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Antimicrobials

Bacteriostatic versus bactericidal, the mechanism-based classification, and the antibiotics that matter to an anaesthetist.

Bacteriostatic vs bactericidal

Static agents stop bacteria multiplying and rely on the host immune system to clear them. Cidal agents kill outright — preferred in endocarditis, meningitis, neutropenia and any deep-seated infection where host defences cannot finish the job.

Bacteriostatic — stop growth	Bactericidal — kill
Macrolides — erythromycin, azithromycin, clarithromycin Tetracyclines — doxycycline Sulphonamides · trimethoprim Clindamycin Linezolid Chloramphenicol	Beta-lactams — penicillins, cephalosporins, carbapenems Aminoglycosides — gentamicin, amikacin Glycopeptides — vancomycin, teicoplanin Fluoroquinolones — ciprofloxacin, levofloxacin Metronidazole Co-trimoxazole (the combination) · isoniazid · rifampicin

Two caveats the examiners like

The distinction is not absolute — it is concentration- and organism-dependent. Chloramphenicol is static against most organisms but cidal against H. influenzae; linezolid is cidal against streptococci.
Sulphonamides and trimethoprim are each bacteriostatic alone, but the combination (co-trimoxazole) is bactericidal — sequential blockade of the same folate pathway.

Classification by mechanism

Target	Class	Examples
Cell wall synthesis	Beta-lactams (inhibit transpeptidase / penicillin-binding proteins) Glycopeptides (bind D-ala-D-ala)	Penicillins, cephalosporins, carbapenems, monobactams Vancomycin, teicoplanin
Protein synthesis	30S subunit 50S subunit	Aminoglycosides, tetracyclines Macrolides, clindamycin, linezolid, chloramphenicol
Nucleic acid / DNA	DNA gyrase (topoisomerase II) RNA polymerase DNA strand breaks	Fluoroquinolones Rifampicin Metronidazole (anaerobes)
Folate pathway	Dihydropteroate synthase Dihydrofolate reductase	Sulphonamides Trimethoprim
Cell membrane	Disrupt membrane integrity	Polymyxins (colistin), daptomycin

Mnemonic for the 50S / 30S split

"Buy AT 30, CCELL at 50" — Aminoglycosides and Tetracyclines act on 30S; Chloramphenicol, Clindamycin, Erythromycin (macrolides) and Linezolid act on 50S.

What matters to an anaesthetist

Interactions with neuromuscular blockade

Aminoglycosides potentiate non-depolarising blockade — gentamicin, kanamycin, neomycin, streptomycin. They reduce presynaptic acetylcholine release and reduce postsynaptic sensitivity.
The same applies to polymyxins, clindamycin and tetracyclines to a lesser degree.
Magnesium acts by the same presynaptic mechanism and is additive — a real risk in the obstetric patient on magnesium who also receives an antibiotic.

Toxicities worth knowing

Aminoglycosides: nephrotoxic and ototoxic (both cochlear and vestibular, potentially irreversible). Concentration-dependent killing with a post-antibiotic effect — hence once-daily dosing and trough-level monitoring.
Vancomycin: "red man syndrome" is a direct histamine release from rapid infusion, not an allergy — slow the infusion. Also nephrotoxic and ototoxic.
Fluoroquinolones: QT prolongation, tendinopathy, seizure threshold reduction.
Macrolides: QT prolongation; erythromycin is a potent enzyme inhibitor.
Rifampicin: a potent enzyme inducer — reduces the effect of warfarin, steroids and oral contraceptives. Colours secretions orange.
Metronidazole: disulfiram-like reaction with alcohol.

Perioperative prophylaxis principles

Give within 60 minutes before incision (120 minutes for vancomycin and fluoroquinolones, which need longer infusions).
Redose for prolonged surgery (roughly every two half-lives) or major blood loss.
Antibiotics are the second commonest cause of perioperative anaphylaxis after neuromuscular blocking drugs — with teicoplanin and co-amoxiclav prominent among them.
Genuine penicillin allergy is far rarer than reported; a documented rash in childhood is not usually a contraindication to a cephalosporin.

Antifungals — the two facts

Candida krusei is intrinsically resistant to fluconazole (and C. glabrata is often resistant) — a classic exam point.
For invasive aspergillosis, voriconazole is first line; liposomal amphotericin B is an alternative. Echinocandins (caspofungin) are used for invasive candidiasis.

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