M.O.A:

Bind to trans-peptidase enzyme   inhibit binding of peptidoglycans   leading to :

1) Change in bacterial cell wall   bacterial autolysis   death of bacteria.

2) Pores in cell wall   intrance of fluids into bacterial cell by osmosis   explosion of bacterial cell   death of bacteria.

So Penicillins are bactericidals.

The previous things occur in (growing and dividing cells) only.