Anaerobic susceptibility data – Australia and New Zealand – updated 2019

This posting summarises all the work that can be found – I would welcome advice about any additional studies.  Image: Bacteroides fragilis Gram stain from


New Zealand

Microbiology reporting comments (Pathology North)

Mixed or Gram negative anaerobic isolate(s)

Antibiotics that are active against Gram negative and Gram positive anaerobes (mixed infections) include metronidazole (oral/IV, dose 12-hrly), lincomycin (IV), clindamycin (IV/oral), piperacillin+tazobactam (IV) or amoxicillin+clavulanate (oral). See Therapeutic Guidelines: Antibiotic Edition 14 for advice on doses.

Gram positive anaerobic isolate(s)

Benzylpenicillin (IV), amoxicillin (oral), di/flucloxacillin (IV/oral) at usual dosage are active against Gram positive anaerobes. Broader spectrum anaerobic agents such as metronidazole (IV/oral, dose 12-hrly), lincomycin (IV), clindamycin (IV/oral) or piperacillin+tazobactam (IV) are also active.

Metronidazole dosing

Metronidazole exhibits a concentration-dependent killing effect against anaerobes and has a post-antibiotic effect for more than 3 hours. Studies show that the metronidazole minimum inhibitory concentration (MIC) for most susceptible anaerobes is <6.2 micrograms/mL. Trough concentrations with 12-hourly dosing (400mg orally & 500mg IV) are in excess of the MICs for the majority of anaerobes. In Australia, 12-hrly dosing has been the routine for some years now and is recommended by Therapeutic Guidelines: Australia.

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