From the Guidelines
Mixed flora in a urine culture typically indicates contamination rather than a true urinary tract infection (UTI), and a repeat culture with proper collection technique is usually required rather than antibiotic treatment. When multiple bacterial species are identified in a urine sample, it usually means the specimen was contaminated during collection with bacteria from the skin, vagina, or external urethral area 1. This finding generally requires a repeat urine culture with proper collection technique rather than antibiotic treatment. To obtain a clean-catch midstream urine sample, patients should thoroughly clean the genital area, begin urinating into the toilet, then collect the middle portion of urine in a sterile container. For accurate diagnosis of a UTI, a culture should show a single predominant organism with colony counts greater than 100,000 CFU/mL 1. If symptoms of UTI are present despite mixed flora results, a repeat culture is recommended before starting antibiotics. In some cases, particularly in catheterized patients or those with recurrent UTIs, mixed flora may represent true polymicrobial infection requiring clinical correlation with symptoms and possibly antibiotic treatment based on susceptibility testing.
Some key points to consider when interpreting urine culture results include:
- The presence of a single predominant organism with colony counts greater than 100,000 CFU/mL is generally indicative of a true UTI 1
- Mixed flora with multiple bacterial species is often indicative of contamination, but may represent true polymicrobial infection in certain cases 1
- Proper collection technique is essential to minimize contamination and ensure accurate results
- Repeat culture may be necessary if symptoms persist despite initial mixed flora results
It's also important to note that the significance of mixed flora in a urine culture can vary depending on the clinical context, such as in catheterized patients or those with recurrent UTIs. In these cases, clinical correlation with symptoms and possibly antibiotic treatment based on susceptibility testing may be necessary 1.
From the Research
Significance of Mixed Flora in Urine Culture
- Mixed flora in urine cultures can represent true mixed infection, particularly in certain clinical settings such as long-term catheterization 2
- The clinical significance of mixed flora is often underestimated, and detailed laboratory analysis of such cultures can provide valuable information for patient management 3
- Mixed flora cultures can contain multiple bacterial isolates, with Enterococcus species and Escherichia coli being the most frequently isolated organisms 3
- The presence of mixed flora in preoperative urine cultures does not necessarily increase the risk of postoperative infection after ureteroscopy 4
Clinical Implications
- Early species identification and antimicrobial susceptibility testing of each urinary isolate can be beneficial for patient care, especially in cases of polymicrobial bacteriuria 2
- Clinicians may be more likely to diagnose and treat a UTI when provided with detailed microbial analysis of mixed flora cultures, including identity and antibiotic sensitivity of organisms 3
- Empiric antibiotic therapy and reporting of mixed cultures based on culture morphology without complete identification or antibiotic susceptibilities may represent a cost-effective solution to the mixed culture problem in the diagnosis and treatment of urinary tract infection 5
Microbiological Analysis
- Mixed flora cultures can contain a variety of bacterial isolates, with varying degrees of antibiotic resistance 6, 3
- Detailed laboratory analysis of mixed flora cultures can provide valuable information on the identity and antibiotic sensitivity of the isolated organisms 3
- The use of antibiograms can help guide antibiotic treatment in patients with suspected UTI, particularly in cases of mixed flora cultures 6