The etiology of the purple urine bag syndrome (PUBS), in which the urinary catheter bag of some elderly patients develops intense purple coloration, was studied. The purple was found to be a mixture of indirubin dissolved in the plastic and indigo on its surface. Six patients with PUBS were studied, and Providencia stuartii was isolated from the urine of five and Klebsiella pneumoniae was isolated from the urine of one. These strains produced indigo in 7.9 mM indoxyl sulfate-containing agar. One hundred and fifty isolates of 41 species of bacteria were tested for their ability to produce indigo on agar containing indoxyl sulfate, but only 17 of 27 strains of P. stuariWi, a single strain of Klebsiella pneumoniae, and Enterobacter agglomerans were positive. All of the indigo-producing bacteria had an indoxyl phosphatase with a pI of 6.4. This enzyme also possessed indoxyl sulfatase activity and was not present in strains that were unable to produce indigo from indoxyl sulfate. We conclude that PUBS results from the decomposition of urinary indoxyl sulfate to indigo and indirubin by bacteria (notably P. stuarti). As elderly catheterized patients often have high urinary indoxyl sulfate levels and colonization of their urinary tract with P. stuartii, the condition is most commonly seen in them.

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Purple urine drainage bags were found in 7 of 71 chronically catheterized elderly women. The purple staining of the bags is due to a violet discoloration (indirubin) of the plastic of the catheter bag and fine blue crystals of indigo in the urine. The colors are formed from the substrate indoxyl sulfate (indican) and all 7 patients had bacteria in the urine that would produce blue colonies on agar enriched with the urine (filter sterilized) of the patients involved. Organisms identified were Providencia or Klebsiella species. Indican excretion was higher in patients with purple urinary catheter bags than in controls.