The emergence of antibiotic‐resistant bacteria (ARBs) has become a threat to the aquatic industry. Virulent bacterial spp. namely Aeromonas hydrophila, A. veronii, Pseudomonas fluorescens and P. aeruginosa that were isolated from Nile tilapia (Oreochromis niloticus) and grey mullet (Mugil cephalus) were collected from freshwater fish farms at three different sites, and their associated antibiotic‐resistant genes (ARBs) were further characterized. The ARGs for sulphonamide and tetracycline were the most prevalent ones. Out of 50 O. niloticus and 50 M. cephalus, A. hydrophila (16.5 ± 5.5 and 11 ± 1.0 respectively) and P. aeruginosa (16 ± 1.0 and 12.5 ± 2.5 respectively) had significantly higher infection rates. At site 3, O. niloticus and M. cephalus showed severe lesions in the hepatic tissues which had higher residues of iron (Fe, 1238.3–1250.0 µg/g) and zinc (Zn, 940.0–078.0 µg/g) compared with sites 1 and 2. The severity of hepatic tissue lesions was associated with bacterial infection and heavy metal residues, and also the presence of ARGs was associated with histopathological alterations of the hepatic tissues. It was concluded that ARGs (sulphonamide and tetracycline) could be developed in bacteria isolated from fish, which subjected to heavy metal pollution (Fe, Mn and Zn).

The present work aimed to investigate the causes of summer mortality syndrome affecting cultured European seabass (Dicentrarchus labrax) by examining physiochemical farm water characteristics, isolation, and identification of recovered bacterial pathogens from diseased fish studying the effect of water temperature on stress biomarkers and disease severity. Studied water parameters were normal except ammonia and dissolved oxygen was higher and lower than the standard value. Sixty-two bacterial isolates were recovered from moribund fish and identified as 31 Vibrio fluvialis, 23 Pseudomonas aeruginosa and 8 Staphylococcus aureus isolates. The calculated LD50 of V. fluvialis, P. aeruginosa and S. aureus for D. labrax fingerlings were 4.67 × 107, 2.37 × 106 and 1.38 × 107, respectively. There was a direct correlation between water temperature and mortality rate of fish challenged with V. fluvialis as the mortality rate was 44.44, 50, 66.66, and 83.33% for fish maintained at 27, 30, 33, and 36°C. Plasma cortisol, superoxide dismutase, catalase and malondialdehyde significantly increased when the water temperature exceeded 30°C. The experimentally infected fish showed similar clinical signs and postmortem lesions of naturally diseased fish with no boundary between different pathogens. Antibiogram test indicated that florfenicol was the most effective antibiotic against all the recovered bacterial isolates while all isolates resisted sulfamethoxazole-trimethoprim. Massive degenerative changes observed in the hepatopancreas, posterior kidney and gill tissues of experimentally infected fish.

Edwardsiella tarda is a bacterial fish pathogen associated with high mortality in aquaculture. Lactobacillus plantarum, which is a probiotic with immunomodulatory properties, was used to improve the disease resistant in Nile tilapia (Oreochromis niloticus). One hundred twenty O. niloticus (60 ± 5 g live body weight) were divided into four groups; the first (control) was fed on an unsupplemented diet, the second and the third group were fed on a diet supplemented with L. plantarum (1012 bacterial cells/kg feed) for two weeks (G1) and four weeks (G2), and the fourth group (G3) was fed on a diet supplemented with antibiotic florfenicol for ten days. Haemogram, liver enzymes (AST, ALT and ALP levels), hepatic antioxidant enzymes (GPx and CAT) and serum creatinine were measured to assess the impacts of the additives on the health status of fish. The level of gene expressions of interleukin beta 1 (IL‐1β), tumour necrosis factor (TNF)‐α and IL‐10 was measured after supplementation and challenge tests. Fish were challenged with the obtained median lethal dose (LD50) of E. tarda (2.54 × 106 CFU/ml). O. niloticus fed on supplemented diets with L. plantarum G1 and G2 showed significantly enhanced immune status even after fourteen days of the experimental infection with E. tarda; however, florfenicol treatment G3 (mortality 20%) had exhibited lower MR% compared with L. plantarum groups (mortality 43.3% for both) and the control (mortality 60%). L. plantarum could protect O. niloticus against E. tarda infection compared to the control with a significant enhancement of fish health that helps in the restoration of normal physiological status.

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