Evaluation of the Disk Diffusion Test for Bacteroides fragilis Group Clinical Isolates

Abstract

Background: Bacteroides fragilis group (BFG) isolates are the most frequently isolated gram-negative anaerobic bacteria and exhibit higher levels of antimicrobial resistance than other anaerobic bacteria. Reliable susceptibility testing is needed because of reports of resistance to the most active antibiotics. Recently, the European Committee on Antimicrobial Susceptibility Testing (EUCAST) introduced disk zone diameter breakpoints. We evaluated the disk diffusion test (DDT) for susceptibility testing of BFG isolates compared with the agar dilution method.

Methods: In total, 150 BFG isolates were collected from three institutes in Korea. The agar dilution method was conducted according to the CLSI guidelines. DDT was performed following the EUCAST guideline. Fastidious anaerobe agar supplemented with 5% defibrinated horse blood was used as the culture medium. Nine antimicrobials were evaluated: penicillin, cefoxitin, cefotetan, imipenem, meropenem, piperacillin-tazobactam, clindamycin, moxifloxacin, and metronidazole.

Results: The categorical agreement (CA) between the two methods was >90.0% for imipenem, meropenem, clindamycin, and metronidazole. However, the CA for piperacillintazobactam was low, at 83.2%. Major errors were found: 5.4% for imipenem, 7.4% for meropenem, and 12.8% for piperacillin-tazobactam. All minor errors were <10%. We propose using the area of technical uncertainty (ATU) zone-overlapping area for susceptible and resistant strains to reduce errors in the DDT. Outside the ATU, the CAs of cefoxitin, cefotetan, and piperacillin-tazobactam were >90.0%, whereas that of moxifloxacin was increased to 88.5%.

Conclusions: The DDT can be a useful alternative antimicrobial susceptibility test for BFG isolates when using the ATU zone to reduce errors.

Keywords: Area of technical uncertainty; Bacteroides fragilis; Disk diffusion antimicrobial tests; Microbial sensitivity tests; Uncertainty.

Fig. 1. Distribution of inhibition zone diameters and MICs of 149 BFG isolates for nine antibiotics. The numbers in the figures represent isolate counts. Red letters on the axis represent MIC or zone diameter breakpoints for resistance. Solid black and dashed lines represent CLSI MIC values for resistance and intermediate resistance, respectively. Dash-dot lines indicate EUCAST MIC values for resistance (A–I). Red vertical lines represent zone diameter breakpoints for resistance (A–F). The gray zone represents the ATU in this study (G–I).

Abbreviations: BFG, B. fragilis group; MIC, minimum inhibitory concentration; EUCAST, European Committee on Antimicrobial Susceptibility Testing; ATU, area of technical uncertainty.

Fig. 1. Distribution of inhibition zone diameters and MICs of 149 BFG isolates for nine antibiotics. The numbers in the figures represent isolate counts. Red letters on the axis represent MIC or zone diameter breakpoints for resistance. Solid black and dashed lines represent CLSI MIC values for resistance and intermediate resistance, respectively. Dash-dot lines indicate EUCAST MIC values for resistance (A–I). Red vertical lines represent zone diameter breakpoints for resistance (A–F). The gray zone represents the ATU in this study (G–I).

Abbreviations: BFG, B. fragilis group; MIC, minimum inhibitory concentration; EUCAST, European Committee on Antimicrobial Susceptibility Testing; ATU, area of technical uncertainty.