**2. Materials and methods**

A *microbial agent* is a microorganism that is capable to cause disease in animal. *Pathogenicity* is the capability to induce disease in an animal. Pathogenic bacteria express their pathological effect by means of their *infectious aggressiveness* (*virulence*), a term which refers to the level of pathogenicity of the microbial organism. Hence, the factors of virulence of a pathogen are any of its genome, biochemical characters, and structure that give the ability to induce disease in an animal. Domestic livestock have always suffered from a wide range of bacterial diseases. As livestock are concentrated in larger numbers, the problems of major epidemic have become more severe.
