2.4 Pathogen Pseudomonas fluorescens was previously isolated from naturally infected O. niloticus and identified according to the standard bacteriological tests. The pathogen was cultured in Tryptic Soya broth (TSB) (Oxoid) for 24 h at 37°C. The broth culture was centrifuged for 10 min at 3000 rpm. The supernatant was discarded and the pellets were re-suspended in phosphate buffered saline at pH 7.4 (PBS 7.4) and the optical density (OD) of the solution was adjusted to 0.5 at 456 nm, which correspond to 1 × 108 cells mL-1. This bacterial suspension was serially diluted using standard dilution technique with PBS 7.4 and used for the challenge experiment.