Penicillin attacks bacterial cells by inactivating an enzyme that is essential for bacterial growth.  The enzyme is peptidoglycan transpeptidase and it catalyses the cross-linking of the peptidoglycan, which forms the cell wall of the bacteria.

The peptidoglycan transpeptidase enzyme is not needed in animals as their cells do not have cell walls. Therefore, the penicillin can safely disrupt the bacterial cell wall biosynthesis without harming existing cells in the body.  The penicillin stops the growth of the bacterial cell wall, causing the pressure inside the cell to rise considerably until the cell lyses and thus the cell is destroyed.4 

Penicillin G and V are only active against Gram Positive bacterial cells, which have an exposed layer of peptidoglycan around the outside of the cell wall, as shown above.  Gram Negative bacteria have a more complicated composition, which Penicillin G and V can not destroy, although there are other antibiotics that can.

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