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

Australia

• Teng-JC et al. Victoria-wide data 2018: 416 isolates (Gram pos and neg), almost all were metronidazole, meropenem and augmentin susceptible. 75%  of Gram neg isolates were clindamycin susceptible.
• Hunter New England 2015 – study of 70 Gram negative anaerobe isolates (Hughes/Pederson et al).
• J Antimicrob Chemother. 1992 Dec;30(6):811-20. Antimicrobial susceptibility of anaerobic bacteria in Australia. Chen SC, Gottlieb T, Palmer JM, Morris G, Gilbert GL.  Cefoxitin and cefotetan inferior- 73 and 65% of 900 isolates susceptible respectively.
• Unpublished study B. fragilis group- phenotypic and genotypic resistance study- Concord Hospital 2004 – Siarakas-Bacteroides-fragilis-2004. Tabled with permission.
• Queen Elizabeth Hospital, Royal Adelaide Hospital, SA Pathology (Nelson et al) presented at ASID ASM meeting 2010: 193 isolates ; 2% of 41 B. fragilis isolates had metronidazole resistance identified
• Austin Health, Victoria (Montgomery et al) 2010: 122 isolates ; no metronidazole resistance in Gram negative anaerobes identified . Abstract only; no published.

New Zealand

• J Antimicrob Chemother. 2006 May;57(5):992-8. Antimicrobial susceptibility of anaerobic bacteria in New Zealand: 1999-2003. Roberts SA, Shore KP, Paviour SD, Holland D, Morris AJ.    No unexpected surprises from this survey with conventional agents retaining activity against most (all) tested organisms.

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.