Failure of bacteriophage typing to detect an inter-hospital outbreak of methicillin-resistant Staphylococcus aureus (MRSA) in Zagreb subsequently identified by random amplification of polymorphic DNA (RAPD) and pulsed-field gel electrophoresis (PFGE)

Objective: To establish the extent of inter-hospital spread of methicillin-resistant Staphylococcus aureus (MRSA) in Zagreb and to determine the most suitable method for typing local strains.
Methods: We analyzed a collection of 33 MRSA isolates from three Zagreb hospitals together with five unrelated British MRSA isolates by antibiogram typing, bacteriophage typing, randomly amplified polymorphic DNA (RAPD) analysis and pulsed-field gel electrophoresis (PFGE) after digestion with Smal restriction endonuclease. Bacteriophage typing was done with the international set of S. aureus typing phages. RAPD and PFGE profiles were analyzed visually and by using the 'GelCompar' computer program.
Results: Antibiogram typing provided eight profiles. Thirty (91%) of the 33 Croatian strains of MRSA were non-typable by phage typing. Visual analysis of RAPD products identified six, and visual analysis of PFGE fragments nine, distinct profiles. Computer analysis of RAPD data separated British isolates from the Croatian ones, but did not cluster the visually determined RAPD types. PFGE computer analysis separated British isolates and clustered isolates in concordance with visual interpretation. Thirty-one of the 38 isolates (82%) were visually grouped in the same clusters by both molecular methods. The dominant strain was present in each of the three hospitals.
Conclusions: Bacteriophage typing was unhelpful for the analysis of Croatian MRSA, since most strains were untypable with the international set of bacteriophages. RAPD and PFGE were more successful in typing the organisms and showed evidence of inter-hospital spread of one predominant MRSA strain in all three Zagreb hospitals. Thus RAPD and PFGE proved to be a useful aid in elucidating the epidemiology of MRSA infection in Zagreb hospitals and should be established in Croatia for typing MRSA.