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Release of SpeA from Streptococcus pyogenes after exposure to penicillin: Dependency on dose and inhibition by clindamycin

The amount and time course of SpeA release from group A streptococci (GAS) was studied at different starting inoculae after exposure to different doses of penicillin, clindamycin or a combination of the 2. The release was related to the bacterial concentration and killing rate. A clinical GAS strain was exposed to benzylpenicillin, 2 and 1000×MIC, clindamycin, 2 and 32×MIC, or combinations of the 2. Samples for viable counts and SpeA analyses were drawn before and after the addition of antibiotics and at 3, 6 and 24h. The SpeA release was higher at low than at high concentrations of penicillin and the combination (both, p<0.05). The addition of clindamycin to penicillin reduced SpeA production at both concentrations (p<0.01). Most SpeA was released before 3 h, and for penicillin and the combination, the amount correlated to the number of killed bacteria during this period (r=0.50; p<0.05). A positive correlation was found between the inoculum size and the SpeA concentration at time zero (r=0.54; p<0.05). The SpeA concentration was dependent on the initial number of bacteria, the class of antibiotic, the dose of penicillin and the killing rate.