Chemical signaling between Salmonella, the intestinal microbiota, and the host. In the complex environment of the gastrointestinal tract there are opportunities for chemical signaling to take place between the microbiota, host cells, and the invading pathogen. The resident microbiota and S. Typhimurium may produce signaling molecules, which modulate the activities of the microbiota or pathogen. Examples of such signaling molecules include AI-1, AI-2, AI-3, and indole. Some of these signals may also modulate the activities of host cells such as AI-1, AI-3, and indole. The host can produce signaling molecules, which can also be detected by the microbiota and pathogens to modulate their activities. These host signals include catecholamine hormones such as norepinephrine.