beta-lactam antibiotic penicillin beta-lactam ARO:3000008 penam antibiotic_resistance Penicilins (Penams) are a group of antibiotics derived from Penicillium fungi that share a skeleton beta-lactam moiety fused with a thiazolidine ring. Penicillin-like antibiotics are historically significant because they are the first drugs that were effective against many previously serious diseases such as syphilis and Staphylococcus infections. Penicillins are still widely used today, though many types of bacteria are now resistant. All penicillins are beta-lactam antibiotics in the penam sub-group, and are used in the treatment of bacterial infections caused by susceptible, usually Gram-positive, organisms.