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Abstract

Background

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.

Methods

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.

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.

Conclusion

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.

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/content/journal/acmi/10.1099/acmi.afm2019.po0011
2019-09-01
2019-10-16
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