Distribution of enzyme-producing bacteria in the digestive tracts of some freshwater fish

Background. The information on gut microflora in fish is scanty and there is a paucity of knowledge regarding microbial enzyme activity in fish gastrointestinal tracts. Although some information is available on the enzyme-producing bacteria in fish digestive tracts, almost nothing is known about their distribution in different regions of the gut. In the present study, an attempt has been made to investigate the distribution of enzyme-producing microflora in the foregut and hindgut regions of seven culturable freshwater teleosts. Materials and methods. Isolation and enumeration of aerobic bacterial flora in the foregut and hindgut regions of the gastrointestinal tracts of seven freshwater teleosts of different feeding habits, namely rohu, Labeo rohita; catla, Catla catla; mrigal, Cirrhinus mrigala; bata, Labeo bata; orange-fin labeo, Labeo calbasu; Nile tilapia, Oreochromis niloticus; and climbing perch, Anabas testudineus, have been carried out. Microbial culture of the gut mucosa on selected nutrient media, following the enrichment culture technique, was done for bacterial isolation. Bacterial isolates were qualitatively screened on the basis of their extracellular enzyme-producing ability. The selected strains were further quantitatively assayed for amylase, cellulase and protease activities. Results. In general, bacterial population was lower in the foregut region of all the seven species of fish examined. Amylolytic strains were present in higher densities in the foregut region of orange-fin labeo and bata (12.20 × 103 CFU·g-1 gut tissue and  11.50 &times 103 CFU·g-1 gut tissue, respectively) in comparison to the hindgut region. The cellulolytic population exhibited maximum densities in the hindgut region of bata (7.20 × 103 CFU·g-1 gut tissue) followed by the foregut region of the same fish (5.50 × 103 CFU·g-1 gut tissue). Amylolytic and cellulolytic bacterial flora was not detected in both the fore and hindgut regions of climbing perch. Proteolytic bacterial flora was found in all the species of fish studied and the maximum count was observed in the hindgut region of bata (13.40 × 103 CFU·g-1 gut tissue), orange-fin labeo (9.00 × 103 CFU·g-1 gut tissue), Nile tilapia (8.30 × 103 CFU·g-1 gut tissue) and climbing perch (7.20 × 103 CFU·g-1 gut tissue). Minimum count of proteolytic bacterial flora was observed in the foregut region of all the fishes studied. Peak amylase and cellulase activities were exhibited by bacterial strains isolated from the foregut of orange-fin labeo (266.43 ± 0.15 U) and the hindgut of bata (64.01 ± 0.42 U), respectively. Maximum protease activity was exhibited by a strain isolated from the hindgut region of orange-fin labeo (44.33 ± 0.09 U), followed by the strains isolated from the hindgut regions of climbing perch (32.87 ± 0.12 U), bata (29.71 ± 0.11 U), and Nile tilapia (29.46 ± 0.11 U). Conclusion. The results of the present study indicate that there is a distinct microbial source of digestive enzymes apart from the endogenous sources in fish digestive tracts. The enzyme-producing bacteria isolated from the digestive tracts can be beneficially used as a probiotic while formulating aquafeeds, especially in the larval stages. However, further investigations are required to determine if the addition of such isolates to fish feeds do, in fact, provide some kind of benefit to the fish involved before advocating their use.