Skin flora refers to the communities of microorganisms that naturally reside on the human skin and external bodily surfaces. While the urinary tract is typically sterile, these microorganisms are commonly found in urine samples. Understanding this distinction is important for interpreting urine test results.
The Body’s Natural Skin Flora
The skin hosts a diverse community of microorganisms, including bacteria, fungi, and viruses, collectively known as skin flora. These microbes primarily live on the superficial layers of the epidermis and within hair follicles. They are generally harmless and can provide benefits to the body.
Many of these microorganisms are considered commensal, meaning they coexist without causing harm, or mutualistic, offering benefits to the host. Resident skin flora can help prevent the colonization of the skin by more harmful pathogens. They achieve this by competing for available nutrients, secreting antimicrobial substances, and supporting the skin’s immune system.
How Skin Flora Appears in Urine Samples
Urine samples are collected from the body’s exterior, an area abundant with skin flora. During the collection process, microorganisms from the skin surrounding the urethra can inadvertently enter the urine collection container. This leads to sample contamination.
To minimize this contamination, healthcare providers often recommend a “clean catch” urine collection method. This technique involves cleaning the genital area with antiseptic wipes and collecting a midstream urine sample. The initial flow of urine helps flush away some of the microorganisms from the urethra’s opening.
Despite careful clean-catch procedures, some skin flora may still find its way into the sample. Common skin bacteria like Staphylococcus epidermidis, Lactobacillus, and certain Corynebacterium species are frequently identified in contaminated urine cultures. Their presence usually indicates external contamination rather than an internal infection.
Distinguishing Contamination from Infection
Medical professionals differentiate between a contaminated urine sample and a urinary tract infection (UTI) by evaluating several factors. A key indicator is the type and quantity of bacteria identified in a urine culture. Contaminated samples often show a low colony count, typically less than 100,000 colony-forming units per milliliter (CFU/mL), of multiple different organisms.
In contrast, a true UTI is indicated by a high count, often 100,000 CFU/mL or more, of a single type of pathogenic bacteria, such as Escherichia coli. The presence of white blood cells (pyuria) and nitrites in the urine, detected through urinalysis, also suggests an infection. However, pyuria alone does not confirm infection, as it can indicate inflammation from other causes.
Patient symptoms provide another distinction. Individuals with a UTI experience symptoms such as painful urination, frequent urges to urinate, cloudy or foul-smelling urine, and sometimes lower back pain or fever. If skin flora is present in a urine sample but the patient has no such symptoms, it suggests sample contamination rather than an active infection.
What to Do About Skin Flora in a Urine Sample
When skin flora is identified in a urine culture without accompanying symptoms, medical professionals generally suspect sample contamination. A doctor might recommend repeating the urine test using a meticulous clean-catch technique to obtain a more accurate sample. This retesting helps confirm whether the initial result was due to external bacteria.
For asymptomatic bacteriuria, which is the presence of bacteria in urine without symptoms, treatment with antibiotics is usually not necessary for most individuals. Unnecessary antibiotic use can contribute to antibiotic resistance and may cause adverse drug effects. Healthcare providers weigh the culture results against the patient’s clinical presentation.
If symptoms of a UTI are present, or if there are specific risk factors, further investigation or treatment with antibiotics may be considered, even if the initial culture showed mixed flora or lower counts. This approach ensures that true infections are addressed while avoiding unnecessary medication for contaminated samples.