Pathological analysis and antimicrobial susceptibility of Chryseobacterium balustinum RTFCP 298 isolated from diseased rainbow trout, Oncorhynchus mykiss.

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.

Biosafety, histological alterations and residue depletion of feed administered anti-parasitic drug emamectin benzoate in golden mahseer, Tor putitora (Hamilton, 1822) as a model candidate fish for sport fishery and conservation in temperate waters.

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 B1a and B1b 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 B1a 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.

Improvement in the growth status and carbohydrate utilization of Labeo rohita (Hamilton, 1822) fingerlings with dietary supplementation of chromium picolinate.

A 60-day feeding experiment was conducted to evaluate the effects of dietary chromium (Cr) on carbohydrate utilization and growth performance of Labeo rohita fingerlings. Fishes were fed with four high carbohydrate (53%), isonitrogenous (crude protein 35%), and isocaloric (415 Kcal, 100 gm-1) experimental diets containing different levels of dietary chromium picolinate (Cr-Pic) viz.0, 400, 800, and 1200 µg kg-1 diet. Weight gain (WG%), specific growth rate (SGR), feed efficiency ratio (FER), and protein efficiency ratio (PER) were significantly increased at 800 µg kg-1 diet chromium supplementation (P < 0.05). Cr-Pic supplementation (800 µg kg-1) also significantly (P < 0.05) enhanced the protein: DNA ratio in muscle, while DNA: RNA and DNA: tissue ratios were significantly (P < 0.05) decreased indicating higher growth. Significantly higher amylase, protease, and lipase activities were recorded in 800 µg Cr-Pic kg-1 diet fed fishes (P < 0.05), while any of the experimental groups showing no significant (P > 0.05) change in hexokinase activity, indicating normal glycolysis in all. Furthermore, significant (P < 0.05) decrease of glucose-6-phospatase activity in 800 µg Cr-Pic kg-1 diet fed group, showcasing an evidence for protein-sparing action with Cr-Pic supplementation. Significantly (P < 0.05) higher serum insulin and liver glycogen in 800 µg Cr-Pic kg-1 diet fed fishes denote an improvement in carbohydrate metabolism. However, significantly (P < 0.05) higher ATPase and SOD activities were also observed when chromium supplementation was more than 800 µg kg-1 diet, indicating stress at higher level. The present study indicates that growth and carbohydrate utilization can significantly (P < 0.05) be improved by feeding the L. rohita fingerlings with Cr-Pic (800 µg kg-1 diet) supplemented diet in laboratory condition.

Characterization and pathogenicity of Aeromonas veronii associated with mortality in cage farmed grass carp, Ctenopharyngodon idella (Valenciennes, 1844) from the Central Himalayan region of India.

In the study, Aeromonas strains (n = 12) were isolated from moribund grass carp fry reared in the cage culture unit from the Central Himalayan region of India. They were identified as Aeromonas veronii, by biochemically and 16S rRNA analysis. The experimental bath infection of grass carp fry was performed using A. veronii GCAFBLC 228, one of the 12 isolates at cell concentrations 106 and 108 CFU mL-1. The infected fry showed varied behavioural characteristics followed by tail rot, black pigmentation and hemorrhage in the body 48-96 h post infection. The post bath challenged demonstrated maximum mortality (23%) at cell concentration 108 CFU mL-1 during 10th and 12th day. Histopathology revealed hypertrophy, hyperplasia, fusion of gill lamellae, detachment and epithelial cell detachment in gill, swelling of hepatocytes, granular deposition in liver and tubular degeneration and yellow pigmented macrophage aggregates in the kidney. The in vitro assays for virulence traits recorded that A. veronii GCAFBLC 228 was ß-haemolytic having strong cell surface hydrophobicity (CHS) characteristic (> 50%), precipitated after boiling, produced slime, non-suicidal and bound to crystal violet. The antibiogram showed that the strain was susceptible to ciprofloxacin (5 µg), cefotaxime (30 µg), ceftazidime (30 µg), cefoxitin (30 µg), ceftriaxone (30 µg), chloramphenicol (30 µg) and tetracycline (30 µg). Negative staining transmission electron microscopy revealed presence of the lateral flagellum-like structure and cell adherence possibly could be correlated with the pathogenicity of A. veronii GCAFBLC 228. The further investigation is warranted to study the transmission, pathogenesis and epidemiology of A. veronii GCAFBLC 228 to develop the best health management practice for cage farmed fish.