Background. Fish gut bacteria may be used as probiotics for fish. Those occurring in the gastrointestinal (GI) tract of rohu, Labeo rohita (Hamilton, 1822), have not been sufficiently studied. This study was intended: to detect bacteria in the intestine of rohu by scanning electron microscopy (SEM); to evaluate the existence of heterotrophic, proteolytic, amylolytic, and cellulolytic bacteria in three different regions of the gut of rohu; and to identify more than 25% of the isolated gut bacteria by 16S rRNA. Materials and Methods. The GI tracts were removed and divided into three regions: foregut, midgut, and hindgut. Tissues used for SEM were fixed in glutaraldehyde, washed in heparinised saline, dehydrated in graded ethanol, fixed in amyl acetate, dried with liquid nitrogen, coated with gold, observed, and photographed. Homogenates of the intestinal segments were spread onto tryptic soy agar plates and selected nutrient media plates to determine heterotrophic, proteolytic, amylolytic, and cellulolytic bacterial populations, respectively. Of the 59 adherent bacterial strains isolated from the GI tract of rohu 16 isolates were tried for identification by 16S rRNA genes. Results. Bacteria were observed to adhere to the gut enterocyte surfaces. Population level of enzyme-producing bacteria was higher in the midgut and hindgut regions than that observed in the foregut region. Furthermore, dense amylolytic and cellulolytic bacterial population in comparison to the proteolytic population was noticed in the GI tract of L. rohita. Highest density of the cellulolytic bacterial population in the hindgut region may indicate that fermentative degradation occurs in this part of the GI tract. Eleven adherent bacterial strains belonged to bacilli, 2 strains to Pseudomonas, 1 strain to Aeromonas, 1 strain was most closely related to Enterobacter, while 1 strain showed < 97% 16S rRNA sequence similarity in BLAST program was treated as unknown. Conclusion. Autochthonous bacteria were present in the GI tract of rohu and that the autochthonous microbiota possess enzymatic activity that might be beneficial for the fish.
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