During normal bacterial growth, bacterial enzymes called autolysins put breaks in the peptidoglycan in order to allow for insertion of peptidoglycan building blocks (monomers of NAG-NAM-peptide). As new monomers are linked to the existing rows of peptidoglycan during cell wall synthesis, transpeptidase enzymes form a peptide bridge that cross-links the peptides coming off of each NAM. These links connect each row of sugars with its adjacent rows and each layer of peptidoglycan with each other layer. This is what gives peptidoglycan its strength. Penicillins and cephalosporins bind to the transpeptidase enzyme and block the formation of the peptide cross-links. This results in a weak cell wall and osmotic lysis of the bacterium.