In vitro antimicrobial susceptibility trends in commonly isolated anaerobic bacteria within the Australian context

Antibiotic susceptibility testing is often not performed for anaerobic bacteria; empiric antibiotics with anaerobic cover are used if indicated. However, resistance has been described to various commonly used anti-anaerobic agents, so it is important to assess whether this approach remains optimal.

To determine phenotypic susceptibility profiles of commonly isolated anaerobic bacteria.

Stored clinical isolates from within Prevotella spp. (n=30), Clostridium perfringens (n=30), Cutibacterium acnes (n=30), underwent susceptibility testing using EUCAST methodology; additionally gram positive anaerobic cocci (GPAC) such as Finegoldia magna (n=11), Parvimonas micra (n=8), and Peptoniphilus spp. (n=9) underwent susceptibility testing using an experimental method, the Thermo Fisher Anaerobe Sensititre broth microdilution plate.

High resistance rates were found in Prevotella spp. to amoxicillin-clavulanate and clindamycin (53.3% and 50% respectively), as well as 43.3% clindamycin resistance in Clostridium perfringens, and notable clindamycin and tetracycline resistance rates in the gram positive anaerobic cocci. Susceptibility was generally otherwise retained for F. necrophorum and the Gram positive anaerobes tested, and to metronidazole for all organisms.

Given relatively high rates of resistance to common agents within Prevotella spp. and with clindamycin to several species tested, anaerobic susceptibility testing should be considered by laboratories in select circumstances, particularly in infections where Prevotella spp. have a significant role or where clindamycin is being considered as the main agent for significant infections.