Resident flora refers to the communities of microorganisms, including bacteria, fungi, and viruses, that naturally inhabit the human body without causing disease. They are a normal part of human biology, residing on surfaces like the skin and within mucous membranes. These microorganisms can perform various functions, some of which may contribute to overall health.
Understanding Resident Flora in Urine
Resident flora in urine cultures refers to microorganisms found in the lower urinary tract, specifically the distal urethra, external genitalia, and surrounding skin. The bladder and upper urinary tract (ureters and kidneys) are considered sterile in healthy individuals.
The flora present in the distal urethra and external areas includes various bacterial species. Common examples include certain Lactobacillus species, which are often found in the female genital tract and can extend to the urethra. Coagulase-negative Staphylococci, such as Staphylococcus epidermidis, are also frequently encountered as normal inhabitants of the skin and urethral opening. Corynebacterium species are another group of bacteria that commonly reside on the skin and can be part of the normal flora of the periurethral area.
These microorganisms are harmless when they remain in their natural environment and do not invade sterile tissues. They contribute to the local microbial balance, and their detection in a urine sample is a result of their normal presence during collection.
Differentiating Normal Flora from a Urinary Tract Infection
Distinguishing between normal resident flora and a urinary tract infection (UTI) in a urine sample involves several factors. Medical professionals consider the “colony count,” the number of bacterial units per milliliter of urine, as a primary indicator. A high colony count, exceeding 100,000 colony-forming units per milliliter (CFU/mL) of a single type of bacteria, suggests an infection. Conversely, a low colony count, such as less than 10,000 CFU/mL, especially with multiple types of bacteria, points towards contamination by normal flora or a non-significant finding.
The identification of specific bacterial species is also a telling factor. Pathogenic bacteria commonly associated with UTIs include Escherichia coli (E. coli), which accounts for the majority of infections, along with Klebsiella species and Proteus mirabilis. The presence of these known pathogens, even at lower counts in symptomatic individuals, can indicate an infection. In contrast, the isolation of common skin contaminants or non-pathogenic bacteria, such as coagulase-negative Staphylococci or Lactobacillus species, without a high colony count, is more consistent with normal flora.
Patient symptoms are an important consideration in diagnosing a UTI. Individuals with an infection experience symptoms like painful urination (dysuria), frequent or persistent urges to urinate, or lower abdominal discomfort. Fever and back pain can also indicate a more severe infection. The detection of bacteria consistent with normal flora in an asymptomatic individual, especially with a low colony count, means there is no active infection requiring treatment.
Ensuring Accurate Urine Culture Results
Proper urine sample collection is important to minimize contamination by resident flora and ensure accurate culture results. The “clean catch” or “midstream” collection method is widely recommended. This technique involves cleaning the external genital area with antiseptic wipes before urination begins. The initial stream of urine is then allowed to pass into the toilet, which helps flush out bacteria from the distal urethra and surrounding skin.
The middle portion of the urine stream is then collected into a sterile container. This midstream collection is less likely to be contaminated by surface bacteria compared to the initial flow. Improper collection, such as touching the inside of the cup or not adequately cleaning the area, can introduce normal flora from the skin or perineum into the sample. This contamination can lead to misleading results, indicating bacteria not truly causing an infection.
Delays in processing the urine sample after collection can also influence the accuracy of results. Bacteria present in the sample can multiply rapidly at room temperature, artificially increasing the colony count and misrepresenting the true bacterial load. Samples should be processed promptly or refrigerated if not immediately possible, to inhibit bacterial growth and preserve integrity.