Denaturing gradient gel electrophoretic multilocus sequence typing of Staphylococcus aureus isolates

To obviate the need for multilocus sequencing, a method using denaturing gradient gel electrophoresis (DGGE) was developed for the multilocus sequence typing (MLST) of Staphylococcus aureus isolates. Sequence types (STs) were obtained on the basis of sequences of polymerase chain reaction (PCR) products from seven housekeeping genes and compared to the reference MLST database. The melt curves, sequences and DGGE profiles were compared for 100 STs (i) to determine PCR conditions with 40-mer GC-clamps attached to the forward and reverse primers; (ii) to choose single restriction enzyme sites for digestion of PCR products into two fragments each with a GC-clamp attached and (iii) to optimize DGGE conditions. When the DGGE types (DT) were analyzed, the majority of DTs (76/100) were accurately classified into one ST (95% of nucleotide changes were detected), 10 DTs were classified into one of two STs corresponding to a single nucleotide ambiguity and 14 DTs were classified into 3 or 4 STs corresponding to 4 or 5 nucleotide ambiguities. A combination of STs and DTs were used to obtain septuplet sets of STs (7-ST) for 25 S. aureus isolates. When compared to the reference MLST database, one methicillin-resistant S. aureus (MRSA) isolate had the same genotype as the first MRSA clone. The DGGE-MLST method can be used as a rapid, accurate and 20-fold less expensive method than DNA sequencing for the detection of all sequence types. This combined laboratory and in silico approach could have wide applicability not only to MLST methods for other bacteria but to the screening of multilocus nucleotide differences deposited in other mutation databases.