Elevated activity and expression levels of group A Streptococcus (GAS) M1T1 5448 strain (WT) virulence factors in the presence of subinhibitory clindamycin (CLI) concentrations. Mid-logarithmic growth-phase supernatants of GAS WT, animal-passaged (AP) GAS, and GAS ΔspeB grown in the presence (CLI) or absence (control) of subinhibitory CLI concentrations (0.0075 μg/mL) were tested for streptolysin O (SLO), extracellular DNase Sda1, and Streptococcus pyogenes cell envelope protease (SpyCEP) activity. Stationary growth–phase supernatants were used for Streptococcal pyrogenic exotoxin B (SpeB) activity quantification. A, SLO activity. B, DNase activity. One representative gel of 3 independent experiments is shown. C, SpyCEP activity. D, SpeB activity. A, C, and D, Data show pooled results from 3 independent experiments and are presented as mean ± standard error of the mean. *P < .05, ***P < .001, and ****P < .0001, by the unpaired t test. Abbreviations: MW, molecular weight; NS, not significant; OD, optical density.