FIG. 8.

Phylogenetic tree of MRSA strains. It shows the divergence of modern clinical strains from the original MRSA strain, which is shown on the top of the figure. It can be seen that from this original strain at least eight major strains (black circles) evolved and diverged but retained this MRSA character. The implication is that mecA arose only once, but variations on it arose much more often, probably in the current antibiotic era. (Reprinted from reference 14 with permission from the publisher.)