Common mechanism for repellents and attractants in bacterial chemotaxis.

The migrational response of Salmonella typhimurium away from compounds such as phenol, indole, acetic acid, and leucine occurs because the bacteria tumble less frequently while descending gradients of repellents. This contrasts with their response of tumbling less frequently while ascending gradients of attractants. The results of competition experiments suggest that repellents, like attractants, operate through specific receptors, and the algebraic additivity experiments indicate that repellents and attractants utilize a common memory mechanism for taxis.