FIG 1.

Factors required and associated with β-lactam resistance by MRSA. (A) SCCmec element. The major features of SCCmec types II and IV are shown as examples of HA-MRSA and CA-MRSA, respectively (based on reference 11). The mecA gene is contextualized within SCCmec of different sizes, by the cassette chromosome recombinase (ccr) gene complex and the joining regions (J1 to J3) that can encode a variety of other functions, including housekeeping and transposons. (B) Roles of Aux and Pot factors. There are two steps required for an MRSA strain to develop high-level resistance to β-lactams. The first is acquisition of the mecA/SCCmec cassette that is required for resistance per se. This, however, provides only the ability to grow in the presence of low concentrations of β-lactams. High-level MRSA is supported by those factors associated with resistance. The second step, leading to high-level resistance, requires a mutation in a potentiator gene (pot). Auxiliary factors (aux) are required to support resistance.