The frequency of resistance to carbapenems in Bacteroides fragilis isolates is rising and is conferred by a metallo-β-lactamase encoded by the cfiA gene. The utilisation of existing technologies such as MALDI-TOF MS (MTMS) may allow for the early detection of this important resistance phenotype.


168 B. fragilis isolates were tested. Meropenem MICs were performed by agar dilution and the results interpreted using EUCAST guidelines. All isolates were identified to species level by MTMS and the presence of CfiA detected using the Bruker MBT subtyping module. The presence of the cfiA gene was confirmed by PCR. Next generation sequencing (NGS) was utilised for discrepant results.


108 isolates (64%) were phenotypically susceptible, with 98% of these testing negative for cfiA by both MTMS and PCR. 40 isolates (25%) were phenotypically resistant, of which 65% were positive by PCR and MTMS with 35% negative by both methods.

35% of positive isolates were found to have an IS element upstream of cfiA, and all had an MIC ≥ 64 mg/L.

Discordant results included two isolates which were susceptible, cfiA positive and IS negative, and three cfiA negative resistant isolates.


Overall, a good correlation was observed between methods. Discrepancies highlight an important distinction between the presence of the cfiA gene and its expression.

Although a negative MTMS result does not rule out resistance, it is a rapid and valuable tool to guide early therapeutic decisions for this important pathogen.

  • This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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