In recent years, many herds have been affected by a new neonatal piglet diarrhoea (NNPD) unresponsive to conventional treatment and preventive measures. We have recently shown that NNPD is not caused by previously well-known infectious agents. Instead, the diarrhoea was found to be associated with intestinal colonization by E. hirae, a bacterium not previously associated with piglet diarrhea. Our findings have also been confirmed in Danish studies suggesting that E. hirae might be a new and widespread cause of diarrhea in newborn pigs. However, a definite, causal relationship is yet to be established. The aim of this study is to characterise the pathogenicity of E. hirae by genomic characterisation and by the development of an infectious challenge model. These studies will either support or dismiss that E. hirae is a cause of piglet diarrhoea and provide information that can be used to develop better diagnostics and ultimately better preventive measures against this common disease.