Fitness cost of SCCmec and methicillin resistance levels in Staphylococcus aureus

Abstract

Transformation of a type I SCCmec element into Staphylococcus aureus yielded highly oxacillin-resistant transformants with a reduced growth rate. Faster-growing variants could again be selected at the cost of reduced resistance levels, demonstrating an inverse correlation between oxacillin resistance levels and growth rate.

FIG. 1.

SmaI restriction patterns of the strains used and their corresponding growth curves. The SmaI fragments carrying SCCmec are indicated by filled triangles, and the corresponding fragments of the cured strain are indicated by open triangles. BB255, naïve, susceptible recipient; RA120, BB255 transformant containing SCCmec; ME23, RA120 cured of SCCmec; ME51, rapidly growing strain derived from RA120; ME57, ME51 cured of SCCmec.

FIG. 2.

Growth competition. Survival of oxacillin-resistant strains versus that of oxacillin-susceptible strains in mixed culture after daily subculturing. (a) Strain RA120 with its cured derivative ME23 inoculated at a ratio of 1:1; (b) strain RA120 with ME23 inoculated at a ratio 100:1; (c) strain RA120 with BB255 inoculated at a ratio of 100:1; (d) strain ME51 with ME23 inoculated at a ratio 1:1.

FIG. 3.

Population analysis profiles. The numbers of colonies that formed after 48 h in the presence of different concentrations of oxacillin after overnight cultures of strain RA120 or ME51 were plated on increasing concentrations of oxacillin are indicated.