Nosocomial infections causedbymethicillin-resistant

Staphylococcus aureus (MRSA)are a significant epidemiological problem. Detecting thesourcesofepidemic strains andpreventing their accesstopatients, however, depend upon theavailability oftechniques toreliably distinguish among MRSA strains. We evaluated restriction enzyme analysis ofplasmid DNA foruse as an epidemiological markerofMRSA strains. The diversity ofplasmid types was assessed byexamining 120clinical andenvironmental MRSA isolates fromfive southern California hospitals andfromtheAmerican TypeCulture Collection. Thirty-seven distinctive EcoRI digestion patterns wereobserved. Wecharacterized eachstrain bythenumberofplasmids itcontained andthe sizes ofthefragments that were generated byEcoRI. Veryfewoftheisolates (4.2 %)lacked plasmids, andsome (6.7%) contained DNA that was notdigested byEcoRI.Several isolates (12.5%) contained twoor more plasmids. We wereable toassessthestability ofMRSAplasmid typesbytracking epidemic strains overa2-year period. Wealso examined successive isolates from10individual patients during their hospitalization. Inau but one case,thepatient's plasmid profiles remained unchanged. We conclude thatthediversity andstability of MRSA plasmid typesmakethemexcellent epidemiological markers. Insupport ofthis conclusion, we found that our dataprovided significant epidemiological insights. Twoepidemic strains, accounting formore than half oftheinfections, were identified inthefive hospitals. Theremaining caseswere sporadic, caused byMRSA strains thatappeared veryinfrequently andthat may haveoriginated fromsourcesoutside thehospitals. Therehasbeena steady increase intheincidence of nosocomial infections caused bymethicillin-resi stant