Use of denaturing gradient gel electrophoresis to detect mutation in VS2 of the 16S-23S rDNA spacer amplified from Staphylococcus aureus isolates.

To develop a double gradient denaturing gradient gel electrophoresis (DG-DGGE) based typing method that rapidly and accurately types clinical isolates of Staphylococcus aureus, the VS2 region of the 16S-23S rRNA spacer region (ISR) was chosen because of its potential high variation. The VS2 region was amplified with a 40-mer GC-clamp attached to the 5'-end of the reverse primer. The 145 bp PCR product was then separated by DG-DGGE using denaturant concentrations of 25-40% and polyacrylamide concentrations of 6-12%. Of the five mutations identified in 336 S. aureus isolates, one mutation was found to be highly specific for 161/171 (94%) of methicillin-resistant S. aureus (MRSA) isolates from different geographic locations and isolation times. This same mutation was found in 15/160 (9%) of penicillin- or methicillin-sensitive S. aureus isolates. In some isolates two mutations occured together in the one genome suggesting some S. aureus isolates have two copies of VS2. In these 336 isolates nine genotypes with different combinations of the five mutations were identified. In 18 coagulase-negative staphylococci (CNS), the MRSA-specific mutation was found along with two other mutations in all isolates demonstrating consistent differences in the presence of these mutations between CNS and S. aureus. The marked differences in VS2 sequences found between MRSA, methicillin- or penicillin-sensitive S. aureus (SSA), and CNS by DGGE in the present study may be useful in evolutionary studies and in the development of a specific assay for MRSA from clinical specimens.