Isolation and Invitro Evaluation of Bacteriophage Therapy Targeting Streptococcus agalactiae in Nile Tilapia (Oreochromis niloticus): A Potential Approach to Sustainable Disease Management in Aquaculture

Abstract

Streptococcus agalactiae (Group B Streptococcus, GBS) poses a significant threat to Nile tilapia (Oreochromis niloticus) aquaculture, causing severe streptococcosis characterised by high mortality and economic losses. This study elucidates the pathogenesis of S. agalactiae through an integrated diagnostic approach and evaluates bacteriophage therapy as a sustainable alternative to antibiotic treatments. Clinical signs in infected tilapia, including erratic swimming, exophthalmia, and haemorrhagic lesions, were accompanied by systemic postmortem findings such as hepatosplenomegaly and ascitic fluid accumulation. Comprehensive identification of S. agalactiae isolates from pond water and diseased fish was achieved using morphological, biochemical, serological, and molecular techniques, confirming its role as the causative agent. Concurrently, three bacteriophages (STRA1, STRA2, STRA3) were isolated from aquaculture ponds and characterised by their lytic efficacy, host specificity, and environmental stability. These phages exhibited tailed morphologies, high lytic activity (up to 83.3% against S. agalactiae), and resilience under pond-like conditions (pH 4.0–10.0, 28°C–50°C), with STRA1 demonstrating efficacy against multidrug-resistant strains. In vitro assays revealed significant bacterial load reductions (e.g., STRA3: 1.03 × 106 CFU/mL vs. control: 6.13 × 106 CFU/mL at 72 h), though resistant mutants emerged at low frequencies (8.11–8.40 × 10−4). Suboptimal water quality parameters, including low dissolved oxygen (3.6 mg/L) and elevated iron (1321 μg/L), likely exacerbated infection severity. These findings underscore S. agalactiae's pathogenicity and position bacteriophage therapy as a promising, eco-friendly biocontrol strategy, warranting further in vivo validation to optimise its application in tilapia aquaculture.