Evolutionary Models of the Emergence of Methicillin-Resistant Staphylococcus aureus

Proposed evolutionary models for the emergence of MRSA in CC5 and CC8 (left panel) and CC22, CC30, and CC45 (right panel). The models were based on MP analyses of the combined sas and housekeeping genes. The SCCmec type and the spa type were added to the models by assuming the fewest additional changes to the trees. Only those branches on which MRSA isolates have emerged are shown. Large circles represent ancestral clones. Smaller circles represent descendant clones. Arrows indicate the directions and relative amounts of change between isolates. Names and countries of isolation are given for all clones. The numbers in parentheses after the country names represent the numbers of clones. Full genotypes are given for the ancestors with their housekeeping gene alleles at arcC-aroE-glpF-gmk-pta-tpi-yqiL (top line), sas gene alleles at sasA-sasB-sasD-sasE-sasF-sasH-sasI (middle line), and spa repeats (bottom line). Previously named clones are indicated in black boxes. Dotted lines represent alternative evolutionary hypotheses. Isolates from the 1950s and 1960s are indicated by asterisks next to the country of isolation; all other isolates are from the 1980s and 1990s. The country abbreviations are as follows: Aus, Australia; Bel, Belgium; Cub, Cuba; Den, Denmark; Fin, Finland; Fra, France; Ger, Germany; Ire, Ireland; Jap, Japan; Net, Netherlands; Pol, Poland; Por, Portugal; Slo, Slovenia; Swe, Sweden; UK, United Kingdom; USA, United States.