Colorimetric method for rapid determination of bacteriuria

Abstract

An inexpensive, rapid, and simple colorimetric test for detection of bacteriuria is described. This test does not require bacterial growth and has the marked advantage of being able to quantify bacteria, even when the organisms are present in the urine of bacteriuric patients who are being treated with antibiotics. The test is carried out with 1 ml of urine, which is processed through a 10-mm-diameter filter than entraps the bacteria on its surface. Safranine dye is passed through the filter to stain the bacteria and the filter fibers. A decolorizer, which removes the dye from the filter fibers but not from the bacteria, is then passed through the filter. If there are greater than or equal to 10(5) colony-forming units of bacteria per ml in the sample, the 10-mm filter disk manifests a pink to red color. If there are less than 10(5)colony-forming units of bacteria per ml, the filter disk remains white or becomes slightly yellow. The entire procedure has been adapted to a semiautomated instrument and the time required per test is less than 1 min. The results obtained on the test card are a permanent record to be filled with the patient's chart. The bacteria can be quickly classified as gram positive or gram negative by selective staining of a second milliliter of urine on the filter. Of 441 urine specimens tested, 430 (98%) were correctly classified as containing more or less than 10(5) colony-forming units per ml. A total of 62 urine specimens were positive by bacterial plating (greater than or equal to 10(5) colony-forming units per ml), and 59 were positive by the colorimetric test. Eight false-positives (colorimetric test positive, plate counts less than 10(5)) were encountered in patients (bacteriuric) being prescribed antibiotics. Removal of the antibiotics from these urine specimens, with subsequent replating of the samples, indicated the presence of greater than or equal to 10(5) colony-forming units of bacteria per ml in three representative cases tested, indicating that the results of the colorimetric tests were not false-positives but that the plate counts were low because of the inhibition of bacterial growth by the residual antibiotics present in the urine.