Effect of an Indigenous Probiotic (<i>Shewanella algae</i>) Isolated from Healthy Shrimp (<i>Penaeus monodon</i>) Intestine on <i>Clarias gariepinus</i>

Abstract

The efficacy of Shewanella algae , isolated from healthy shrimp ( Penaeus monodon ) intestine, as a biocontrol agent against Aeromonas hydrophila infection in Clarias gariepinus was evaluated. Spread plated technique was employed for bacterial isolation. The bacterial isolates were screened for in vitro antimicrobial activity against pathogenic Aeromonas hydrophila using agar well diffusion assay. The most active isolate (B2) was identified using phenotypic and molecular methods. The safety of the indigenous probiotic (B2) and the pathogenicity of the Aeromonas hydrophila on Clarias gariepinus were investigated. Feeding experiment was designed to evaluate the in vivo antibacterial efficiency of the most active strain using three groups of fish ( Shewanella algae - supplemented feed group, control infected and control non-infected groups). At the end of the experiment, five fish from each group were randomly selected for haematological and serum biochemical response investigation. The parameters analysed were haemoglobin, packed cell volume (PCV), red blood cell (RBC), white blood cell (WBC), lymphocytes, monocytes, total protein and albumin. A total of four genera belonging to Pseudomonas, Shewanella, Vibrio and Proteus which were isolated from shrimp intestine had antagonistic ability against Aeromonas hydrophila with zone of inhibition of 8.0 ± 0.0 mm, 10.0 ± 0.0 mm, 8.0 ± 0.0 mm and 8.0 ± 0.0 mm respectively. Molecular analysis conducted on the most active isolate (B2) revealed that it is closely related to Shewanella algae strain KJ-W32 gi: 385880930. The virulence test carried out on Clarias gariepinus using Aeromonas hydrophila which was injected intraperitoneally, showed 50% lethal dose value at 6.4 x 104 cfu ml-1. The result from the probiotic trial showed that fish fed with incorporated Shewanella algae strain KJ-W32 exhibited better resistance to pathogenic Aeromonas hydrophila than the control infected fish. The mortality rate during the in vivo challenge experiments observed for Shewanella algae - supplemented feed group, control infected and control non -infected fish groups were 0%, 50% and 5% respectively. Fish fed with supplemented diet showed better haematological and serum biochemical performance than those fed with normal fish feed. The study showed that Shewanella algae strain KJ-W32 is beneficial to Clarias gariepinus . Therefore, this indigenous bacterium could be used as effective biocontrol agent for management of aeromonasis in aquaculture.