Assessing multi-drug resistance in Streptococcus agalactiae infecting farmed Nile Tilapia: Findings from Kerala, India.

Streptococcus agalactiae infections in tilapia are indeed a major concern in the global aquaculture industry, leading to significant economic losses. This study describes the isolation, virulence factors, pathogenicity and antibiotic susceptibility pattern of S. agalactiae in cultured Nile tilapia (Oreochromis niloticus) from aquaculture farms in Kerala, India. The diseased fish showed erratic swimming, lethargy, eye opacity, exophthalmia, darkened body, ascites and haemorrhages. Histopathological findings revealed hepatocytic vacuolization and meningitis. Molecular serotyping of the S. agalactiae isolates identified the serotype Ia. In terms of virulence characteristics, the S. agalactiae isolate obtained from tilapia sample had fbsA, cfB and pbp1A/ponA genes, and they were moderate biofilm producers. It is a matter of concern that the isolates were resistant to the tested macrolides, glycopeptides, chloramphenicol, tetracycline, sulphonamides, lincosamides, oxazolidinones and beta lactam group of antibiotics. Pathogenicity of the isolated strain was tested by experimental challenge through intraperitoneal injection of the isolated strain in Nile tilapia.100 %, 80 %, 40 % and 20 % mortality at doses of 1.0 × 10, 1.0 × 10, 1.0 × 10 and 1.0 × 10 CFU/ml, respectively were recorded in the challenged fish. Level of liver enzymes such as Aspartate aminotransferase (AST) and Alanine Aminotransferase (ALT), and glucose were significantly increased compared to that in control. The haematological indices such as RBC and haemoglobin counts were significantly reduced, while WBC count increased in the challenged fish. The haemolysis test on blood agar plate showed beta haemolysis (β). The emergence of multidrug-resistant pathogen S. agalactiae in tilapia farms in the state is an early warning for appropriate preventive measures to be taken to control their spread across farms as tilapia culture is widely popular in the state.