Distribution and phenotypic and genotypic detection of a metallo-β-lactamase, CphA, among bacteraemic Aeromonas isolates

The objectives of the study were to investigate the distribution of cphA-related genes (cphA) encoding a CphA metallo-β-lactamase (MBL) among 51 consecutive Aeromonas blood isolates and to compare different phenotypic methods for detecting CphA. The presence of cphA was detected by PCR. Four phenotypic methods, the imipenem-EDTA combined disc test, imipenem-EDTA MBL Etest, agar dilution test and modified Hodge test (MHT), were used to detect imipenem susceptibility and MBL production. The results showed that 35 (69 %) blood isolates had cphA. All (100 %) of 16 Aeromonas aquariorum isolates and 12 Aeromonas veronii isolates, and 4 (80 %) of 5 Aeromonas hydrophila isolates, carried cphA, but none of 15 Aeromonas caviae isolates did. With the standard inocula, irrespective of the presence or absence of cphA, all but one (50, 98 %) isolates were susceptible to imipenem tested by disc diffusion, Etest and agar dilution (104 c.f.u. spot inocula), and did not exhibit MBL production by the imipenem-EDTA combined disc test and MBL Etest. By the agar dilution test using large inocula (107 c.f.u.), 34 (97 %) of 35 cphA+ isolates had imipenem MICs of ≥16 µg ml−1, higher than the susceptible breakpoint (4 µg ml−1), and demonstrated positive results for the MHT, while one cphA+ and all 17 cphA− isolates had imipenem MICs of ≤4 µg ml−1. In conclusion, the distribution of cphA among aeromonads is species-specific, found in A. aquariorum, A. veronii and A. hydrophila, and the MHT may be a phenotypic screening test for CphA production.