17.8%; Sn48h=7 CFU/100cm2). However, the swab sampler performed well when corrected for area actually sampled (SE48h=24.3%; Sn48h=76 CFU/100cm2), but is greatly limited by the surface area which can be effectively sampled. Among the contact-based methods, the roller sampler outperformed the contact plate (roller: SE48h=9.9%; Sn48h=17 CFU/100cm2; contact plate: SE48h=0.04%; Sn48h=1,412 CFU/100cm2), and both contact samplers performed better at higher inoculating concentrations. Statistically significant differences were observed between the samplers only at the lowest two and highest inoculation concentrations (p conc 1= 0.009; p conc 2= 0.007; p conc 5= 0.029). Overall, the wipe resulted in the greatest number of replications that resulted in positive growth, with 73.8% at 24 hours, and 90.5% at 48 hours. Using this laboratory data, a field-based surveillance study was conducted to determine whether S. aureus (both methicillin-susceptible Staphylococcus aureus [MSSA] and MRSA) could be isolated from bus surfaces. A total of 237 samples were collected from 40 buses from a major urban transportation agency. Sixty-eight percent of buses were contaminated with S. aureus (both MSSA and MRSA), and 63% with MRSA. In total, 17% of the 237 samples were positive for S. aureus (both MSSA and MRSA) and 15% for MRSA. Categorical analysis indicated that at the sample level, sampling date (p S. aureus, July= 0.075) and sample location (p MRSA= 0.000; p S. aureus= 0.000) on a vehicle were associated with vehicle contamination. At the bus level, no factors were significantly associated with vehicle contamination, but ridership (p S. aureus= 0.088) and sampling date (p MRSA, July= 0.074; p S. aureus, July= 0.074) indicated potentially important associations. Finally, the antibiotic resistance profiles and molecular characteristics of positive bus isolates were evaluated. Multiple resistance patterns were observed, with high levels of multi-class resistance. Sixty-five percent (65%) of isolates were of the community-associated staphylococcal cassette chromosome mec (SCCmec) type IV, and 20% were healthcare-associated SCCmec type II. Multiple pulsed-field gel electrophoresis patterns were noted, with the majority being CA-types USA300/400; the HA-type USA100 was also found. The hospital-associated strain types were resistant to more classes of antibiotics, although multi-class resistance was also observed among the community-associated strains. High levels of clindamycin resistance were noted across all study isolates, for both MSSA and MRSA. Clonality results demonstrated that buses may be effective mixing vessels for MRSA strains, as multiple unique strains were noted in some study vehicles. |
