Objectives: Recent population genetic studies suggest that the Staphylococcal
Chromosome Cassettes mec (SCCmec) was acquired at a global
scale much more frequently than previously thought. We hypothesized
that such acquisitions can also be observed at a local level. In the present
study, we aimed at investigating the diversity of SCCmec in a local
MRSA population, where the dissemination of four MRSA clones has
been observed (JCM 2007, 45: 3729).
Methods: All the MRSA isolates (one per patient) recovered in the
Vaud canton of Switzerland from January 2005 to December 2008 were
analyzed in this study. We used the Double Locus Sequence Typing
(DLST) method, based on clfB and spa loci, and the e-BURST algorithm
to group the types with one allele in common (i.e. clone). To increase the
discriminatory power of the DLST method, a third polymorphic marker
(clfA) was further analyzed on a sub-sample of isolates. The SCCmec
type of each isolate was determined with the first two PCRs of the Kondo
scheme.
Results: DLST analysis indicated that 1884/2036 isolates (92.5%)
belong to the four predominant clones. A majority of isolates in each
clone harboured an identical SCCmec type: 61/64 (95%) isolates to
DLST clone 1−1 SCCmec IV, 1282/1323 (97%) to clone 2−2 SCCmec
II, 237/288 (82%) to clone 3−3 SCCmec IV, and 192/209 (92%) to clone
4−4 SCCmec I.
Unexpectedly, different SCCmec types were present in a single
predominant DLST clone: SCCmec V plus one unusual type in 3 isolates
of clone 1−1; SCCmec I, IV, V, VI plus two unusual types in 41 isolates
of clone 2−2; SCCmec I, II, VI plus three unusual types in 51 isolates of
clone 3−3; and SCCmec II, IV, V plus one unusual type in 17 isolates of
clone 4−4. Interestingly, adding a third locus generally did not change
the classification of incongruent SCCmec types, suggesting that these
SCCmec elements have been acquired locally during the dissemination
of the clones.
Conclusion: Although the SCCmec diversity within clones was
relatively low at a local level, a significant proportion of isolates with
different SCCmec have been identified in the four major clones. This
suggests that the local acquisition of SCCmec elements is not a rare
event and illustrates the great capacity of S. aureus to quickly adapt to
its environment by acquiring new genetic elements.