Rarely, there can be purple discoloration of the urine, collecting bag, and tubing (the purple urine bag syndrome). The purple color of the urine is due to metabolic products of 
biochemical reactions formed by bacterial enzymes in the urine. Gastrointestinal tract flora break down the amino acid tryptophan into indole, which is subsequently absorbed into the portal circulation and converted into indoxyl sulfate. Indoxyl sulfate is then excreted into the urine, where it can be broken down into indoxyl if the appropriate alkaline environment and bacterial enzymes (indoxyl sulfatase and indoxyl phosphatase) are present. The breakdown products, indigo and indirubin, appear blue and red, respectively. Bacteria capable of producing these enzymes include Providencia spp, Klebsiella, and Proteus.
REFERENCES
- Peters P, Merlo J, Beech N, et al. The purple urine bag syndrome: a visually striking side effect of a highly alkaline urinary tract infection. Canadian Urological Association Journal. 2011;5(4):233-234. doi:10.5489/cuaj.10177.
- Lin C-H, Huang H-T, Chien C-C, Tzeng D-S, Lung F-W. Purple urine bag syndrome in nursing homes: Ten elderly case reports and a literature review. Clinical Interventions in Aging. 2008;3(4):729-734.