* diffuses through cell wall of gram-positive bacteria → can enter some gram-negative bacteria through porins (surface molecules)
* penicillin binds to DD-transpeptidase
* bacteria continually make and break down parts of their cell walls
* enzyme DD-transpeptidase catalyses cross-link formation between amino acid side chains and peptidoglycan molecules
* breakdown by hydrolysis continues → more cell wall lost than gained
* cell wall weakened so when water enters by osmosis, increased pressure potential causes cell lysis