Experimental pathogenesis and host immune responses of Enterococcus faecalis infection in Nile tilapia (Oreochromis niloticus)

Enterococcus faecalis, an opportunistic fish pathogen, has been recognized as a causative agent of hemorrhagic septicaemic disease in fish with high mortalities. Knowledge about host immune response following E. faecalis infection in fish is still limited. To address this issue, Nile tilapia (Oreochromis niloticus), were infected intraperitoneally with 0.2 mL of E. faecalis suspension containing 3 x 10(8) CFU (6 x 10(7) CFU/fish, 12 x 10(5) CFU/g). Experiment was conducted for 14 days. Fish were sampled at different time points for innate immune response, oxidant / antioxidant parameters, gene expression, and histopathology. Signs of septicemia were evident in infected fish and total mortalities were up to 80% in challenged fish compared to controls 11%. Post-infection, phagocytic and respiratory burst activities significantly increased at days 7 and 14, but bactericidal and lysozyme activities decreased at day 7, and days 7, 14, respectively. In terms of oxidant/antioxidant enzymes; MDA (malondialdehyde) level was significantly increased, but GSH (glutathione) level was decreased at days 7 and 14; SOD (superoxide dismutase) and TAC (serum total antioxidant capacity) were significantly decreased only at day 14 post-infection. However, Interleukin-8 (IL-8) and Tumor necrosis factor-alpha (TNF alpha) mRNA transcriptional levels showed no statistical changes. Histopathological and transmission electron microscope (TEM) examination revealed degenerative and necrotic changes in the heart and liver at all time-points post-challenge. These findings improve understanding the host immune response at cellular and molecular levels and show the potential to develop successful management strategies to manage this serious problem.