Edwardsiella tarda in Tambaqui (Colossoma macropomum): A Pathogenicity, Antimicrobial Susceptibility, and Genetic Analysis of Brazilian Isolates

Abstract

Edwardsiella tarda is a crucial pathogenic bacterium in tropical aquaculture. This bacterium was recently isolated from tambaqui (Colossoma macropomum), a commercially important fish species in Brazil. This study assessed the antimicrobial susceptibility, pathogenicity, and genetic diversity of the tambaqui-derived E. tarda isolates. Fourteen bacterial isolates isolated from tambaqui were identified as E. tarda by using matrix-assisted laser desorption/ionization–time-of-flight mass spectrometry and dnaJ gene sequencing. Antimicrobial susceptibility tests were conducted against seven drugs using the disc diffusion assay. The pathogenicity test conducted by intraperitoneal injection of 2.4 × 107 colony-forming units (CFU) fish−1 of E. tarda (ED38-17) into tambaqui juveniles eventually revealed that neither clinical signs nor death were present. However, splenomegaly and whitish areas in the spleen and kidneys were observed. The histological investigation also revealed granulomatous splenitis, nephritis, and hepatitis occurring internally. Repetitive extragenic palindromic-PCR fingerprinting separated the 14 isolates into three genetic groups. The antibiogram revealed that all E. tarda isolates were wild-type (WT) to florfenicol (FLO), norfloxacin (NOR), neomycin (NEO), erythromycin (ERY), and oxytetracycline (OXY); however, some were non-wild-type to sulfamethoxazole/trimethoprim (7.1%) and amoxicillin (21.4%). Therefore, through experimental infection, E. tarda ED38-17 could induce pathogenic effects in C. macropomum. Additionally, three distinct genetic types were found, and the E. tarda isolates were WT to FLO, NOR, NEO, ERY, and OXY. These findings raise awareness of a bacteria causing unseen lesions, a pathogen that will potentially impact tambaqui aquaculture in the future.

Antimicrobial Resistance in Aquaculture: Risk Mitigation within One Health Context

“…In a study by de Oliveira et al [100] MIC assay based on the protocol by Clinical and Laboratory Standards Institute (CLSI, 2014) was used and provisional ECV for Streptococcus (agalactiae) in tilapia farming, evaluating the profile of florfenicol resistance, was established observing that ECV was 8 µg/mL for 94% of the tested strains classified as a wild-type; ECV was calculated using two methodologies: the normalized resistance interpretation (NIR) [101] and ECOFFinder MS (https://clsi.org/meetings/susceptibilitytesting-subcommittees/ecoffinder/). The MICs for Edwardsiella in aquatic culture was conducted using disk-diffusion test and provisional ECVs that were established were 8 µg/mL for erythromycin, 10 µg/mL for neomycin, 18 µg/mL for sulfamethoxazole-trimethoprim, 23 µg/mL for amoxicillin, 25 µg/mL for oxytetracycline, 26 µg/mL for norfloxacin and 27 µg/mL for florfenicol [102]. Further investigation is needed to establish more precisely ECVs for afore mentioned four most frequent bacteria associated with AMR in aquaculture.…”