COMPARISON OF MIDI SHERLOCK SYSTEM AND PULSED-FIELD GEL-ELECTROPHORESIS IN CHARACTERIZING STRAINS OF METHICILLIN-RESISTANT STAPHYLOCOCCUS-AUREUS FROM A RECENT HOSPITAL OUTBREAK

An outbreak of methicillin-resistant Staphylococcus aureus infections at the University of Utah Health Sciences Center occurred over a 7-mon th period, While the isolates phenotypically appeared to be similar in gross morphology and have similar Vitek antibiotic susceptibility pat terns, two additional methods of strain characterization were evaluate d to enhance the epidemiological investigation: pulsed-field gel elect rophoresis and gas chromatography with the MIDI Sherlock system. Sherl ock uses gas chromatography to qualitatively and quantitatively analyz e the cellular fatty acid composition of organisms and creates two-dim ensional plots based on principal-component analysis to define groups of closely related organisms. All isolates were also evaluated by dige sting their chromosomal DNAs with the low-frequency-cutting enzyme Sma I and separating the restriction fragments by contour clamped homogene ous electric held gel electrophoresis, Sample preparation for this pul sed-field gel electrophoresis included a novel cell lysis procedure in volving achromopeptidase, greatly reducing the turnaround time, Isolat es tested were recovered from the following: 45 suspected outbreak pat ients, 6 hospitalized patients believed to be unrelated to the outbrea k, 6 patients from outside the hospital, and one health care, practiti oner implicated in the outbreak Of 45 phenotypically similar suspect s trains, 43 clustered tightly on the Sherlock two-dimensional plot, All outbreak patient isolates were also identical by pulsed-field gel ele ctrophoresis with the exception of the same two outliers identified by Sherlock In this epidemiologic investigation, we found an excellent c orrelation between the Sherlock and pulsed-field gel electrophoresis r esults for strain characterization of methicillin-resistant S. aureus.