Ann Lab Med.  2014 Nov;34(6):439-445. 10.3343/alm.2014.34.6.439.

Comparison of Supplemented Brucella Agar and Modified Clostridium difficile Agar for Antimicrobial Susceptibility Testing of Clostridium difficile

• Kim GH
• Kim J
• Pai H
• Kang JO

• KMID: 2129569
• DOI: http://doi.org/10.3343/alm.2014.34.6.439

Abstract

BACKGROUND
Antimicrobial susceptibility testing (AST) of Clostridium difficile is increasingly important because of the rise in resistant strains. The standard medium for the AST of C. difficile is supplemented Brucella agar (sBA), but we found that the growth of C. difficile on sBA was not optimal. Because active growth is critical for reliable AST, we developed a new, modified C. difficile (mCD) agar. C. difficile grew better on mCD agar than on sBA.
METHODS
C. difficile isolates were collected from patients with healthcare-associated diarrhea. sBA medium was prepared according to the CLSI guidelines. Homemade mCD agar containing taurocholate, L-cysteine hydrochloride, and 7% horse blood was used. For 171 C. difficile isolates, we compared the agar dilution AST results from mCD agar with those from sBA.
RESULTS
No significant differences were observed in the 50% minimal inhibitory concentration (MIC50) and 90% minimal inhibitory concentration (MIC90) of clindamycin (CLI), metronidazole (MTZ), moxifloxacin (MXF), piperacillin-tazobactam (PTZ), and rifaximin (RIX), but the values for vancomycin (VAN) were two-fold higher on mCD agar than on sBA. The MICs of CLI, MXF, and RIX were in 100% agreement within two-fold dilutions, but for MTZ, VAN, and PTZ, 13.7%, 0.6%, and 3.1% of the isolates, respectively, were outside the acceptable range.
CONCLUSIONS
The MIC ranges, MIC50 and MIC90, were acceptable when AST was performed on mCD agar. Thus, mCD agar could be used as a substitute medium for the AST of C. difficile.

Keyword

Clostridium difficile; Antimicrobial susceptibility test; Minimum inhibitory concentration; Susceptibility testing media

MeSH Terms

Anti-Infective Agents/*pharmacology
Clostridium Infections/microbiology
Clostridium difficile/*drug effects/isolation & purification
Diarrhea/*microbiology
Humans
Microbial Sensitivity Tests/*methods
Anti-Infective Agents

Figure

• 

  Fig. 1 Distribution of the minimum inhibitory concentrations (MIC) (x-axis, units are µg/mL) of six antimicrobial agents against the 171 clinical isolates of Clostridium difficile on supplemented Brucella agar (gray bars) and modified CD agar (black bars). The dotted lines represent the breakpoints for the antimicrobials, with the exception of rifaximin, which does not presently have breakpoint criteria.

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