Intrinsic and extrinsic light responses of Salmonella typhimurium and Escherichia coli.
Exposure to intense light in the region between 390 and 530 nm has been shown to have three effects on the motility of Salmonella typhimurium and Escherichia coli. Short pulses of light initiate continuous tumbling. Longer exposures to light induce smooth swimming, and prolonged exposures induced paralysis. The tumbling response is intimately connected with the chemical gradient-sensing apparatus of the bacterium and can be overcome by strong temporal gradients of attractant. Some mutants of S. typhimurium which are defective in the tumble-generating mechanism for chemotaxis are also unable to tumble in intense light. This intrinsic light effect can be mimicked by the addition of external dyes (the classical photodynamic effect), but it can be shown that the two phenomena are distinct. The extrinsic (photodynamic) effect can be inhibited by histidine or by anaerobic conditions, whereas the intrinsic effect is not. The observation that the extrinsic effect can also produce the three types of light responses listed above suggests a common pathway after an intial event on either an endogenous or an externally added photoreceptor.