Staphylococcal Cassette Chromosome mec (SCCmec) Natural Excision Frequencies and Its Contributing Factors in Variant SCCmec Type Prototypic Strains.

BACKGROUND

Staphylococcus aureus acquires methicillin resistance genes through the SCCmec element. Although spontaneous SCCmec excision has been observed, its frequency, type-specific variation, and responsiveness to environmental conditions remain undefined. Here, we systematically quantified SCCmec excision across diverse prototypic types/subtypes and evaluated the factors that contribute to excision variability.

METHODS

Twenty five prototypic MRSA strains (SCCmec types I-VIII, XI-XIII and defined subtypes) were examined under standard growth temperature (37 °C), elevated temperature (42 °C), desiccation, prolonged continuous culture (30 days), and sub-lethal oxacillin pressure. Excision frequencies were quantified using qPCR, normalized to the gyrB housekeeping gene using the formula: 10-((Ct,orfX-Ct,gyrB)/3.32). Statistical analyses included one-way ANOVA, t-tests, and OLS regression for time-dependent trends.

RESULTS

At 37 °C, excision frequencies ranged from 2.40 × 10-6 to 1.32 × 10-3 and varied among representative SCCmec types/subtypes but were unrelated to SCCmec size (R2 = 0.027, p = 0.44). Type I showed no detectable excision due to a truncated ccrB gene. At 42 °C, excision increased in 14 of 24 types (median +11.2%; eight significant) and decreased in 10 (median -7.4%; four significant). Desiccation produced similar effects, with nine types increasing (median +7.1%; four significant), 14 decreasing (median -8.2%; five significant), and one unchanged. Continuous culture exhibited progressive increases in excision across multiple types (R2 = 0.3-0.94), whereas sub-lethal oxacillin uniformly maintained low detectable excision frequencies across all SCCmec types.

CONCLUSIONS

Excision varied among representative SCCmec types and was influenced heterogeneously by distinct stress conditions. Continuous culture promoted excision, whereas oxacillin exposure maintained low detectable excision. This work quantitatively confirms spontaneous SCCmec excision and provides new insights into MRSA genome plasticity.