The most important fish pathogen is Aeromonas hydrophila (syn. A.liquifaciens, A.formicans), and this group is often referred to A.hydrophila complex. The prevalence of bacterial pathogen occurs in organically polluted waters. Predisposing risk factors include high temperature, overcrowding, organic pollution, and hypoxia. Motile aeromonads often invade skin wounds, commonly with water molds or ectoparasites. A.hydrophila is often associated with the protozoan Epistylis in causing widespread epidemic skin lesions known as red-sore disease. They are opportunistic pathogens of many immune compromised poikilotherms and homeotherms. A.hydrophila, Pseudomonas sp, Enterobacter sp., Serratia and Micrococci are some of the organisms highly abundant in the infected fishes. Among this, the maximum occurring A. hydrophila has been chosen for further microbiological and immunological studies. The direct bacterial count showed the maximum of 37.1 x 106 CFU/ml at 48th hour whereas by the total plate count the maximum bacterial count of only 21 x 106 CFU/ml was observed at 48th hour. The LD50 value was calculated as 3 x 106 cells for 16 gm average weight of experimental fishes. In the present study, potential bacterial pathogen like A.hydrophila was isolated and microbial characterizations were carried out to find out the impact of water pollution on the bacterial pathogenic potential.
