Molecular surveillance of methicillin-resistant Staphylococcus aureus by multiple-locus variable number tandem repeat fingerprinting (formerly multiple-locus variable number tandem repeat analysis) and spa typing in a hierarchic approach.

In this study, clonal relatedness of 202 Staphylococcus aureus (mostly methicillin-resistant) isolates recovered in 29 Polish hospitals was investigated by multiple-locus variable number tandem repeat fingerprinting (MLVF) and spa typing. Our analysis yielded 69 MLVF patterns and 34 spa types. Almost all isolates (97.4%) identical by MLVF were also indistinguishable by spa typing. Therefore, the MLVF method can be a cheap and fast screen before spa typing. Moreover, results obtained by MLVF suggest a set of simple criteria for grouping of spa types. The proposed algorithm groups isolates into the same cluster when spa sequences differ by a single mutation event: i) a single deletion or insertion of repeat unit(s) at the X region of the protein A gene or ii) a single nucleotide polymorphism within a repeat sequence. The combined use of these 2 methods, MLVF in local laboratories and spa typing of selected isolates in reference centers, can improve the monitoring of hospital-to-hospital strain transmission events and public health interventions on a huge scale.