Staphylococcal Chromosomal Cassette mec Type I, spa Type, and Expression of Pls Are Determinants of Reduced Cellular Invasiveness of Methicillin-Resistant Staphylococcus aureus Isolates

We have shown previously that pls, which codes for the surface protein Pls of methicillin-resistant Staphylococcus aureus (MRSA), reduces adhesion to immobilized fibronectin, fibrinogen, laminin, and immunoglobulin G as well as invasion of host cells. Here, we tested a collection of 66 clinical MRSA isolates—48 negative for pls/Pls (pls-/Pls-), 15 positive for pls/Pls (pls+/Pls+), and 3 harboring the pls gene but not expressing Pls (pls+/Pls-)—for cellular invasiveness. Invasion of 293 cells by pls+/Pls+ strains was lower than that by the pls-/Pls- strains (median [range], 36% [22%–70%] vs. 93% [25%–162%]). The 3 pls+/Pls- strains (median [range], 95% [63%–103%]) were as invasive as the pls-/Pls- strains. In addition, we identified a pls+/Pls+ staphylococcal chromosomal cassette mec (SCCmec) IV strain. In conclusion, 3 properties—pls/Pls, SCCmec type, and spa type—strongly predicted the cellular invasiveness of MRSA strains, as indicated by good clustering. In contrast, the spa type–deduced multilocus sequence typing clonal complex (MLST-CC) was not able to predict the invasiveness of MRSA strains equally well. The underlying mechanism remains to be elucidated.