To establish infection, pathogens should overcome host defenses and at the same time exploit the presence of host factors to gain access and persist in the host. To succeed in this this process, bacterial pathogens undergo a series of adaption processes which include the expression of virulence factors that enable the establishment and their persistence in the host. For several bovine mastitis pathogens, microbial virulence strategies are based on the attachment and internalization into host cells. Such a strategy allows bacterial pathogens to reach an intracellular environment where most of the host mechanisms are not effective. To achieve that, bacterial mastitis pathogens express factors that specifically bind to host factors (milk proteins, extracellular matrix proteins) present in milk that are taken and or produced by bovine mammary cells. Such a pathogenic mechanism allows a close contact between host cells and the invading bacterial pathogen that ends in the entry and persistence on the pathogen in the mammary gland.