Molecular characterization and antimicrobial resistance profile of atypical Citrobacter gillenii and Citrobacter sp. isolated from diseased rainbow trout (Oncorhynchus mykiss)

Abstract

Aim: Over the last decade, Citrobacter species have been responsible for infections in fish and many species and also new Citrobacter species have been identified. In this study, molecular identifications and the phenotypic and genotypic antimicrobial-resistance characteristics of atypical and typical Citrobacter species were determined. Materials and methods: Seven Citrobacter isolates were investigated from rainbow trout of different lengths with signs of disease. Biochemical characteristics were determined using conventional tests and rapid test kits; moreover, molecular identifications were conducted with 16S rRNA and the gyrB housekeeping gene region. The sequencing results obtained from the gyrB gene region were deposited in the GenBank database and compared with isolates from different countries that were registered in the database. Resistances to florfenicol, sulfonamides, and tetracycline antimicrobials were determined using the broth micro dilution method, and molecular resistance genes against these antimicrobials were identified. All detected resistance genes were confirmed by sequence analyses. Results: It was determined that three of the Citrobacter species with biochemical characteristics were atypical and showed oxidase-positive reactions. All the Citrobacter species were identified as Citrobacter sp. using the 16S rRNA gene; three isolates were identified as Citrobacter gillenii and four as Citrobacter sp. based on gyrB gene sequence analysis. Some isolates were found in the same group as other countries' isolates in the GenBank database, while isolates with high identities were found in different genogroups. All isolates were found to be phenotypically resistant to sulfamethoxazole and susceptible to tetracycline; these isolates' resistance genes included sulI, tetA, tetB, and tetD.