Emergence of Aeromonas salmonicida subsp. masoucida MHJM250: unveiling pathological characteristics and antimicrobial susceptibility in golden mahseer, Tor putitora (Hamilton, 1822) in India.

Aeromonas salmonicida subsp. masoucida, designated as laboratory strain MHJM250, was characterized from a naturally infected farmed golden mahseer, Tor putitora. The infected fish exhibited clinical signs of erosion at the caudal fin and hemorrhage onx the ventral body surface. Molecular identification through 16 S rDNA and phylogenetic analysis revealed 100% similarity with a known strain A. salmonicida subsp. masoucida (MT122821.1). MHJM250 exhibited positive reactions for oxidase, catalase, esculin, MR-VP, O/F and utilized arginine and lysine. It also demonstrated siderophore activity, thrived at various NaCl concentrations, hydrolyzed gelatinase, skimmed milk and casinase. In vitro studies exhibited its hemolytic nature, significant biofilm production in glucose-rich tryptone soya broth and beta-hemolysis. MHJM250 didn't produce slime and was non-precipitated upon boiling. It showed crystal violet binding characteristics and auto-agglutination with relatively weak hydrophobicity (25%). In the challenge assay, intraperitoneal administration of MHJM250 to T. pitutora fingerlings at 108 CFU mL-1 resulted in pathogenicity with 3% mortality and mild hemorrhagic symptoms. Histopathological analysis revealed degenerative changes in gill, kidney, liver, muscle, and intestine samples. The bacterium displayed resistance to several antibiotics (µg/disc); ampicillin (10 µg), ampicillin/ sulbactam (10/10 µg), clindamycin (2 µg), linezolid (30 µg), penicillin G (10 µg) and rifampicin (5 µg) and varied minimum inhibitory concentrations against oxytetracycline, erythromycin and florfenicol. Transmission electron microscopy showed its rod-shaped structure with single polar flagellum and lophotrichous flagella. An investigation on the molecular basis for virulence factors of A. salmonicida subsp. masoucida MHJM250 may offer crucial understandings to formulate disease prevention and control strategies in aquaculture.