Antibiotic Pharmacology and Resistance Mechanisms (incl. Biofilm)

These are the toxicities that change orthopaedic practice and require monitoring:

• Fluoroquinolones: tendinopathy and tendon rupture (classically the Achilles) - highly relevant to the orthopaedic patient - plus QT prolongation, dysglycaemia, C. difficile, and a cartilage/arthropathy caution in children (relative contraindication in the growing patient).
• Aminoglycosides (gentamicin): dose-related nephrotoxicity and ototoxicity (vestibular and cochlear) → therapeutic drug monitoring; relevant to systemic use and to high local cement/bead loads.
• Vancomycin (glycopeptide): nephrotoxicity and the rate-related "red man"/infusion reaction (not a true allergy) → level/AUC monitoring.
• Rifampicin: a potent hepatic enzyme inducer (drug interactions - reduces warfarin, oral contraceptives and many drugs), hepatotoxicity, and orange discoloration of secretions; and rapid resistance if used alone.
• Linezolid: with prolonged use, myelosuppression (thrombocytopenia), peripheral/optic neuropathy, and serotonin syndrome (it is a weak MAO inhibitor).
• Clindamycin: the classic association with C. difficile colitis.
• Beta-lactams: allergy/anaphylaxis (and a low penicillin-cephalosporin cross-reactivity).

The surgeon-facing high-yield ones are fluoroquinolone tendinopathy and aminoglycoside/vancomycin nephro-ototoxicity needing level monitoring.

• The PK/PD indices: concentration-dependent killing is optimised by a high peak relative to the MIC (Cmax/MIC) and the AUC/MIC ratio - so aminoglycosides are given as a high once-daily dose (also exploiting the post-antibiotic effect), and fluoroquinolones are dosed to AUC/MIC. Time-dependent killing depends on the time the concentration stays above the MIC - so beta-lactams benefit from more frequent dosing or extended/continuous infusions, and vancomycin is dosed to an AUC/MIC target.

• The definitions: the MIC (minimum inhibitory concentration) and MBC (minimum bactericidal concentration) underpin "bactericidal versus bacteriostatic".

• Bone penetration (orthopaedically critical): fluoroquinolones, clindamycin, rifampicin, linezolid and fusidic acid penetrate bone well (useful oral options for osteomyelitis/PJI and oral switch), whereas beta-lactams and especially glycopeptides (vancomycin) penetrate bone relatively poorly - which informs agent and route choice and the appeal of bone-penetrating oral agents.

• Oral bioavailability - why an oral switch is even possible. Several of the bone-penetrating agents are almost completely absorbed: fluoroquinolones, linezolid, metronidazole and fusidic acid approach near-complete oral bioavailability, and clindamycin and rifampicin are well absorbed too. For these drugs the oral route achieves serum concentrations close to intravenous, so an oral regimen is a pharmacological equivalent rather than a compromise. By contrast vancomycin is not absorbed orally at all - oral vancomycin treats C. difficile within the gut lumen and has no systemic activity whatever.

• Half-life drives redosing. Intra-operative redosing is a pharmacokinetic decision, not a ritual: a further dose is due once the operation exceeds roughly two half-lives of the agent, or after major blood loss, because the aim is a tissue concentration above the MIC for the whole time the wound is open. Obesity raises the volume of distribution and is the usual reason a standard dose under-doses.

• The monitoring target. Vancomycin is now dosed to an AUC/MIC of roughly 400-600, which has largely replaced trough-only monitoring because it tracks efficacy while limiting nephrotoxicity.

• Synergy. Some combinations kill better than either agent alone - classically a beta-lactam (or vancomycin) plus an aminoglycoside in enterococcal infection, where cell-wall disruption lets the aminoglycoside reach its ribosomal target.

(Prophylaxis regimens, timing and the penicillin-allergy pathway are developed in our Perioperative Antibiotics topic; treatment regimens, duration and the oral-versus-intravenous evidence in Orthopaedic Antibiotic Therapy; cement and beads in Local Antibiotic Delivery. The pharmacology that drives all three sits here.)