The significance of urine culture with mixed flora

Urine cultures that contain more than one organism are usually considered contaminated. The frequency with which such growth truly represents mixed infection is unknown. Surprisingly few studies have evaluated the clinical significance of polymicrobial growth from urine. Such significance was demonstrated in these studies either by recovering the same combination of microorganisms from blood and urine, in cases of urosepsis, or by the reproducibility of the same mixture of bacteria from sequential urine cultures. In certain clinical settings, polymicrobial bacteriuria is not only frequently significant but its overall clinical impact seems to be substantial. Bacteriuria associated with long-term catheterization, the most common nosocomial infection in American medical care facilities, is predominantly polymicrobial. Symptomatic urinary tract infection is a common outcome of such bacteriuria and has been associated with increased risk for bloodstream infections and excess mortality. Early species identification and antimicrobial susceptibility testing of each urinary isolate may be of paramount benefit to the care of these patients. We believe that in properly collected urine samples, multiple growth often represents true mixed infection and should therefore be completely evaluated.