Group A Streptococcus pyogenes (GAS) bacteria, illustration. These bacteria, which are are Gram-positive and coccus shaped (spherical), are commonly found on the skin and in the throat. GAS can cause sore throat (strep throat) and soft tissue infections such as impetigo and cellulitis. Occasionally GAS can be a more serious infection known as invasive Group A Streptococcus (iGAS), which can cause bacteremia, septic arthritis and meningitis. GAS can be treated with antibiotics such as penicillin or erythromycin.