Antimicrobial susceptibility testing of Bacteroides fragilis using the MALDI Biotyper antibiotic susceptibility test rapid assay (MBT-ASTRA).

This study evaluated the MBT-ASTRA for antimicrobial susceptibility testing of Bacteroides fragilis with different classes of antibiotics. MALDI-TOF MS peak AUCs from suspensions with B. fragilis with and without an antibiotic were used to calculate the relative growth (AUC "with antibiotic" divided by "without antibiotic"). Antimicrobial susceptibility testing of B. fragilis ATCC 25285 (susceptible) and B. fragilis O18 (resistant) was demonstrated with a clear difference of the relative growth between susceptible and resistant. The MBT-ASTRA needs further development and assessment but could be a relatively easy and inexpensive method for rapid antimicrobial susceptibility testing in specific cases of infection with B. fragilis.

Phenotypic detection of the cfiA metallo-ß-lactamase in Bacteroides fragilis with the meropenem-EDTA double-ended Etest and the ROSCO KPC/MBL Confirm Kit.

OBJECTIVES: To investigate the performance of the meropenem and imipenem double-ended Etest ±â€ŠEDTA and the tablet-based (meropenem and meropenem + dipicolinic acid) KPC/MBL Confirm Kit to detect cfiA metallo-ß-lactamase (MBL) in Bacteroides fragilis. METHODS: Well-characterized B. fragilis isolates, most from previously published studies, harbouring the cfiA gene and covering a wide range of meropenem MICs were included (n = 21). RESULTS: The imipenem double-ended Etest showed an indeterminate result in 95% of the included isolates with the cfiA gene (20 of 21), whereas the meropenem double-ended Etest gave an MIC ratio ≥8 (positive test) with all the isolates. All isolates that were meropenem intermediate or resistant had a zone diameter difference ≥6 mm with the KPC/MBL Confirm Kit. CONCLUSIONS: The meropenem double-ended Etest and not imipenem should be preferred for phenotypic detection of MBLs in B. fragilis. The KPC/MBL Confirm Kit could be an alternative with isolates that are meropenem intermediate or resistant (MIC >2 mg/L).