Epidemiology of methicillin-resistant Staphylococcus aureus in a Warsaw hospital.

Methicillin-resistant Staphylococcus aureus (MRSA) isolates were collected during two eight-month periods in 1991 and 1994, respectively. In order to study the epidemiology, all 74 strains were characterized by phage-typing, antibiotic resistance patterns and DNA-restriction map after cleavage with SmaI enzyme, and pulsed-filed gel electrophoresis (PFGE). These investigations confirmed that MRSA in the hospital, 1991 and 1994, was not due to the spread of one or two clones, but by the simultaneous occurrence of a few well characterized strains and sporadic, occurring strains of different phage-types. Some of these might have developed from the more commonly occurring strains. Isolates from 1994 were more resistant to antibiotics in vitro, than the 1991 isolates. The typing results also indicated that whilst most of the MRSA strains in 1994 were different compared with those of 1991, some of the strains might have been present in both years. The PFGE-typing was more discriminatory and gave a higher typability than the phage-typing, especially among the multiply resistant isolates of MRSA from 1994. Among the less resistant strains the phage-typability was high and with only few exceptions, there was a good correlation between PFGE-type and phage-type.