Abstract
Previous surveillance studies carried out by our laboratories, primarily in Southern and Eastern Europe, Latin America, and the United States, have characterized 3,067 methicillin-resistant
Staphylococcus
aureus
(MRSA) hospital isolates by a combination of molecular typing methods. Nearly 70% of these isolates could be classified into five clonal types showing extensive geographic spread. Representative
isolates of these clonal types were now reexamined for their genetic relatedness by multilocus sequence typing (MLST) and by sequencing the polymorphic region of protein A (
spaA
typing), and also
for the type of the Staphylococcal Chromosomal Cassette (SCC
mec
) resident in the bacteria. Three of the previously classified clonal types (Iberian, Brazilian, and Hungarian clones) shared a common
or closely related genetic background A, which was the same as the background of the earliest European isolates of MRSA from England and Denmark. The Pediatric and New York/Japan clones belonged to a completely
different genetic background B. The three recently described SCC
mec
types were specifically associated with different pandemic clones: types I and III with isolates of genetic background A and type
II with isolates of genetic background B. A novel SCC
mec
related to type I, called SCC
mec
type IV, was identified in some MRSA strains belonging to genetic background A as well as B. Structural
variations in SCC
mec
types I and III were also observed. The data allow tentative identification of an evolutionary pathway for the emergence of pandemic MRSA clones and also provide evidence for
the multiple, yet restricted, numbers of acquisition of the
mec
element by
S. aureus
.