Bacteria reproduce by dividing to produce two new cells. They enlarge to about twice
their size before the DNA chromosome is copied. The two new chromosomes move
apart and a cell wall forms between them. But if penicillin is present, the new cell
wall won't be able to form. It doesn't harm old bacterial cell walls, but it stops new
ones forming. This means the bacteria can't reproduce, so the disease can't spread.
Penicillin acts by blocking the activity of the enzyme transpeptidase, which cross
connects long polymers of sugars that form the bacterial cell wall. The β-lactam ring
on penicillin (see structure) irreversibly blocks the activity of the enzyme by
covalently bonding with the functional end of the enzyme. As a result, newly-formed
cell walls will be structurally weak in some areas, causing water to rush in and rupture
the cell.