Unraveling the pathogenic potential, virulence traits, and antibiotic resistance genes of multidrug-resistant Streptococcus agalactiae strains retrieved from Nile tilapia

BACKGROUND: Streptococcus agalactiae is implicated in severe infections in humans and causes considerable financial losses in the aquaculture industry, particularly in tilapia farming. Given its dual threat to public health and food security, this study was conducted to comprehensively assess the prevalence, molecular identification, multidrug resistance (MDR) patterns, virulence traits, antimicrobial resistance genes, and pathogenic potential of S. agalactiae strains isolated from Oreochromis niloticus. Notably, this study highlights the detection of critical virulence genes (cfb, hylB, lmb, and cylE), which play essential roles in host invasion and immune evasion. In parallel, the presence of resistance determinants such as pbp1A, aac(6')-aph(2''), tetM or tetK, and ermB or ermA underscores the growing threat of antimicrobial resistance in aquatic environments. METHODS: A total of 180 Oreochromis niloticus samples-comprising 90 apparently healthy and 90 moribund fish-were collected from private fish farms in Ismailia, Egypt. Clinical and postmortem (PM) examinations, along with bacteriological analysis, were subsequently performed. The recovered isolates were then subjected to molecular identification via 16 S rRNA gene amplification, antibiotic susceptibility testing via standard protocols, and PCR-based screening for key virulence and antimicrobial resistance genes. RESULTS: All recovered Streptococcus agalactiae isolates were confirmed by the presence of the species-specific 16 S rRNA gene. The overall prevalence of S. agalactiae among the examined O. niloticus was 10% (18/180), with the brain identified as the primary target organ for infection. PCR analysis revealed that the most frequently detected virulence-associated genes among the isolates were cfb (100%), hylB (82.1%), lmb (78.5%), and cylE (57.1%). Additionally, 35.7% of the S. agalactiae isolates exhibited multidrug resistance (MDR) to seven different antimicrobial classes and harbored resistance genes including pbp1A, aac(6')-aph(2''), tetM, and ermB. Another 17.8% of the isolates were MDR to six antimicrobial classes and carried the pbp1A, tetM, and ermA genes. Pathogenicity assays demonstrated a strong correlation between the presence of virulence genes and the observed mortality rates in experimentally infected fish. Notably, mortality sharply increased within the first 7 days post-inoculation, reaching approximately 73.3%, underscoring the high virulence potential of the recovered strains. CONCLUSION: Briefly, this study highpoints the development of MDR S. agalactiae in O. niloticus, affirming a public health risk. The emerging MDR S. agalactiae strains in O. niloticus frequently harbor the cfb, hylB, lmb, and cylE virulence genes, and the pbp1A, aac(6')aph(2''), tetM or tetK, and ermB or ermA resistance genes.