Receiving a laboratory result that includes the term “mixed flora” can be confusing and often leads to concern about a serious infection. This term refers to the community of microorganisms, or microbiota, normally living on or within the body, and its interpretation is entirely dependent on the source of the sample and the clinical context. The seriousness of the finding is not uniform; what is an expected, harmless result in one type of sample can signal a severe infection in another.
Decoding the Term “Mixed Flora”
“Flora” describes the population of bacteria, fungi, and other microbes residing naturally in a specific area of the body. These microorganisms are often beneficial or harmless, forming a balanced ecosystem. A “mixed flora” result means the laboratory culture detected the growth of multiple different types of bacteria, typically three or more, within the collected specimen.
In a mixed flora report, none of the individual organisms are dominant in number, and they usually appear in low or moderate quantities. This finding is distinct from a positive culture that isolates a single, specific pathogenic organism, such as Escherichia coli in a urinary tract infection, which would be present in high concentrations. The report indicates a community of microbes rather than a single infectious agent.
Mixed Flora as a Normal Finding or Contamination
In many cases, a mixed flora result is not a sign of disease and can be attributed to either normal colonization or specimen contamination. The human body is naturally home to diverse microbial communities on surfaces like the skin, mouth, and gastrointestinal and urogenital tracts. Sampling from these sites will naturally yield a mixed flora result, which is considered normal colonization.
The more common scenario, especially in urine culture reports, is specimen contamination. During collection, bacteria from the skin, genital area, or surrounding environment can enter the sample. For example, in a midstream urine collection, microbes from the periurethral or vaginal area can easily contaminate the specimen, leading to a result that does not reflect a true infection. Contamination is suggested if the culture shows multiple organisms at low colony counts, typically less than 10,000 colony-forming units per milliliter (CFU/mL). The lab may also note the presence of epithelial cells, further suggesting external contamination.
When Mixed Flora Indicates a True Infection
A mixed flora finding becomes a serious concern when the specimen is taken from a site that should normally be sterile. Sites like the bloodstream, cerebrospinal fluid, or joint fluid are considered sterile, and the presence of any microorganism is a significant medical finding. In these sterile sites, a mixed culture can signal a serious, systemic infection like sepsis or meningitis.
A true polymicrobial infection involves two or more microorganisms actively working together to cause disease at the infection site. These infections often occur in deep-seated abscesses, where oxygen levels are low and multiple species can thrive collaboratively. Examples include perforated bowel infections, which involve aerobic and anaerobic bacteria from the gut, and aspiration pneumonia, caused by a mixture of microbes from the mouth and throat. In these cases, the mixture of organisms is directly responsible for the pathology, often leading to more complex and difficult-to-treat conditions.
Next Steps After a Mixed Flora Result
Interpreting a mixed flora report requires correlating the laboratory finding with the patient’s symptoms and physical examination. The report is rarely interpreted in isolation. A patient with a mixed flora result but no symptoms, such as fever or pain, is much more likely to have a contaminated sample. Conversely, strong symptoms like burning on urination or flank pain make the possibility of a true infection more likely.
If contamination is suspected, the most common next step is to repeat the culture, often emphasizing proper sterile collection technique. If the patient is symptomatic, the healthcare provider may proceed with antibiotic sensitivity testing on the isolated organisms to determine the most effective treatment. This targeted approach is reserved for cases where the clinical picture strongly suggests an active infection requiring treatment for multiple organisms.