Establishment and evaluation of the new typing systems for the epidemiological investigation in MRSA (Methicillin Resistant Staphylococcus Aureus) infections

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This study has been carried out to know the most useful way to " type " the MRSA strains for the epidemiological servey in the infections caused by the bacterium. Six typing systems of S. aureus have been evaluated for their utility for the each 96 MRSA and 12 MSSA (Methicillin Se-nstitive S. aureus) strains of known origins. Analysis by Pulsed Field Gel Electrophoresis (PFGE) for the genomic DNA cut with a restriction enzyme, SmaI, enabled us to classify the strains into 14 types based on the DNA bands separated by the electrophoresis. Ninty-eight per cent strains were revealed to have a plsmid or plasmids and molecular size of the DNA ranged from about lkb to 40kb. A very interesting finding was that 87% of the MRSA have, in common, a plsmid of 24, 3 kb. This plasmid was rarely detected in MSSA. Results of the biochemical tests, i. e., fermentation of lactose and sucrose, could classify the strains into the four biotypes which seemed to be of limited epidemiological value. In the phage typing, only 10% strains of MRSA were sensitive to one or more typing phage (s) diluted to IRTD. By contrast, 73% strains of MSSA were typable. Typing by coagulase (cell free) neutralization test (antisera from type I to type VIII inclusive were employed) gave a disappointing results for the purpose of epidemiological investigation, as the 98% strains belonged to a same type, type ii. All of the results summarized above led us to conclude as follows ; In the epidemiological investigation of the infections due to MRSA, at least when we are going to clarify the spread of contamination, the most useful way to " type " an isolate is combination of the results presented by analysis, by PFGE, for genomic DNA digested with SmaI, analysis of plasmic DNA, especially those which is larger than 20kb and biotyping based on their ability to ferment lactose and sucrose.