Application of the CDS to Anaerobic bacteria

7.1. Introduction

Susceptibility testing of anaerobic bacteria is not performed routinely unless infections such as osteomyelitis, infected prosthetic devices and brain abscesses are present and the knowledge of the antimicrobial susceptibility may be crucial for treatment. The CDS has been calibrated for Propionibacterium sp., Peptostreptococcus sp., Clostridium sp., Bacteroides sp., Prevotella sp., Fusobacterium sp.  For the performance of the CDS Test and quality assurance using the reference strains Clostridium perfringens ATCC 13124 and Bacteroides fragilis ATCC 25285, see Chapters 2 and 3 of this Manual.

7.2. Inoculum, Medium and Incubation conditions

Anaerobes are tested on supplemented Brucella Medium Base (Oxoid CM0169), (Section 2.2.1) and incubated anaerobically at 35‑37°C for 24 hours (slow growing organisms require 48 hours). The inoculum is prepared to an equivalent 0.5 McFarland standard as described in section 2.2.2 subsection Anaerobes.

7.3. Antibiotic calibrations

Amoxycillin-clavulanic acid 3 μg, benzylpenicillin 0.5 units, cefoxitin 10 μg, clindamycin 2 μg, meropenem 5 μg, metronidazole 5μg, moxifloxacin 2.5μg and piperacillin-tazobactam 55μg were calibrated for testing anaerobic bacteria. Table 11.1.c lists the antibiotics calibrated, the disc potencies, the annular radius of the inhibitory zones and the antibiotic MIC’s of susceptible strains.

7.3.1. Clindamycin and anaerobes.

When clindamycin is tested against anaerobes, an erythromycin 5μg disc must be placed 13mm edge to edge from the clindamycin 2μg disc. This is to detect inducible clindamycin resistance (ICR) (See Section 4.7 for an explanation of ICR).