Enzyme discovery causes a stink

There’s nothing quite like the sweet stench of success. Researchers have identified the enzyme that some bacteria use to produce the profoundly malodorous molecule 3-methylindole, also known as skatole (Nat. Commun. 2018, DOI: 10.1038/s41467-018-06627-x). “Our discovery explains why animal feces stink,” says geneticist and biochemist Yan Zhang of Tianjin University, part of the research team. Besides giving manure its aroma, skatole also lends a funky smell to some pork products, known as boar taint. People can discern skatole’s distinctive reek at concentrations of less than 1 ppb. Bacteria make skatole by degrading tryptophan, and the last step of this biochemical pathway relies on a previously unknown indoleacetate decarboxylase (IAD). In principle, identifying this enzyme could help develop an inhibitor to block its activity and curb skatole production in meat or dung, Zhang says. Zhang’s team pinpointed the enzyme by searching the genomes of two known skatole-making bacteria. When the researchers expressed this enzyme in Escherichia coli and then fed it some indoleacetate, they were rewarded by a truly noxious stench, which they confirmed as skatole by gas chromatography/mass spectrometry. The team then looked for the enzyme in a huge database of bacterial genomes and found 12 other bacteria that produce the same IAD. One of these was isolated from the crevice between tooth and gum in a human’s mouth. “It could explain some of the smell of halitosis,” Zhang says.