Bacterial Pathogens and Their Antimicrobial Resistance in Farmed Nile Tilapia Experiencing “Summer Mortality” in Kafr El-Sheikh, Egypt

During four outbreaks in 2023 and 2024, samples from pond-reared Nile tilapia were taken from different farms located in Kafr Elsheikh governorate, Egypt. Samples were submitted for laboratory examinations. Diseased fish exhibited bacterial septicemia and some cases died without showing any clinical signs. A total of 30 bacterial isolates were isolated and identified. Of these isolates, 57% were identified as Gram-positive bacteria, whereas the remaining 43% were identified as Gram-negative bacteria. PCR targeting the 16S rRNA gene and genome sequencing confirmed five bacterial isolates as <i>Aeromonas veronii</i> (30%), <i>Vibrio alginolyticus</i> (13.3%), <i>Enterococcus faecalis</i> (23.3%), <i>Aerococcus viridans</i> (16.7%)<i>,</i> and <i>Staphylococcus epidermidis</i> (16.7%). The NCBI GenBank accession numbers of these strains were (PV018985) for <i>A. veronii</i>, (PV016854) for <i>V. alginolyticus</i>, (PV013413) for <i>E. faecalis</i>, (PV032005) for <i>A. viridans</i>, and (PV012491) for <i>Staph. epidermidis</i>. The antibiogram revealed that the bacterial strains showed resistance to most of the antibiotics tested. <i>A. viridans</i> exhibited resistance to nearly all the antibiotics except for intermediate sensitivity to ciprofloxacin and amoxycillin/clavulanic acid. However, <i>A. veronii</i> showed high sensitivity to amoxycillin/clavulanic acid, oxytetracycline, kanamycin, and trimethoprim/sulfamethoxazole and intermediate susceptibility to ciprofloxacin. Similarly, <i>E. faecalis</i> showed high susceptibility to amoxycillin/clavulanic acid, ciprofloxacin, and trimethoprim/sulfamethoxazole in addition to intermediate sensitivity to ampicillin and kanamycin. Furthermore, <i>Staph. epidermidis</i> strain was highly susceptible to ampicillin, amoxycillin/clavulanic acid, oxytetracycline, novobiocin, and trimethoprim/sulfamethoxazole and was partially sensitive to kanamycin and ciprofloxacin. To conclude, summer mortalities recorded in farmed tilapia were closely related to a multifactorial bacterial origin with different sensitivity to antibiotic discs.