Discrepancy between reference phenotypic and genotypic detection of cphA metallo-β-lactamase in Aeromonas spp

Infection with Aeromonas spp. can be difficult to treat due to the common presence of multiple β-lactamase genes. Although the presence of β-lactamase genes appears to be mostly species-specific, species-level identification of Aeromonas spp. is currently unreliable. Routine phenotypic susceptibility testing is unreliable for the detection of chromosomal cphA metallo-β-lactamase-mediated carbapenem resistance, which can lead to adverse outcomes. A reliable, easy-to-use phenotypic method for the detection of carbapenem resistance in Aeromonas spp. would allow laboratories to better guide treatment of these organisms. This study used whole-genome sequencing for the detection of β-lactamase genes in 43 Aeromonas spp. isolates as a reference to evaluate different phenotypic methods for the detection of cphA. Standard Kirby Bauer disk diffusion (KB) and broth microdilution (BMD) were least sensitive, detecting carbapenem resistance in 3 to 5 of 24 cphA-positive isolates, depending on the drug and method. Increased-inoculum KB detected resistance in 3 to 14 of 24 isolates, whereas increased-inoculum BMD detected resistance in 11 to 18 of 24 isolates depending on the carbapenem. The modified carbapenem inactivation method (mCIM) was positive for all 24 isolates, whereas EDTA-modified mCIM was positive for 19/24 isolates. None of the 19 cphA-negative isolates tested positive by mCIM or resistant by any phenotypic method. The mCIM is an easy-to-implement method that detected cphA-mediated carbapenem resistance with 100% sensitivity and specificity and could help guide effective treatment of Aeromonas sp. infection.IMPORTANCESerious infections with Aeromonas species are associated with high rates of morbidity and mortality. Some species can have a carbapenemase that is not detectable by typical phenotypic methods. It is important for clinical laboratories to be aware and have a method for identifying and reporting this phenotype. Clinicians should also be aware of this resistance phenotype when choosing an antibiotic for therapy. Here, we show the difficulty in using broth microdilution or disk diffusion to identify carbapenem resistance mediated by a specific carbapenemase, cphA, in Aeromonas spp. Through genotypic analysis, we also found cphA to be associated with certain species of Aeromonas. Unfortunately, identification to the species level using typical methods like MALDI-TOF is also difficult. This study presents data for a previously well-established method, modified carbapenem inactivation method, that can be utilized to reliably detect this resistance phenotype in commonly encountered Aeromonas species.