Flucloxacillin is active against Gram-positive organisms including penicillinase producing strains. It has little activity against Gram-negative bacilli. Flucloxacillin acts by inhibiting the formation of cell wall of bacteria. Flucloxacillin is isoxazolyl penicillin which combined the properties of resistance to hydrolysis by penicillinase, gastric acid stability and activity against gram-positive bacteria. Flucloxacillin is a bactericidal antibiotic that is particularly useful against penicillinase-producing staphylococci. Flucloxacillin kills bacterial cellwall, thus interfering with peptidoglycan synthesis. Peptidoglycan is a heteropolymeric structure that provides the cell wall with its mechanical stability. The final stage of peptidoglycan synthesis involves the completion of the cross-linking with the terminal glycine residue of the pentaglycin bridge linking to the fourth residue of the pentapeptide (D-alanine). The transpeptidase enzyme that performs this step is inhibited by Flucloxacillin. As a result the bacterial cellwall is weakened, the cell swells and then ruptures. Flucloxacillin resists the action of bacterial penicillinase probably because of the steric hindrance induced by the acyl side chain which prevents the opening of the _- lactam ring.