Isolation of Aeromonas hydrophila infecting the fresh-water fishes in the Jammu region of Indian Himalayas

The bacterial infection, hemorrhagic septicemia is one of the causes of heavy economic losses in the fish farms across Jammu & Kashmir. Its principal causative agent is a Gram-negative bacterium, Aeromonas hydrophila. The present study was undertaken to characterize A. hydrophila isolated from freshwater carps being reared in various private fish farms of rural Jammu district under the active fish disease surveillance program. In this study, a total of 120 swab samples were taken from the suspected moribund fish (common carp and Indian major carps) for bacteriological analysis during November and December 2020. The modified Rimler-Shotts (RS) medium was used as a selective presumptive bacterial isolation medium. Under the microscope (100X) after Gram staining, typical red-colored, rod-like cells (bacilli) of A. hydrophila were observed. IMVIC biochemical identification procedures were employed for preliminary identification of the bacterial strain. Indole, citrate, and Voges-Proskauer were all found to be positive, but the methyl red test (MRT) was determined to be negative. For the molecular detection of isolated bacteria, a polymerase chain reaction (PCR) assay was used against two virulence factors of A. hydrophila: aerolysin (aer-2) and hemolysin (hyl-2). The results showed the band pattern of the amplified genes of aer-A and hyl-A in positive samples corresponding to the molecular weight of 416bp and 597bp, respectively. PCR detection showed that 110 samples (91.66%) were positive for the bacterium out of the total 120 swab samples. The antibiotic sensitivity test was performed against some commonly used antibiotics, and among them, Amoxyclav (AMC) was found to be the best medicine in combating this bacterial infection in-vitro.