Epidemiological analysis of a methicillin-resistant Staphylococcus aureus outbreak using restriction fragment length polymorphisms of genomic DNA.

The genomic DNA of 58 isolates of methicillin-resistant Staphylococcus aureus (MRSA) obtained during an infection outbreak at two major Canberra hospitals was analysed for restriction fragment length polymorphism (RFLP) by digestion with the endonuclease SmaI and resolution of the fragments by pulsed-field gel electrophoresis. Based on the fraction of common fragments generated by the endonuclease, DNA similarities among the isolates were estimated. Distance matrix analysis showed that the MRSA isolates could be divided into two major clusters (RFLP types I and II) and one minor one (type 46). A fourth group of miscellaneous isolates was found to be heterogeneous in terms of DNA sequence similarity. The epidemiological data indicated that RFLP type I was most common in the intensive care units in the two hospitals, with particular subtypes of RFLP type I concentrated in individual units. RFLP type II and the miscellaneous group were more generally distributed. Type 46 isolates appear to be related to a group which was present in epidemics in Melbourne hospitals in the early 1980s. Using the standard phage set, the RFLP type I group was largely untypable. However, type II isolates were all phage typable, with a shared susceptibility to phages 29/85/95/90; type 46 isolates had a shared susceptibility to phages 85/90. The miscellaneous isolates were of variable phage types.