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The  mef (A) gene was originally identified as the resistance determinant responsible for type M resistance to macrolides, a phenotype frequently found in clinical isolates of  Streptococcus pneumoniae  and  Streptococcus pyogenes . MefA was defined as a secondary transporter of the major facilitator superfamily driven by proton-motive force. However, when characterizing the  mef (A)-carrying elements Tn 1207.1  and Φ1207.3, another macrolide resistance gene,  msr (D), was found adjacent to  mef (A). To define the respective contribution of  mef (A) and  msr (D) to macrolide resistance, three isogenic deletion mutants were constructed by transformation of a  S. pneumoniae  strain carrying Φ1207.3: (i) Δ mef (A)–Δ msr (D); (ii) Δ mef (A)– msr (D); and (iii)  mef (A)–Δ msr (D). Susceptibility testing of mutants clearly showed that  msr (D) is required for macrolide resistance, while deletion of  mef (A) produced only a twofold reduction in the minimal inhibitory concentration (MIC) for erythromycin. The contribution of  msr (D) to macrolide resistance was also studied in  S. pyogenes , which is the original host of Φ1207.3. Two isogenic strains of  S. pyogenes  were constructed: (i) FR156, carrying Φ1207.3, and (ii) FR155, carrying Φ1207.3/Δ msr (D). FR155 was susceptible to erythromycin, whereas FR156 was resistant, with an MIC value of 8 μg/ml. Complementation experiments showed that reintroduction of the  msr (D) gene could restore macrolide resistance in Δ msr (D) mutants. Radiolabeled erythromycin was retained by strains lacking  msr (D), while  msr (D)-carrying strains showed erythromycin efflux. Deletion of  mef (A) did not affect erythromycin efflux. This data suggest that type M resistance to macrolides in streptococci is due to an efflux transport system of the ATP-binding cassette (ABC) superfamily, in which  mef (A) encodes the transmembrane channel, and  msr (D) the two ATP-binding domains.

Type M Resistance to Macrolides Is Due to a Two-Gene Efflux Transport System of the ATP-Binding Cassette (ABC) Superfamily