Susceptibility of Pacific white snook Centropomus viridis to Vibrio species

To examine the pathogenicity of Vibrio strains, several doses of Vibrio harveyi (CAIM 1622 and CAIM 1508), Vibrio ponticus (CAIM 1751) and Vibrio anguillarum (CAIM 8) were used to challenge Pacific white snook Centropomus viridis Lockington, 1877 juveniles, and survival, gross signs and histological lesions were observed. Susceptibility of pathogenic vibrios CAIM 1508 and CAIM 1751 to antibiotics used in aquaculture was also evaluated. The growth ability of the tested strains was not related to their pathogenicity. One of the V. harveyi strains (CAIM 1508) was the most virulent, causing per-acute septicaemia in C. viridis even at a low dose (1.4 × 104 CFU g-1). Although the V. ponticus strain (CAIM 1751) was less virulent, this is the first report of it as a pathogen of white snook. Fish challenged with V. ponticus displayed external, generalized haemorrhaging. Necrosis of the digestive tract and intravascular haemosiderosis were the most remarkable histological lesions in fish challenged with both strains. Multifocal necrosis of the internal organs and bacterial masses was also observed. The lowest minimum inhibitory concentration of the pathogenic strains (CAIM 1508 and CAIM 1751) was calculated for enrofloxacin (20 and 10 µg ml-1, respectively), and both bacteria were resistant to amoxicillin, ampicillin and trimethoprim-sulfamethoxazole.

Virulence of the fish pathogen Aeromonas dhakensis: genes involved, characterization and histopathology of experimentally infected hybrid tilapia

Aeromonas dhakensis (Ad) CAIM 1873 growth was evaluated at different conditions and antibiotic susceptibility. Mortality and histopathological damages in hybrid tilapia Oreochromis niloticus × O. mossambicus, and virulence factors caused by Ad bacterial cells and extracellular products (ECPs) were evaluated, and the whole genome was obtained. Ad grew between 0.0 and 5.5% NaCl at a pH of between 4 and 10 and from 4 to 37°C. The lowest minimum inhibitory concentration was found for enrofloxacin (<5 µg ml-1), and bacteria were resistant to erythromycin, amoxicillin and ampicillin. Ad bacterial cells (1.86 × 105 cells g-1) and ECPs (0.462 µg protein fish-1) were highly virulent to challenged hybrid tilapia and caused over 80% mortality at 24 h. The primary clinical sign caused was haemorrhage, and damage was most marked in the spleen, liver, kidney and brain of fish challenged with bacterial cells. To our knowledge, this is the first report that Ad causes pyknotic and karyorrhectic nuclei of erythrocytes in the internal organs of hybrid tilapia, which was the most striking histopathological observation. The virulence of Ad to hybrid tilapia may be primarily related to the activity of haemolysins (hlyA genes) and cytotoxins (aerolysin aerA), along with the production of siderophores and proteases. We also found β-lactamase, tetracycline and multiple antibiotic resistance genes, as well as adherence, iron acquisition, toxins (aerolysin family, haemolysins) and diverse protease genes.