The present invention relates to a novel approach to prevent or treat mastitisboth in humans and animals through the use of mammal milk-derived microorganismsobtained from healthy hosts of the homologous species; to those new probioticmicroorganisms obtained from milk able to reduce infectious mastitis and themethod of screening used for their obtaining; to the use of these probiotic bacteriafor the prophylaxis or treatment against mastitis and other diseases; and finally,to compositions comprising these compounds.