Isolation and enumeration of heterotrophic bacteria, lactic acid producing bacteria such as Lactobacillus species and fish pathogens such as Salmonella species, Vibrio species, Pseudomonas species and Aeromonas species from the gastrointestinal tract of mrigal, Cirrhinus cirrhosus has been carried out to find out their importance in the nutrition of the host fish. Amylolytic, proteolytic and cellulolytic bacteria were detected in the mrigal fish gut. Among specific enzyme producing bacteria, amylolytic and proteolytic bacteria were present at higher number with in the gut of mrigal fish, respectively selected intestinal isolates were analyzed for extracellular enzyme producing capacities. Amylase and protease activities were exhibited by bacterial the strains isolated from the mrigal fish gut. The information generated from the present investigation might contribute to the utilization of these extracellular enzyme-producing bacteria in commercial aquaculture.
    Lactic acid bacteria (LAB) produce many kinds of metabolites, which might affect the other microbes in the mrigal fish mid gut, lactic acid produced by both homolactic acid and heterolactic strains, which will reduce pH in the luminal contents in the stomach of neonatal piglets of fish. Isolated lactic acid bacteria were used as a probiotic in freshwater mrigal fish Cirrhinus cirrhosus against the most common mrigal fish gut pathogen Aeromonas species.  Lactic acid bacteria are highly appreciated as a biological enhancers-probiotics, because of beneficial effect of live microorganisms.
    Bacteriocins are proteinaceous antimicrobial compounds with a bactericidal or bacteriostatic mode of action against bacteria closely related to the producer strain, fish gut borne pathogen. Bacteriocin-producing bacteria are frequently isolated from various natural resources. For in this study, bacteriocins produced by lactic avoid bacteria such as Lactobacillus species which isolated from the gut of mrigal fish was described. Lactic acid bacteria (LAB) produce a variety of antagonistic factors that include metabolic end products, antibiotic-like substance and bactericidal proteins, termed bacteriocins. Bacteriocins of lactic acid bacteria have potential for use as food bio preservatives to control spoilage and pathogenic bacteria.
    The natural presence of lactic acid bacteria in fish may be of great interest in producing fermented fish products worldwide. The aim of this study was to characterization of lactic acid bacteria isolated from the gastrointestinal tract of mrigal fish (Cirrhinus cirrhosus) of commercial farms. The two lactic acid producing bacterial isolates were Gram-positive, produce gas from dextrose. These isolates were divided into genera of the Lactobacillus by sugar fermentation patterns. In this study characterization of eight fish pathogens and two beneficial lactic acid bacteria isolates through bio-chemical and carbohydrate fermentation tests have been reported. Eight of fish gut pathogens were Gram-negative, rod-shaped, identified as Salmonella species, Vibrio species, Pseudomonas species and Aeromonas species, while the remaining two being Gram-positive, rod-shaped and were found to be Lactobacillus species. The bacteriocin has broad range of antibacterial activity against major food-borne pathogens. Maximum bacteriocin production and activity was observed at the pH of 8. This study revealed the possibility of using bacteriocin as a food preservative and the Lactobacillus species strain as probiotic.