a, A diverse and non-disturbed microbiota confers resistance to colonization by enteric pathogens in the intestinal epithelium. b, Treatment with antibiotics decreases the diversity of the microbiota and leads to expansion of the C. difficile population. Toxins that are released from C. difficile (TcdA and TcdB) enter and damage the cells of the epithelium, which leads to inflammation (colitis) and cell death. c, Treatment with antibiotics also leads to an increase in the levels of free sialic acid (from the host) and succinate (from the microbiota) in the lumen of the intestine. Elevated sialic acid promotes the expansion of the S. Typhimurium population, which can lead to inflammation (gastroenteritis) if the bacterium invades the cells of the intestinal epithelium. Elevated levels of sialic acid and succinate further promote the expansion of the C. difficile population and the development of colitis and cell death.