This document discusses different classes of antibiotics, their sources, mechanisms of action, and examples. It focuses on penicillins, providing details on: 1) Penicillin G is the first antibiotic used clinically in 1941 and is obtained from fungus Penicillium notatum. 2) Penicillins work by inhibiting cell wall synthesis in bacteria. 3) Semisynthetic penicillins were developed to address limitations of Penicillin G like stability, spectrum and resistance. These include methicillin, amoxicillin, and piperacillin. 4) Beta-lactamase inhibitors like clavulanic acid are used to overcome bacterial resistance to penicillins from beta-lactam