Virulence factors of microorganisms are described as the molecules that support the colonization of the host at the cellular level , and can be either secretory, associated with the membrane, or cytosolic. Virulence factors include, amongst others, proteins like toxins, adhesins, pili, proteases, hemolysins, or proteins from secretion systems, organized often as pathogenicity islands and regulatory pathways . Their function is to allow nutrient acquisition, to protect against the host's immune response, or to enable adhesion, invasion, and intoxication of host cells . The expression of these molecules is of outstanding importance for the health of humans, animals, and environmental systems. The transmission of virulence genes between or within species has the potential to create pathogen populations of enhanced virulence and favor the emergence of new diseases .