In two experimental trials; florfenicol pharmacokinetics following a single dose oral administration at 15 mg kg⁻¹ fish body weight and biosafety through extended medicated feeding were studied in the rainbow trout, Oncorhynchus mykiss. The pharmacokinetic trial was conducted for 5 days, whereas the biosafety experiment lasted for a 30-day safety margin followed by a 20-day residual period analysis at 3, 5 and 10 times greater than the therapeutic dose 10 mg kg⁻¹ biomass day⁻¹. C_max µg kg⁻¹ calculated for florfenicol were found to be 5,360 in intestine, 2,890 in gill, 2,250 in kidney, 973 in liver and 273 in plasma, obtained at T_max of 16 h. Intestine had utmost area under the concentration–time curve _{(tissue/plasma)} of 13.83 h μg kg⁻¹ and a prolonged half life (t_1/2ß) of 28.62 h. The highest apparent metabolic rate value in the kidney (0.327) showed a high level of biotransformation of florfenicol to its metabolite florfenicol amine. The apparent distribution rate of florfenicol amine in muscle, in comparison to the parent drug florfenicol, indicated elimination of the medication mostly in the form of florfenicol amine with t_1/2 of 16.75 h. The biosafety of florfenicol orally administered to rainbow trout recorded effective feed consumption, physiological responses, drug tolerance and significantly low drug concentrations in muscle of rainbow trout, thus its usage at 10 mg kg⁻¹ fish body weight is recommended. In the study, the rapid absorption, greater bioavailability, enhanced dispersion, slower elimination and biosafety of the drug form a significant basis for the florfenicol and its metabolite florfenicol amine as a useful antibacterial agent in aquaculture.

The study focused on the evaluation of the probiotic potency of Bacillus velenzensis STPB10, isolated from the digestive tract of the hillstream fish Snow trout (Schizothorax richardsonii). Ten B. velezensis strains were identified through colony morphology, biochemical analysis, and 16 S rRNA methods. The representative laboratory strain STPB10, was subjected to various in vitro assessments including tolerance to pH, bile salt resistance, NaCl, temperature, biofilm, endospore formation, antibiotic susceptibility and antagonistic activity to determine its potential as a probiotic strain. B. velezensis STPB10 exhibited positive results for catalase, glucose, arginine, lysine, and endospore tests, while it yielded negative results for oxidative/fermentative, DNase, and urease tests. This test isolate was γ-hemolytic in nature and demonstrated growth across a temperature range of 15 to 35 °C. Moreover, it exhibited survival rates of 31, 39.7, 47, 63, and 75% at pH values of 2, 2.5, 3, 3.5, and 4, respectively, after 6 h of incubation. At a bile salt concentration of 0.3%, the bacterium exhibited a survivability of more than 99%. A strong biofilm production by B. velezensis STPB10 was detected in tryptone soya broth enriched with 0.45% glucose (p < 0.05). It also exhibited significantly greater adhesion to intestinal mucus (63.67%). Following exposure of the intestinal mucus to B. velezensis STPB10, the adhesion of A. hydrophila, A. veronii, (A) salmonicida and V. anguillarum to the mucus was notably reduced. It was susceptible to several antibiotics and produced an antagonistic effect against pathogenic bacteria Aeromonas salmonicida, Aeromonas veronii, Vibrio anguillarum, and Aeromonas hydrophila isolated from diseased fish. The pathogenicity of (B) velezensis STPB10 through intraperitoneal injection and immersion challenge at cell concentrations of 10⁸ and 10⁹ CFU mL⁻¹ revealed that the strain did not produce any pathogenic risk to common carp. These findings highlight the resilience and adaptability of B. velezensis STPB10 as a candidate probiotic in aquaculture.

In the present experiment, the attempt has been made to study the biosafety, toxicity, residue depletion and drug tolerance of graded doses of emamectin benzoate (EB) in juveniles of golden mahseer, Tor putitora as a model candidate fish for sport fishery and conservation in temperate waters through an extended medicated feeding. The graded doses of EB viz., 1× (50 μg/kg fish/day), 2 × (100 μg/kg fish/day), 5 × (250 μg/kg fish/day) and 10 × (500 μg/kg fish/day) were administered to golden mahseer juveniles through medicated diet for 21 days at water temperature of 18.6°C. The higher doses of EB did not cause any mortality during and 30 days after the end of medication period, but considerable variations in feeding and behavior were observed. Severe histological alterations observed after EB-diets (5 × and 10×) were vacuolation, pyknotic nuclei, melanomacrophage centre and necrosis in liver; Bowman’s capsule dilation, degenerated renal tubules in kidney; myofibril disintegration, muscle oedema, splitting of muscle fibres, migration of inflammatory cells in muscle; and abundant goblet cells, dilated lamina propria and disarrangement of mucosa in intestine tissues. The residual concentrations of EB metabolites Emamectin B_1a and B_1b were analyzed using muscle extracts and were found to be peaked during medication period followed by gradual depletion in post-medication period. The outcome of this study showed that the Emamectin B_1a residual concentration in fish muscle in 1×, 2×, 5×, and 10× EB treatment groups were 1.41 ± 0.49, 1.2 ± 0.7, 9.7 ± 3.3, and 37.4 ± 8.2 μg/kg at 30 days of post-medication period, respectively, which falls under the maximum residue limits (MRLs) of 100 μg/kg. The results support the biosafety of EB at recommended dose of 50 μg/kg fish/day for 7 days. As residue of EB is recorded falling within the MRL, no withdrawal period is recommended for golden mahseer.

In this study, six isolates of Chryseobacterium balustinum were characterized from diseased rainbow trout fingerlings. The virulence characteristics, pathogenicity, and antimicrobial susceptibility pattern of these isolates were investigated. The bacterium showed positive results for catalase, cytochrome oxidase, and aesculin hydrolysis, while negative results were obtained for DNase, gelatinase, methyl red, Voges-Proskauer's reaction, Simon citrate, Hydrogen sulphide, and starch hydrolysis. Amino acid metabolism analysis revealed the inability to metabolize arginine, lysine, and ornithine decarboxylase. Molecular characterization (16S rRNA) and phylogenetic analysis revealed the test isolates as C. balustinum, closely related to strain WLT (99.85% similarity) and C. balustinum P-27 (99.77%). Virulence assay indicated haemolytic activity and biofilm formation by the test bacterium. The challenge test confirmed moderate pathogenicity in rainbow trout and established Koch's postulates. The clinical manifestations of infection included fin erosion, eye and body surface haemorrhage, exophthalmia, and organ liquefaction. The minimum inhibitory concentrations of various antimicrobials ranged from 1 to > 256 µg mL⁻¹. The novel synthetic antimicrobial peptides exhibited MICs of 8 to > 256 µg mL⁻¹, suggesting a potential control method. These findings suggest that C. balustinum is an opportunistic pathogen with moderate pathogenicity in rainbow trout. Further research on the host–pathogen relationship is necessary to understand virulence characteristics and pathogenicity in aquaculture.