Utility of spa typing for investigating the local epidemiology of MRSA on a UK intensive care ward.

In the UK, meticillin-resistant Staphylococcus aureus (MRSA) is frequently endemic on intensive care units (ICUs), yet our understanding of the local epidemiology of MRSA within the ICU is poor and the best methods for preventing MRSA acquisition remain controversial. Newer molecular typing methods may aid epidemiological investigation of local MRSA strains. We applied Staphylococcal Protein A (spa) typing to MRSA strains collected from patients in a UK ICU. spa typing allowed better discrimination than multilocus sequence typing (MLST) but 73% of strains were either spa type t032 or t018 (associated with the prevalent UK MRSA strains, EMRSA-15 and EMRSA-16). MRSA infections were preceded by MRSA colonisation in 72% of patients, and in 88% of these, both commensal and disease-causing strains had identical MLST and spa types. spa typing helped elucidate the transmission of MRSA between patients for 19 strains with unusual spa types, although the high incidence of EMRSA-15 and -16 types t032 and t018 prevented its use for the majority of strains. Surprisingly, only four (9%) of 45 new MRSA isolates occurring within 28 days of isolation of an unusual spa type could have been due to cross-contamination. These results suggest that prompt transmission of MRSA between patients is rare in our ICU, at least for those strains with unusual spa types.