Background: Streptococcus pyogenes, also known as group A streptococcus (GAS) or “the flesh-eating bacteria”, is a Gram positive human pathogen that causes many diseases, ranging in severity from milder infections such as pharyngitis to life-threatening necrotizing fasciitis/myonecrosis and streptococcal toxic shock syndrome (1). Although GAS remains susceptible to penicillin, this beta-lactam antibiotic (a cell wall-synthesis inhibitor) is not efficacious in treating severe GAS myonecrosis, whereas treatment with clindamycin (a lincosamide that inhibits protein synthesis) is 80-100% protective. We have previously demonstrated that sub-inhibitory concentrations of the beta-lactam antibiotic, nafcillin, increases and prolongs extracellular protein production in Staphylococcus aureus, another Gram positive pathogen (2). The present study seeks to examine the effects of sub-inhibitory concentrations of the antibiotics clindamycin, nafcillin, and penicillin on the expression of toxin genes and toxin gene regulators in S. pyogenes. Methods:GAS strain 88-003 (M type 3) was isolated from a fatal case of necrotizing fasciitis and Strep TSS. The minimum inhibitory concentrations (MICs) of clindamycin, nafcillin, and penicillin were established through standard microbroth dilution assay. To determine the effects of antibiotics on gene expression, bacteria were cultured in the presence of sub-inhibitory concentrations of antibiotics (i.e., 0.5 x MIC) until the late-log phase of growth (4 h). Expression of toxin genes and global gene regulators were examined by northern analysis and reverse transcriptase (RT)-PCR. Results: Clindamycin increased the expression of the genes coding for two exotoxins, namely streptolysin O (SLO,slo) and nicotine adenine dinucleotidase (NADase; nga). It also increased expression of two global toxin gene regulators, csrR and mga. By contrast, 0.5 x MIC of nafcillin decreased the expression of these genes. The effects of penicillin at 0.5 x MIC were similar to that of nafcillin, except that penicillin caused a slight increase in csrR expression. Conclusions: Despite its well known ability to prevent translation of bacterial mRNA into exotoxins, clindamycin at sub-inhibitory doses augments toxin gene transcription in GAS. The effects of beta lactam antibiotics on toxin gene regulation appears to be species-, and perhaps, strain-specific.