The first visible-light-activated carbon-monoxide-releasing molecule (CO-RM) to exhibit a potent effect against Escherichia coli is described. The easily prepared tryptophan-derived manganese-containing complex (TryptoCORM) released 1.4 moles of CO at 465 nm, and 2 moles at 400 nm. A comprehensive synthetic, mechanistic and microbiological study into the behaviour of TryptoCORM is reported. The complex is thermally stable (i.e., does not release CO in solution in the absence of light), shows low toxicity against mammalian cells and releases tryptophan on photoinduced degradation, all of which point to TryptoCORM being therapeutically viable.