The colonization of body surfaces, notably of the intestine, by a complex microbiota is generally highly mutualistic, where vital functions are provided by the commensal microbiota to the host, including the synthesis of vitamins, the degradation of complex polysaccharides into small chain fatty acids (which are essential for the maintenance of the intestinal epithelial barrier), and, finally, the outcompetition of pathogens that accidentally gain access to the body ("colonization resistance") (Chow et al. 2011; Backhed 2005). However, under certain conditions, such as changes of environmental factors in a genetically predisposed host, some of these normally symbiotic bacteria may act as pathogens and induce pathologies. Hence, the term "pathobionts" was coined for these bacterial species with ambiguous biological properties (Round et al. 2009).