Evaluation of single and double-locus real-time PCR assays for methicillin-resistant Staphylococcus aureus (MRSA) surveillance

Parallel evaluation of several published single and double-locus rt-PCR MRSA assays of 150 pure culture strains, followed by analysis of 460 swab-derived clinical samples which included standard identification, susceptibility testing, followed by PCR detection of staphylococcal suspected isolates and in-PCR mixed bacterial populations analysis indicated the following findings.Pure cultures analysis indicated that one of the single-locus assay had very high prevalence of false positives (Positive predictive value = 77.8%) and was excluded from further analysis. Analysis of 460 swab-derived samples indicated that the second single-locus assay misidentified 16 out of 219 MRSA's and 13 out of 90 methicillin-sensitive S. aureus's (MSSA) were misidentified as MRSA's. The double-locus detection assay misidentified 55 out of 90 MSSA's. 46 MSSA containing samples were misidentified as MRSA and 9 as other than S. aureus ending with low positive predicted value (<85%) and very low specificity (<62%).The results indicate that high prevalence of false-positive and false-negative reactions occurs in such assays.Staphylococci are ubiquitous colonizers of the skin and mucous membranes. Among them, Staphylococcus aureus is the most pathogenic species and a leading cause of several life-threatening diseases [1]. S. aureus represents a major public health threat, as MRSA strains are the leading cause of nosocomially acquired infections. MRSA infections represent a challenge to both infection control and treatment strategies, resulting in increased morbidity, mortality, length of hospitalization and health care costs [2-4]. Conventional MRSA screening takes up to three days, during which patients and staff carriers can spread MRSA. Development of rapid accurate detection techniques should contribute to the prevention of transmission. rt-PCR is the fastest method fulfilling this task.All currently published or commercially available rt-PCR MRSA detection assays rely on single or double-