Is a urine culture with over 10,000 Colony-Forming Units (CFU) of Alpha streptococcus indicative of a urinary tract infection (UTI)?

Alpha Streptococcus at >10,000 CFU: Not Typically a UTI

Alpha streptococcus (viridans group streptococci) at >10,000 CFU/mL is generally considered a contaminant or colonizer rather than a true urinary tract infection, as these organisms are not recognized uropathogens and typically represent perineal/genital flora contamination. 1, 2

Why This Organism is Problematic

• Alpha streptococcus is not a clinically relevant urine isolate even at colony counts exceeding standard thresholds, as it represents normal skin and genital flora rather than a true uropathogen 1
• The most common true uropathogens are E. coli (>90% of cases in young healthy women), other gram-negative bacilli, and occasionally gram-positive organisms like Staphylococcus saprophyticus 3, 4
• Gram-positive organisms at low-to-moderate colony counts are significantly more likely to represent contamination than gram-negative organisms 5

Diagnostic Criteria for True UTI

To diagnose a legitimate UTI, you need both of the following 1, 6:

• Pyuria (≥10 leukocytes/mm³ or ≥10 WBC/HPF on microscopy)
• ≥50,000 CFU/mL of a recognized uropathogen from a properly collected specimen 3, 1, 6

The presence of bacteriuria alone without pyuria typically indicates contamination, asymptomatic bacteriuria, or colonization—none of which warrant treatment 1, 2

Critical Collection Method Considerations

• Specimen quality is paramount: The presence of elevated squamous epithelial cells (>10-20/HPF) strongly suggests contamination and explains discordant results 2
• Properly collected specimens via transurethral catheterization or suprapubic aspiration are most reliable 1, 6
• Clean-catch midstream specimens have higher contamination rates, particularly with skin/perineal flora like alpha streptococcus 3

Clinical Decision Algorithm

If alpha streptococcus is isolated at >10,000 CFU/mL:

1. Check for pyuria: If absent, this is almost certainly contamination—do not treat 1, 5
2. Assess specimen quality: Look for squamous epithelial cells as a contamination marker 2
3. Evaluate clinical symptoms: True UTI presents with dysuria, urgency, frequency, or systemic symptoms (fever, flank pain) 3
4. Consider recollection: If clinical suspicion remains high despite negative pyuria, obtain a properly collected specimen rather than treating empirically 2

When NOT to Treat

• Asymptomatic bacteriuria should never be treated, even with higher colony counts of recognized uropathogens, as treatment causes more harm than benefit 1, 2
• Colony counts <50,000 CFU/mL of non-standard organisms like alpha streptococcus, especially without pyuria, represent contamination 2, 5
• Mixed flora or multiple organisms typically indicate contamination in routine outpatient settings 7

Rare Exceptions Requiring Caution

Consider treatment despite atypical organisms only in 2:

• Severely immunocompromised patients
• Pregnant patients
• Patients with significant urological abnormalities or instrumentation

Even in these populations, the presence of pyuria remains essential to distinguish infection from colonization 5

Common Pitfalls to Avoid

• Do not treat based solely on colony count without confirming pyuria and appropriate clinical symptoms 1, 6
• Do not assume all bacteria in urine represent infection—the organism identity matters as much as the quantity 1
• Avoid ordering urine cultures for non-specific symptoms like altered mental status (especially in elderly/nursing home patients) or isolated urine odor/color changes, as this drives unnecessary antibiotic use 8
• Recognize that alpha streptococcus is fundamentally different from S. saprophyticus, which IS a recognized uropathogen in young women 4