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

<i>Edwardsiella tarda</i> is a crucial pathogenic bacterium in tropical aquaculture. This bacterium was recently isolated from tambaqui (<i>Colossoma macropomum</i>), a commercially important fish species in Brazil. This study assessed the antimicrobial susceptibility, pathogenicity, and genetic diversity of the tambaqui-derived <i>E. tarda</i> isolates. Fourteen bacterial isolates isolated from tambaqui were identified as <i>E. tarda</i> by using matrix-assisted laser desorption/ionization–time-of-flight mass spectrometry and <i>dnaJ</i> 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 <i>E. tarda</i> (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 <i>E. tarda</i> 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, <i>E. tarda</i> ED38-17 could induce pathogenic effects in <i>C. macropomum</i>. Additionally, three distinct genetic types were found, and the <i>E. tarda</i> 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.