Pathogenic potential of MDR Enterococcus faecalis in tilapia and mullet with insight into virulence traits, antibiotic resistance genes, and multidrug resistance patterns.

Enterococcus faecalis represents a potential public health threat and leads to significant economic losses in the fish producing sectors. To investigate the prevalence, multidrug resistance profiles, virulence characteristics, antibiotic resistance genes, and pathogenicity of E. faecalis, 240 fish samples, including 120 Oreochromis niloticus and 120 Mugil cephalus, were obtained from fish farms in Ismailia, Egypt, and subjected to clinical, postmortem, and bacteriological investigations. The total prevalence of E. faecalis in the examined fish was 14.17% (34/240), with rates of 16.67% (20/120) in O. niloticus and 11.67% (14/120) in M. cephalus. From these positive fish, 62 organ-specific isolates were obtained for further characterization. The liver was the most frequently infected organ. PCR analysis revealed that the predominant virulence-related genes in the recovered E. faecalis strains were gelE (100%), esp (88.70%), hyl (74.19%), and cylA (70.97%). Furthermore, 32.20% of E. faecalis isolates exhibited multidrug resistance (MDR) to seven classes of antimicrobials and harbored the ermB, tetM, vanA, blaZ, and aac(6')-Ie-aph(2″)-Ia genes. Additionally, 19.30% (12/62) of E. faecalis isolates were MDR across six antibiotic classes and harbored the ermB, tetM, vanA, and blaZ genes. The pathogenicity of the recovered E. faecalis isolates was evaluated by experimental challenge in O. niloticus and T. zillii. Kaplan-Meier survival analysis demonstrated significant differences in survival probabilities among the challenged groups, reflecting marked variation in isolate virulence. Mortality outcomes were associated with the distribution of virulence-associated genes, suggesting that the pathogenicity of E. faecalis is governed by isolate-specific virulence determinants and varies between host species. In summary, the study highlights the emergence of MDR E. faecalis in both O. niloticus and M. cephalus, underscoring a potential public health concern that warrants further investigation. MDR E. faecalis strains in fish increasingly possess the resistance genes ermB, tetM, vanA, blaZ, and aac(6')-Ie-aph(2″)-Ia, as well as the virulence genes gelE, esp, hyl, and cylA.