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Disk diffusion susceptibility testing of the Bacteroides fragilis group
R Horn1, A M Bourgault, F Lamothe
1Service de Microbiologie et Maladies Infectieuses, Hôpital Saint-Luc, Montreal, Quebec, Canada.
Antimicrobial Agents and Chemotherapy
|October 1, 1987
Summary
A new disk diffusion test accurately determined antibiotic susceptibility for Bacteroides fragilis group bacteria. This method offers reliable results for cefoxitin, clindamycin, moxalactam, and ticarcillin, aiding clinical decisions.
Area of Science:
- Clinical Microbiology
- Antimicrobial Susceptibility Testing
- Bacteriology
Background:
- The Bacteroides fragilis group is a significant cause of anaerobic infections.
- Accurate antibiotic susceptibility testing is crucial for effective treatment of these infections.
- Disk diffusion methods offer a potentially faster alternative to agar dilution for determining Minimum Inhibitory Concentrations (MICs).
Purpose of the Study:
- To evaluate a new disk diffusion test for determining the susceptibility of Bacteroides fragilis group isolates to six antibiotics.
- To compare the results of the disk diffusion test with the standard agar dilution method.
- To establish reliable zone diameter breakpoints for the disk diffusion test.
Main Methods:
- 225 isolates of the Bacteroides fragilis group were tested against cefoxitin, chloramphenicol, clindamycin, moxalactam, imipenem, and ticarcillin.
- A novel disk diffusion method using Wilkins-Chalgren agar was employed, with direct bacterial suspension and immediate swabbing.
- Results were compared to the standard agar dilution method using regression analysis and the error rate-bounded method.
Main Results:
- The disk diffusion test showed good correlation with MICs for cefoxitin, clindamycin, moxalactam, and ticarcillin.
- Reliable zone diameter breakpoints were established for these four antibiotics, with low rates of false-susceptible (<1%) and false-resistant (<4%) strains.
- Correlation was not established for chloramphenicol and imipenem due to a lack of resistant strains in the tested isolates.
Conclusions:
- The new disk diffusion test is a reliable and accurate method for determining antibiotic susceptibility in the Bacteroides fragilis group for key antibiotics.
- Established zone breakpoints facilitate the practical application of this disk diffusion method in clinical laboratories.
- This method can aid in guiding appropriate antibiotic therapy for infections caused by these common anaerobic bacteria.