The group of antibiotics known as the β-lactams include penicillins, cephalosporins, monobactams and the carbapenems. 

These all share a common β-lactam ring, shown in blue.  The ring is very strained and the bond between the carbonyl and the nitrogen in the β-lactam ring is very labile and hence makes the molecule reactive.  The R-group substituent of the penicillin nucleus can be changed to give the molecule different antibacterial properties.  The two naturally occurring penicillins from Penicillium notatum are Penicillin G,  [Benzyl penicillin, R = C6H6] and  Penicillin V, [Phenoxymethyl penicillin, R = CH2O(C6H6)].2

Want to know how to modify the R-group?

The β-lactam structure is derived from two covalently bonded amino acid residues; cysteine and valine.  This forms via a tripeptide intermediate where the third amino acid is replaced by the variable R-group.3