Penicillin G is a naturally occurring beta lactam antibiotic produced by the Penicillium fungus. It is manipulated and used as the basis for many other important semi-synthetic compounds. Like other beta lactams, it works by inhibiting cell wall production in non-resistant cells by breaking the links between peptidogylocans and causing the production of cell wall damaging autolysins. By binding to specific penicillin-binding proteins (PBPs) located inside the bacterial cell wall, penicillin G inhibits the third and last stage of bacterial cell wall synthesis. Cell lysis is then mediated by bacterial cell wall autolytic enzymes such as autolysins; it is possible that penicillin G interferes with an autolysin
inhibitor.