How do we treat mixed urogenital flora greater than 100,000 colony-forming units (CFU)?

Treatment of Mixed Urogenital Flora >100,000 CFU

Do not treat mixed urogenital flora with antibiotics—mixed growth typically represents contamination or colonization rather than true infection, and treatment should be reserved only for cases with a single uropathogen, pyuria, and clinical symptoms. 1, 2

Key Diagnostic Principles

The presence of mixed flora fundamentally changes the interpretation of your urine culture:

• Mixed bacterial cultures are usually contaminated specimens, not true infections requiring treatment 3, 4
• True UTI requires both a single uropathogen at ≥50,000 CFU/mL (from catheterized specimen) AND evidence of pyuria or bacteriuria on urinalysis 1
• The threshold was lowered from 100,000 to 50,000 CFU/mL based on reexamination of evidence, but this applies only to single organism growth 1

Specific Organisms That Should Never Be Treated

Lactobacillus species, coagulase-negative staphylococci, and Corynebacterium species are explicitly not considered clinically relevant urine isolates, even at high colony counts 2

• These organisms are part of normal urogenital flora 2, 5
• Lactobacillus in particular is a protective commensal organism 5, 6
• Treating these organisms is potentially harmful and disrupts normal protective flora 2, 5

Clinical Algorithm for Mixed Flora >100,000 CFU

Step 1: Assess Clinical Context

• Is the patient symptomatic? (dysuria, urgency, frequency, fever) 2
• Is there pyuria on urinalysis? 1
• How was the specimen collected? (clean catch vs. catheterized vs. suprapubic) 1, 4

Step 2: Determine if This Represents True Infection

For clean-catch specimens with mixed flora:

• 67% represent contamination requiring no treatment 4
• Only 11% represent probable treatable mixed infection 4
• Request repeat specimen collection (preferably catheterized) 4

For catheterized specimens with mixed flora:

• 77% represent contamination or colonization 4
• Only 3% represent probable treatable infection 4

Step 3: Treatment Decision

Do NOT treat if:

• Patient is asymptomatic 2
• No pyuria present on urinalysis 1
• Mixed flora includes Lactobacillus, coagulase-negative staph, or Corynebacterium 2
• Clean-catch specimen without confirmatory catheterized sample 4

Consider treatment ONLY if:

• Symptomatic patient with fever and clinical illness 1
• Pyuria documented on urinalysis 1
• Repeat culture confirms same mixed organisms (suggesting true polymicrobial infection) 3, 4
• Long-term catheterization present (polymicrobial bacteriuria is common and may be significant in this specific population) 3

Important Caveats

The catheterized patient exception: Polymicrobial bacteriuria in long-term catheterized patients may represent true infection and has been associated with increased risk of bloodstream infections and mortality 3. However, this is a distinct clinical scenario from routine mixed flora cultures.

Avoid treating asymptomatic bacteriuria: This is explicitly harmful and contributes to antimicrobial resistance without benefit 2

Antibiotic therapy disrupts protective flora: Treatment eliminates protective Lactobacillus populations and allows uropathogenic bacteria to dominate the urogenital tract, potentially increasing recurrence risk 5

Recommended Action

Request a repeat urine specimen (preferably by catheterization or suprapubic aspiration) rather than empirically treating mixed flora 4. If repeat culture shows single organism >50,000 CFU/mL with pyuria and symptoms, then treat with 7-14 days of antimicrobials based on local sensitivity patterns 1.