In France and Europe the first source of financial impairment in bovine dairy cattle is mastitis, an inflammation of the mammary gland generally induced by bacteria. When Escherichia coli (E. coli) infects the udder, the gravity of the clinical signs are variable, but the infection is generally brief, and the pathogen rapidly cleared. In many cases, E. coli infection results in systemic reaction in the cow, that alters the udder functionality. However in some cases, it leads to death. Thus, this study tends to characterize and understand how some E. coli strains can trigger such diverse responses. Three E. coli strains responsible for mastitis showed, in vivo and in vitro, very distinct inflammatory responses, as for cellular recruitment and pro-inflammatory cytokines, chemokines and anti-microbial peptides production. In a mouse model, P4 and B117 induced a rapid and massive recruitment of leucocytes, especially neutrophils, the key cellular population for an efficient bacterial clearance. As for K08, it induced a milder inflammation associated with weaker production of chemokines and inflammatory cytokines. Intrinsic characteristics like resistance to bovine serum, acute multiplication in the milk and a weak stimulatory effect on mammary epithelial cells may explain how some E. coli strains like P4 and B117 induce hyper acute and acute mastitis. On the contrary, strains like K08 showing serum sensitivity, weak multiplication potential in milk and strong inflammatory responses would be implicated in subacute mastitis. This study underlines the importance of considering mastitis in terms of clinical signs, but also of taking into account the bacterial strains implied.