The relevance of correct identification and interpretation of susceptibility testing of Aeromonas spp. bacteremia isolates

Objective: To assess the relevance of correct identification and interpretation of susceptibility testing of Aeromonas spp. bacteremia isolates using newly developed molecular methods in comparison to previous conventional methods. Material and methods: The study included 22 patients with bacteremia due to Aeromonas hydrophila group, microbiologically characterized using the MicroScan system. Further identification to species level was performed by mass spectrometry, and confirmed by sequencing the rpoB gene. The MIC of imipenem, cefotaxime, piperacillin-tazobactam, ciprofloxacin and cotrimoxazole was studied using a commercial broth microdilution and antibiotic gradient strips with low and high inocula. Detection of carbapenemase production was performed using the modified Hodge test, and was confirmed by amplifying the cphA gene by PCR. Results: A total of 9 (40.9%) isolates were identified as Aeromonas hydrophila, 8 (36.4%) as Aeromonas veronii, and the remaining 5 (22.7%) isolates as Aeromonas caviae. Resistance to beta-lactams according to both the commercial microdilution and MIC gradient strips methods was: 36%-50% to imipenem; 4%-56% to cefotaxime, and 27%-56% to piperacillin/tazobactam. The agreement between results generated by the automated system and the diffusion antibiotic gradient strip was, for all 3 species, 68% for imipenem, 50% to cefotaxime, and 46% to piperacillin/tazobactam. No resistance to cotrimoxazole and ciprofloxacin was found by either of the two methods, although 22.7% of the strains were resistant to nalidixic acid. Conclusions: It is essential to identify the isolates of Aeromonas spp. at the species level, due to the fact that beta-lactam resistance is species- and method-dependent. The high rate of resistance to beta-lactam and quinolones reduce their application as empiric treatments for invasive infection by Aeromonas ssp. (C) 2015 Elsevier Espana, S.L.U. and Sociedad Espanola de Enfermedades Infecciosas y Microbiologla Clinica. All rights reserved.