IN-VIVO STABILITY AND DISCRIMINATORY POWER OF METHICILLIN-RESISTANT STAPHYLOCOCCUS-AUREUS TYPING BY RESTRICTION-ENDONUCLEASE ANALYSIS OF PLASMID DNA COMPARED WITH THOSE OF OTHER MOLECULAR METHODS

We evaluated test discriminatory power and DNA type alterations among methicillin-resistant Staphlococcus aureus strains by testing 199 sequ ential isolates from 39 patients collected over 30 to 228 days, Isolat es were typed by one or three different methods (restriction endonucle ase analysis of plasmid DNA [REAP] with or without pulsed-held gel ele ctrophoresis of genomic DNA [PFGE] and immunoblotting [IB]), REAP was highly discriminatory compared with PFGE and IB. However, the initial isolates from 4 of the 39 patients lacked detectable plasmid DNA and c ould not be typed by REAP. Typing of individual patient isolates showe d that a different REAP type was identified only once every 138 days. Among 25 comparisons, seven sequential isolate pairs demonstrating REA P differences were also different by PFGE and IB, This likely represen ted the presence of more than one strain. Eighteen other pairs with RE AP differences were identical or related to one another by PFGE and IB typing, and 17 of these differences were likely caused by a single ge netic alteration within the same strain or clone. The rate of PFGE dif ferences explicable by single genetic alterations among sequential iso lates identical by REAP was similar to the overall rate for REAP diffe rences in the whole collection. We conclude that REAP and PFGE typing differences explicable by single genetic alterations are relatively in frequent but not rare. These isolates should be examined by alternativ e typing systems to further support or refute clonality.