ABC of Anti-microbials: Flucloxacillin

ANTIMICROBIAL: Flucloxacillin  (also called Floxacillin)

CLASS: Penicillin (More specifically a beta-lactamase resistant narrow spectrum penicillin)

SOME OTHER ANTIMICROBIALS IN THE SAME CLASS: There are lots of penicillins, but the ones most similar to flucloxacillin are cloxacillin and oxacillin. The flucloxacillin wikipaedia page has a great table at the bottom with a succinct classification of all the beta-lactam antibiotics. Click here to access.

ORIGINS: Developed by Beecham as a semi-synthetic penicillin around 1970. I am not sure of the exact date of commercial release.

MECHANISM OF ACTION: It inhibits synthesis of bacterial cell walls by binding to penicillin binding proteins (PBPs) blocking cross linkage of peptidoglycan chains. The presence of the isoxazolyl side chain hinders binding of beta-lactamases (penicillinase) thus gives it activity against penicillinase producing strains of Staphyloccus aureus. (the majority).

ROUTE OF ADMINISTRATION: Oral, intra-venous.

MAIN CLINICAL USES: Soft tissue infections. Invasive infections caused by Staphylococcus aureus eg Infective endocarditis, osteomyelitis, septic arthritis, discitis, epidural abscess etc. it has no effect against Gram negative organisms

MAIN SIDE EFFECTS OF NOTE: Allergic rashes in those with a penicillin allergy. Cholestatic hepatitis is an occasional complication of flucloxacillin treatment, presenting as deranged liver function tests, or jaundice. Flucloxacillin probably has more cholestatic but less renal side effects than its sister drug, cloxacillin.

RESISTANCE: Methicillin Resistant Stapylococcus aureus (MRSA) are resistant to flucloxacillin due to the production of an altered PBP2a encoded by the mecA gene.

OTHER POINTS OF NOTE: Although flucloxacillin has less activity against Streptococcus pyogenes than benzylpenicillin, its serum level still far exceeds the MIC for Streptococcus pyogenes. thus (in the vast majority of cases) it is reasonable to use flucloxacillin to treat infections caused by both Staphylococcus aureus and Streptococcus pyogenes.

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