Background. Previously, we isolated an MSSA from a patient followed a few weeks later by the isolation of an MRSA. PFGE and southern hybridization demonstrated that the fingerprints were identical except for the fragment carrying SCCmec. Also the presumptive donor of the SCCmec was isolated from the same patient. Both isolates belong to ST30.
The aim of this study was to sequence the whole genomes of both the MSSA and MRSA isolate and compare the genomes [Wielders et al, Lancet 2001, 358:424].
Methods. The whole genome of the MSSA and MRSA was sequenced using a Genome Sequencer FLX system with 454 sequencing technology (Roche, Germany) at KeyGene (The Netherlands). The sequences were also compared with that of MRSA252, an EMRSA-16 which belongs to ST36 and CC30.
Results. A total of 45 and 136 contigs >1 kb were obtained with a total genome coverage of 23.5 and 9.4 times for the MSSA and MRSA, respectively. The size of both genomes is 2.8 Mb compared to 2.9 Mb for MRSA252. The order of contigs has been established, but intervening sequences still need to be determined. Mapping of predicted coding sequences showed that the only major difference between the MSSA and MRSA is SCCmec (20 kb, type IV). Approximately 40 minor differences, such as length of coding regions, were observed. The majority can be easily explained by the fact that these are located at edges of the contigs. Additional sequencing will be required to verify these differences.
The sequences of both isolates are similar that of MRSA252. The phages present in both strains closely resemble each other indicating that these may not be exchanged easily. But differences in νSaα (encoding mainly exotoxins and putative lipoproteins) and νSaß (encoding a serine protease cluster and several enterotoxins) are observed. Both elements appear to lack the modification enzyme of the restriction/modification system compared to MRSA252. In addition, both the MSSA and MRSA lacked Tn554 compared to MRSA252. However, the MSSA and MRSA contained a SaPI2-like element containing the tst-1 gene encoding toxic shock syndrome toxin, which is usually absent in ST30 isolates. In addition, the MSSA and MRSA encode SdrD which is probably involved in adhesion.
Conclusion. The genomes of the MSSA and its presumed progeny MRSA are nearly completely identical except for the SCCmec. The isolates are largely similar to MRSA252, but some additional genes are present in the MSSA and MRSA such as tst-1.