*2.1.3 Coliform mastitis*

Coliform bacteria such as *Escherichia*, *Klebsiella,* and *Enterobacter* are a common cause of mastitis in dairy cows [37]. The most common species, isolated in more than 80% of cases of coliform mastitis, is *Escherichia coli* [38, 39]. *E. coli* usually infects the mammary glands during the dry period and progresses to inflammation and clinical mastitis during the early lactation with local and sometimes severe systemic clinical manifestations. Some reports indicated that the severity of *E. coli* mastitis is mainly determined by cow factors rather than by virulence factors of *E. coli* [40]. However, recent molecular and genetic studies showed that the pathogenicity of *E. coli* is entirely dependent on the FecA protein that enables *E. coli* to actively uptake iron from ferric-citrate in the mammary gland [41]. The severity of the clinical mastitis and peak *E. coli* counts in mammary secretions are positively correlated. Intramammary infection with *E. coli* induced expression and release of pro-inflammatory cytokines [42, 43]. Recently, it has been shown with mouse mastitis models that IL-17A and Th17 cells are instrumental in the defense against *E. coli* intramammary infection [44, 45]. However, the role of IL-17 in bovine *E. coli* mastitis is not well defined. The result of recent vaccine efficacy study against *E. coli* mastitis suggested that cell-mediated immune response has more protective effect than humoral response [46]. However, the cytokine signaling pathways that lead to efficient bacterial clearance are not clearly defined.
