Microbiology laboratory reports often contain technical language that can be confusing. One such phrase is “multiple organisms isolated, none in predominance.” This statement is shorthand for a specific finding that requires careful interpretation by a healthcare provider. It signals the presence of a diverse microbial population where no single organism has overgrown the others. This result suggests that no clear cause of a potential illness has been identified.
Deconstructing the Phrase: A Definition of Mixed Flora
The first part, “multiple organisms isolated,” indicates that the laboratory successfully grew more than one distinct type of microbe from the sample. This mixture can include various species of bacteria, yeast, or fungi. This finding suggests a complex microbial community, unlike the single-organism growth typically seen in a straightforward infection.
The second half, “none in predominance,” is quantitative and relates to the growth levels of the isolated organisms. Predominance means a single organism significantly outnumbers all others in the sample. When the lab reports “none in predominance,” the multiple organisms are growing in roughly equal or very low numbers. No single species has reached a threshold high enough to be considered the primary cause of a disease.
This overall state is often referred to as “mixed flora” or “polymicrobial growth.” This finding is common in samples collected from areas of the body that naturally contain diverse microorganisms, such as the skin, mouth, gut, or genital tract. Therefore, this result frequently reflects the body’s normal microbial environment rather than a single invading pathogen.
The Process of Culturing and Quantification
The laboratory arrives at the “none in predominance” conclusion through culturing and quantification. A sample, such as urine or a swab, is plated onto specialized nutrient-rich media in a petri dish. The dish is then incubated, allowing microbes to multiply and form visible clusters called Colony Forming Units (CFUs).
The goal of this method is quantification, which is achieved using calibrated inoculating loops that deposit a precise volume of the sample onto the plate. By counting the resulting CFUs and factoring in the volume plated, the lab calculates the concentration of organisms in the original sample, expressed as CFUs per milliliter (CFU/mL). This quantitative measure allows for an objective determination of the microbial load.
A true, uncomplicated infection is typically diagnosed when a single organism grows above a high threshold, often 100,000 CFU/mL, particularly in samples like clean-catch urine. When the lab observes three or more different types of colonies, but none reach that high concentration, the result is reported as “mixed flora, none in predominance.” This means the organisms are present in relatively low, balanced amounts. This observation confirms that no single microbe has multiplied out of control to meet the criteria for a disease-causing agent.
Clinical Interpretation: Contamination Versus Infection
Interpreting the clinical meaning of “multiple organisms isolated, none in predominance” depends heavily on the sample source. For many common specimens, particularly those collected from non-sterile sites, this finding often indicates contamination. During collection, the sample can easily pick up normal flora—harmless bacteria—from the surrounding skin or mucosal surfaces.
For instance, a midstream urine sample might pick up small numbers of bacteria from the genital area, resulting in a mixed, low-count culture. In this scenario, the result is not useful for diagnosis, as it reflects the body’s natural colonization. The physician will typically suspect the sample was compromised during collection. A high presence of epithelial cells (shed skin cells) in the sample also supports the interpretation that the culture is contaminated and not representative of a true infection.
However, in certain clinical situations, “mixed flora” can represent a true, complex polymicrobial infection. This is more likely when the sample is taken from a site that should be sterile, such as a deep wound, an abscess, or a peritoneal cavity. In these cases, multiple pathogens are genuinely working together to cause the illness, and the result is not due to collection error.
The patient’s symptoms are key in distinguishing between contamination and true infection. An asymptomatic patient with a mixed flora result is highly likely to have a contaminated sample. Conversely, a patient with severe symptoms, such as fever or localized pain, may truly have a complex infection requiring treatment. For example, in individuals with long-term indwelling catheters, multiple organisms can signal a true infection or significant colonization, even without a single dominant organism.
Next Steps for Diagnosis and Treatment
When a physician receives a report of “mixed flora, none in predominance,” the first action is to correlate the result with the patient’s clinical presentation. If the patient has no or mild symptoms, and the sample came from a non-sterile source, the result is usually dismissed as contamination. In this common scenario, the physician will likely ignore the culture or request a repeat sample using proper collection techniques to obtain a more reliable result.
If the patient’s symptoms are significant and strongly suggest an infection, the medical response shifts to addressing a polymicrobial issue. This is especially true if the sample came from a normally sterile site. The physician may proceed with empirical treatment using broad-spectrum antibiotics. These are designed to target a wide range of bacteria, covering the possibility that one or more isolated organisms are causing the disease.
In cases where a true infection is suspected, the laboratory may perform susceptibility testing on the isolated organisms to guide treatment. If the clinical picture remains unclear, further diagnostic steps may be ordered, such as imaging studies or additional, less invasive specimen collection methods, to confirm the infection source. The decision to treat is rarely based on the mixed flora result alone unless the patient exhibits strong clinical signs of severe illness.