The mention of “commensal flora” or “mixed flora” on a urine test result can often be a source of confusion, leading people to believe they have an infection that requires treatment. Commensal flora refers to the normal, healthy microorganisms that live on or in the body without causing disease. When these microorganisms appear in a urine sample, it is typically a benign finding that does not indicate a true urinary tract infection (UTI). This result, in most cases, is a simple byproduct of the collection process.
Understanding Commensal Flora in the Urinary System
The urinary system is designed with protective mechanisms, meaning that most internal structures are naturally sterile. The upper tract, including the kidneys, ureters, and bladder, is generally free of permanent bacterial colonies. The constant flushing action of urine helps maintain this sterile environment, preventing microbes from establishing a foothold.
However, the lower part of the urinary tract is not sterile. The distal urethra and surrounding genital skin naturally harbor a population of commensal bacteria. These organisms, which include species like Lactobacillus and Staphylococcus epidermidis, are part of the body’s normal micro-ecosystem. They live in a balanced state and do not cause illness in a healthy person.
The presence of this flora is actually beneficial, as these microbes can help prevent disease-causing pathogens from colonizing the area.
Why Commensal Flora Appears in Urine Samples
The reason commensal flora frequently appears in a urine culture is almost always due to contamination during the collection process. As the urine passes out of the body, it must travel through the distal urethra and past the surrounding skin, which is covered in these normal bacteria. If the collection technique is not precise, these harmless organisms are washed into the sample cup.
To prevent this contamination, a specific protocol called the “midstream clean-catch” technique is used. This method requires cleaning the area around the urethra with a sterile wipe before collection. The person then voids the first small amount of urine into the toilet, which flushes out any bacteria residing in the urethra’s opening.
Only the “midstream” portion of the urine is then collected in the sterile cup. When this cleaning and midstream collection sequence is skipped or performed incorrectly, the resulting sample is considered “dirty” because it contains a high concentration of the skin’s commensal flora. This failure to collect a clean sample is the primary reason laboratory reports mention mixed flora.
Interpreting Results: When Treatment is Not Needed
The fundamental rule for interpreting a urine culture is to correlate the laboratory findings with the patient’s symptoms. If a person is asymptomatic, the finding of commensal flora is rarely treated. This is considered asymptomatic bacteriuria, which is a common and usually harmless finding.
Laboratory analysis further supports this decision by looking at the type and amount of bacteria present. Contamination is typically indicated by a low colony count, often less than 10,000 colony-forming units per milliliter (CFU/mL), and the presence of multiple, mixed types of bacteria. A true infection, conversely, usually shows a high count, often greater than 100,000 CFU/mL, of a single, specific pathogenic organism.
If the report indicates “mixed flora of doubtful significance” or “normal urogenital flora” in an asymptomatic patient, the result is interpreted as contamination. Clinicians will generally ignore the finding or, if the initial collection was clearly compromised, may request a repeat sample with strict adherence to the clean-catch method. Treating this finding with antibiotics is avoided to prevent unnecessary side effects and antibiotic resistance.
Protocols for Diagnosing True Urinary Tract Infections
A true urinary tract infection requires a clinical diagnosis that links a patient’s symptoms with specific laboratory evidence of a pathogen. When a UTI is suspected, the laboratory looks for signs of inflammation, such as an elevated number of white blood cells (pyuria), in addition to bacterial growth. The diagnosis is strongly supported by the presence of a high concentration of a single type of uropathogen, such as Escherichia coli.
For a true infection, the treatment plan centers on targeting the identified pathogen with appropriate antibiotics. If a patient with UTI symptoms has a culture showing mixed flora, it may simply mean the sample was contaminated, and a repeat test is necessary to isolate the actual infectious agent.
In some cases, if symptoms are severe, a physician may begin treatment based on clinical judgment while awaiting definitive culture results. However, the goal remains to confirm a single, high-level pathogen, not to treat the mixed, low-level commensal organisms. This process ensures antibiotics are used only when a genuine, symptomatic infection is present.