The antibiotic Penicillin is an enzyme inhibitor which interferes in an enzyme activity that occurs in bacteria but not in humans. This makes it useful as an antibiotic but at the same time it is safe for human consumption as it does not cause damage to our body.
The biologically active part of penicillin binds with an enzyme called DD- transpeptidase which helps link the cells in the cell-wall of bacteria. Due to this, the bacterial cell wall is weakened and it breaks up. As bacteria grow, the cell wall is constantly broken and reformed, the enzyme inhibiting action of penicillin interferes in this process and as a result the bacteria can no longer survive.
This action of penicillin has made it very useful in curing illnesses caused due to bacteria.