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Indole then becomes indoxyl sulfate, also known as indican, after it's metabolized by the liver.

When indoxyl sulfate and certain bacteria — specifically Klebsiella pneumoniae in the woman’s case, a urine culture showed — mix in the urinary bag after expelled from the body, a purple color can occur.

“When excreted in the urine, indoxyl sulfate can be broken down by bacterial enzymes to form indigo and indirubin, which are blue and red, respectively, creating the color purple when combined,” the doctors explained in the case study, noting other bacteria such as Pseudomonas aeruginosa, Providencia stuartii, and Escherichia coli can also produce a similar effect.

Mention may also be made of indican, the glucoside of the indigo plant; this is hydrolysed by the indigo ferment, indimulsin, to indoxyl and indiglucin.