VARIATION AND PERSISTENCE OF METHICILLIN-RESISTANT STAPHYLOCOCCUS-AUREUS STRAINS AMONG INDIVIDUAL PATIENTS OVER EXTENDED PERIODS OF TIME

To determine the strain variation and persistence among isolates of me thicillin-resistant Staphylococcus aureus (MRSA) cultured from patient s with colonization over extended time spans, pulsed-field gel electro phoresis was used to analyze the isolates from 47 patients for whom at least two mecA-positive isolates collected a minimum of six months ap art were available. For 22 (47 %) patients, the isolates represented m ultiple distinct strains of Staphylococcus aureus, while 20 (43 %) pat ients had only a single strain detected; five (11 %) patients had simi lar, genetically related isolates. MRSA were frequently associated wit h mucocutaneous abnormalities; 29 (62 %) patients had focal cutaneous defects, and ten (21 %) had chronic dermatitis. Multiple strains of MR SA were detected more frequently than single strains among patients in whom the initial focus of MRSA resolved clinically and another mucocu taneous defect subsequently developed compared to patients with clinic ally persistent foci (11/15 versus 9/23, respectively; p = 0.05, Fishe r's exact test). Among the 21 patients in this series for whom isolate s cultured within a two-month time span were available, there were sev en (33 %) patients with multiple strains of MRSA, including one patien t with polyclonal bacteremia. In summary, patients with long-term MRSA colonization often have several different strains of MRSA, which typi cally change overtime in association with removal or resolution of a c olonized focus and the recurrence of mucocutaneous defects.