A urine culture (Urine C&S) identifies and grows microorganisms in a collected urine sample. When the result states “multiple organisms in urine,” the lab detected two or more distinct types of bacteria or fungi. This finding challenges healthcare providers, requiring careful interpretation to determine if it represents a genuine infection or a common collection issue.
Understanding Polymicrobial Growth in Urine
The presence of multiple organisms is called polymicrobial growth or “mixed flora.” This result is suspicious because most simple urinary tract infections (UTIs) are monomicrobial, caused by a single, predominant type of bacteria, usually Escherichia coli. The simultaneous growth of several different organisms suggests a deviation from the typical infection pattern. While the bladder contains a natural community of microbes, the growth of numerous different species with similar, low counts on a standard culture suggests contamination. This requires distinguishing a true infection from a contaminated sample.
Collection Errors and Sample Contamination
Contamination is the most frequent explanation for a urine culture showing multiple types of organisms. The skin, genitals, and distal urethra host diverse, harmless bacteria (commensal flora). If these external microorganisms are inadvertently introduced into the collection cup, they grow on the culture plate alongside any bacteria present in the bladder.
The standard prevention method is the “clean catch” technique, which involves cleansing the genital area and collecting a midstream sample. Failing to clean properly allows skin bacteria, such as Staphylococcus species or Corynebacterium, to enter the sample. In women, not separating the labia or allowing urine to pass over the vaginal area can introduce normal vaginal flora, like Lactobacillus species.
Another indicator of contamination is the presence of epithelial cells in the urine analysis, which are cells shed from the skin lining the urinary tract and genital area. A contaminated sample often shows multiple bacterial species, but usually with low colony counts for each one. This combination of low counts and diverse organisms is the laboratory’s primary signal that the sample was likely not collected correctly. Contamination does not reflect a urinary tract infection and does not require antibiotic treatment.
When Multiple Organisms Indicate a True Infection
Although contamination is common, multiple organisms can signify a genuine, complicated polymicrobial infection. True mixed infections occur in patients with underlying health issues that compromise the urinary tract’s defense mechanisms. These include individuals with structural abnormalities, such as kidney stones or obstructions that prevent complete bladder emptying.
A significant setting for true polymicrobial growth is in catheter-associated urinary tract infections (CAUTIs), which affect people with long-term indwelling urinary catheters. The catheter provides a surface for multiple species of bacteria to colonize and form a biofilm, leading to a complex infection. Patients with neurogenic bladder dysfunction or those who have undergone surgical procedures like an ileal conduit are also at higher risk. In these scenarios, the multiple organisms detected represent an active disease process requiring aggressive management.
Diagnostic Follow-Up and Treatment Decisions
To distinguish between contamination and a true infection, providers correlate laboratory findings with the patient’s clinical status. The presence or absence of symptoms (e.g., burning during urination, frequent urgency, or fever) is the primary factor. If the patient is asymptomatic, multiple organisms, especially with low bacterial counts, are almost always disregarded as contamination.
The quantitative analysis, or colony count, measures colony-forming units (CFU) per milliliter of urine. Contamination typically results in low counts, often below 10,000 CFU/mL, with no single organism dominating. A true infection, even polymicrobial, often presents with high counts (sometimes exceeding 100,000 CFU/mL) or a significant count alongside clear symptoms.
If contamination is suspected, the standard procedure is to repeat the urine culture, providing detailed instructions on the clean-catch method. Antibiotic treatment is initiated only if the patient has clear symptoms of a UTI and the culture confirms significant growth, either of a single organism or a high-count, dominant polymicrobial culture. Treating low-count mixed flora in an asymptomatic patient is avoided to prevent unnecessary antibiotic use and drug resistance.