Rapid identification of Escherichia coli from routine urine specimens based on macroscopic criteria

In our laboratory, selected strains of Gram-negative rods from urine samples are identified as Escherichia coli on the basis of smell and morphology on lactose agar. To investigate the accuracy of this routine practice, 211 consecutive strains were tested in the urea-indole tube of the Three-tube method (3-TM), in the PGUA test detecting beta-glucuronidase activity and in the Simmons' citrate test, to select the strains that were non-E. coli. Additional 1022 strains were tested by the indole and urease tests of the 3-TM only. The identification of E. coli based on the macroscopic evaluation of colonies on lactose agar gave correct results for 99.1% of the strains. Citrobacter freundii was the most frequent cause of erroneous identification.