Penicillin was the first antibiotic to be discovered. Is is an irreversible inhibitor. It permanently inactivates the key enzymes in bacterial cell walls. This is acheived by covalently bonding with a serine residue in glycopeptide transpetidase. If bacteria can no longer sythesise cell walls, they are prevented from growing and dividing, therefore killing the infection. Our cells do not have a cell wall so we are not harmed by Penicillin .
It was in 1928 that Alexander Flemming first noted the effect of penicillium on the growth of staphyloccoci; yet it was not until 1941 that it was first used for treatment against bacterial infection.