Diversity analysis of hilsa (Tenualosa ilisha) gut microbiota using culture-dependent and culture-independent approaches

AIMS

The bacterial communities associated with the gastrointestinal (GI) tract are primarily involved in digestion, physiology, and the immune response against pathogenic bacteria for the overall development and health of the host. Hilsa shad (Tenualosa ilisha), a tropical anadromous fish, found predominantly in Bangladesh and India, has so far been poorly investigated for its gut bacterial communities. In this study, both culture-based and metagenomic approaches were used to detect intestinal isolates of hilsa, captured from both freshwater and seawater to investigate the community structure of intestinal microbiota.

METHODS AND RESULTS

Culture-dependent approach allowed to isolate a total of 23 distinct bacterial species comprising 16 Gram-negative, and 7 Gram-positive isolates, where Proteobacteria and Firmicutes were identified as the two most dominant phyla. While metagenomic approach explored a wide range of important GI bacteria, primarily dominated by Proteobacteria, Firmicutes, and Bacteroidetes, with Proteobacteria and Firmicutes, being the most abundant in freshwater and seawater samples, respectively.

CONCLUSIONS

A combination of these approaches provided the differential GI-associated bacterial diversity in freshwater and seawater hilsa with the prediction of overall functional potential.

IMPACT STATEMENT

The study explored the diversity of gut microbiota in hilsa, one of the most preferred nutritious dietary fish, captured from freshwater and seawater habitats, which may encourage to comprehend the composition of the gut microbiome in relation to the migratory behavior and polyunsaturated fatty acid profile of anadromous fish in general.

Broodstock development and captive maturation of hilsa (Tenualosa ilisha) in a brackishwater pond-based system

Successful captive broodstock development of clupeid fish, hilsa, is one of the major challenges for the artificial propagation of this economically valued fish. The present study provides novel insights into the pond-reared captive broodstock development of hilsa using artificial feed. In the present study, wild collected hilsa fry (weight 1.35 ± 0.13 g, length 53.84 ± 0.95 mm) were reared in grow-out ponds for 6 months followed by maintaining them as broodstock in other ponds using formulated feed in both cases. After 6 months of pond rearing, the average body weight and length of fish were 66.76 ± 4.53 g and 184.75 ± 3.65 mm, respectively. In broodstock ponds, hilsa subadults fed with formulated feed (crude protein 41.06%) ensured significant advancement in ovarian maturity with gonado somatic index of 9.09%, enhanced secondary yolk stage oocytes of 91.66% and enlargement of oocyte diameters up to 570 μm. The broodstock reached somatic growth such as average body weight and average body length of 383.80 ± 27.38 g and 339.33 ± 9.68 mm, respectively, with a condition factor of 1.06 ± 0.15. Gut content analysis revealed the better acceptability of the artificial feed. Lipid profiling of muscle and ovary during gonadal maturation revealed mobilization of fat from muscle to ovary and selective retention of some fatty acids (i.e., C14:0, C18:0, C20:5, C22:6 and C20:4) in the ovary, which might help in gonadal maturation. STATEMENT OF SIGNIFICANCE: Successful rearing of wild-caught hilsa juvenile in captive condition using formulated feed showed its potential as aquaculture species in low saline conditions. Advance stage of maturation in low saline captive conditions showed the possibility for induced breeding of hilsa, a high priced anadromous fish. Mobilization of fat from muscle to ovary and selective retention of some fatty acids in the ovary showed the scope of dietary manipulation for broodstock development of hilsa.

Microbiome patterns reveal the transmission of pathogenic bacteria in hilsa fish (Tenualosa ilisha) marketed for human consumption in Bangladesh

AIMS

This study conducted bacterial community, virulence and antibiogram profiling inside the hindgut and skin of freshly caught hilsa fish and those sold at markets.

METHODS AND RESULTS

The results of 16S rRNA-based high-throughput sequencing showed a higher number of bacterial genera in marketed fish samples than in fresh fish samples. The total operational taxonomic units, genus counts and diversity index were significantly higher (P > 0·05) in marketed fish, which also had abundant pathogenic bacterial groups. Skin samples had a lower profusion of pathogenic bacteria than gut samples. A total of 52 bacterial isolates from nine species were identified in this study, of which 25 were from a Chittagong market and 22 were from a Dhaka market, whereas only five were from fresh hilsa. The polymerase chain reaction amplification of 12 species-specific virulence genes in the 52 isolates, namely, aer, hly, chxA, toxB, rtxC, sfa, uge, norB, trx, toxA, ipaH, sigA and coa, indicated a high number of positive samples containing Vibrio cholerae, Aeromonas spp., Klebsiella pneumoniae, Escherichia coli and Staphylococcus aureus. Antibiogram profiling of these bacteria against 10 commercial antibiotics showed high-resistance patterns of the isolates against sulfamethoxazole, kanamycin, neomycin, ampicillin and tetracycline.

CONCLUSION

The results reveal the spread of multidrug-resistant bacteria in hilsa fish marketed for human consumption in Bangladesh.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study highlights the risk of spreading environmentally and clinically pathogenic bacteria in fish sold for human consumption in Bangladesh. Such bacteria come from aquatic pollution and poor handling, storage and transportation practices that may predispose fish to major outbreaks of infectious and waterborne diseases.