Aquaculture has been a growing activity for the last 20 years worldwide and this impressive development has been attended by some practices potentially damaging to animal health. The bacterial infections are considered the major cause of mortality in aquaculture. Among the common fish pathogens, A. hydrophila and Y. ruckeri as gram-negative and S. agalactiae, L. garvieae and E. faecalis as grampositive bacteria cause infectious diseases.