The gut ecosystem and enteric infections

The mammalian intestine encompasses a highly complex bacterial consortium, the microbiota. Generally, the host and its microbiota live in peaceful co-existence, despite only a single-layered epithelium separates the microbiota (10 e12 bacteria/g in the lower intestine) from sterile deeper tissues. Co-evolution of the intestinal microbiota with its host has resulted in state of mutual benefit: The host developed a fine-tuned innate and adaptive defense system which enabled it tolerating the dense bacterial population in its gut that, in return exerts beneficial effects on its host. Besides contributing to host nutrition, physiology and immunity the gut flora naturally protects from infection of a wide variety of bacterial pathogens.
Diarrheal diseases caused by enteropathogenic bacteria are a major health concern, both in developing and in industrialized countries. The commensal microflora, populating the intestine to high levels, contributes to protection against these infections (= colonization resistance) by so far poorly understood mechanisms. Enteropathogenic bacteria like Salmonella, pathogenic E. coli and Clostridium difficile can overcome this colonization resistance and out-compete the commensal microflora by the induction of gut inflammation. They ‘abuse’ the host's immune responses for their own benefit. Accordingly, the outcome of gut infections is influenced by different parameters: the host's health status and the microflora competing with the virulence strategies of the pathogen.