Why is penicillin toxic to a bacterium, but safe for a human cell?

Answer 1

Penicillin is a Beta-Lactam Antibiotic (Like the Cephalosporins), as Such it Binds PBPs (Penicillin Binding Proteins), and Therefore Interferes With Bacterial Replication, Specifically, the Cell Wall, Peptidoglycans. They Are Not Present In Eukaryotic Cells.

Answer 2

Bacteria depend upon a rigid cell wall to maintain their cell membrane. If the cell wall is disrupted or removed the bacterial cell absorbs too much water and will burst. Penicillin specifically interferes with the bacterium's ability to synthesize cell wall. Human cells do not need a cell wall and are not normally affected by penicillins.