Phenotypic, genotypic and virulence traits analysis of aeromonads causing massive mortality in farmed Oreochromis niloticus

Motile aeromonads were identified in earthen-pond-farmed Oreochromis niloticus that suffered massive mortalities in Egypt during the summer of 2020. The fish showed hemorrhagic septicemic signs. Poor management practices and inadequate water quality measures were observed in the affected earthen ponds. Motile aeromonads (n = 31 isolates) were identified from 70 fish specimens. Based on their phenotypic and molecular characteristics, isolates were identified as: Aeromonas hydrophila (n = 12), A. veronii (n = 10), A. caviae (n = 5), and A. sobria (n = 4). Bacteriological examination of farm water samples also revealed aeromonads (n=9) and some fish-nonpathogenic bacteria. The aeromonad isolates recovered from fish exhibited lipase (52.5%) and protease (47.5%) activities and harboured some virulence genes: Ser (62.5%), Aer (55%), ela (37.5%), gcaT (32.5%), Hyl (25%), laf-A (22.5%), and Act (20%). They also harboured numerous antibiotic-resistance genes, including aadA (37.5%), tetC (32.5%), tetA (27.5%), sul 1 (20%), and blaTEM (10%). Virulence and antibiotic resistance genes were also noted in some of the Aeromonas spp. isolates obtained from farm water. Aeromonads were highly resistant to ampicillin, amoxicillin, and gentamicin but highly susceptible to ciprofloxacin and florfenicol antibiotics. Aeromonas spp pathogenicity was confirmed by the experimental infection of Oreochromis niloticus. Our results indicate a positive correlation between excessive tilapia mortalities, motile Aeromonas septicemia and adverse water quality parameters measured during the summer. This study provides data on the virulence, pathogenicity, and antibiotic resistance of motile aeromonads affecting fish and humans, which will be useful for developing efficient therapies.