Emergence of USA300 MRSA in a tertiary medical centre: implications for epidemiological studies.

Community-associated meticillin-resistant Staphylococcus aureus (CA-MRSA) has become a major pathogen, particularly in outbreaks of skin and soft-tissue infection (SSTI). A preliminary study conducted at our institution in 2004 revealed that up to 45% of inpatient and 70% of outpatient MRSA isolates tested were the USA300 genotype. In this report, we used pulsed-field gel electrophoresis (PFGE) in a retrospective analysis to determine the time when CA-MRSA USA300 moved from the community to the inpatient population. During the five-year period 2000 to 2004, unique MRSA isolates (N=253) were selected from inpatients in surgical and medical intensive care units, the general hospital population and outpatients. The most common PFGE types found in all populations from 2000 to 2003 were USA100, USA200 and USA600. USA300 was absent from all inpatients from 2000 to 2003 and only sporadic numbers found in the outpatient group. However, in 2004 the USA300 strain emerged in both outpatient and hospitalised patients. There was no difference in the distribution of USA300 between ICUs and the general inpatient population. The emergence of CA-MRSA has resulted in a shift of the MRSA strains that are implicated in healthcare-associated infections in our institution. This has been a recent development that has implications as to the use of PFGE to determine transmission of MRSA in the inpatient setting. Further evaluation of these data in the context of the epidemiology of these infections is needed to determine if more discriminatory approaches to typing will be required for monitoring the spread of the more virulent CA-MRSA phenotype within the inpatient population.