Do Not Treat This Culture Result
A urine culture showing 2000 colonies/ml (2 × 10³ CFU/ml) of Micrococcus luteus in a female patient should not be treated, regardless of symptoms, as this represents contamination rather than true infection. 1
Why This Should Not Be Treated
Micrococcus luteus is Not a Uropathogen
- Micrococcus luteus is a skin commensal organism that is not recognized as a causative agent of urinary tract infections in any major guideline 1
- The most common organisms causing UTIs in females are Escherichia coli (causing approximately 75% of cases), Enterococcus faecalis, Proteus mirabilis, Klebsiella, and Staphylococcus saprophyticus 1, 2
- This culture result represents specimen contamination during collection, not true bacteriuria 3
The Colony Count is Below Diagnostic Threshold
- Even for true uropathogens, 2000 CFU/ml falls below the diagnostic threshold for UTI 1, 3
- While symptomatic women with acute cystitis may have infections with colony counts as low as 10² CFU/ml (100 CFU/ml) for recognized uropathogens like E. coli or S. saprophyticus, this applies only to established urinary pathogens, not skin contaminants 4, 5
- Asymptomatic bacteriuria requires ≥10⁵ CFU/ml in two consecutive samples in women, which this result does not meet 1
Clinical Decision-Making Algorithm
If the Patient is Asymptomatic:
- Do not treat asymptomatic bacteriuria in women without risk factors (strong recommendation) 1
- Do not obtain surveillance urine cultures 1
- Exception: Only screen and treat asymptomatic bacteriuria before urological procedures breaching the mucosa or in pregnant women 1
If the Patient is Symptomatic:
- Obtain a repeat urine culture with proper midstream clean-catch technique to avoid contamination 1, 3
- Diagnosis of uncomplicated cystitis can be made clinically based on dysuria, frequency, urgency, and absence of vaginal discharge 1
- If symptoms are typical for UTI, empiric treatment with first-line agents can be initiated while awaiting proper culture results: nitrofurantoin, fosfomycin, or trimethoprim-sulfamethoxazole 1, 3
- The repeat culture should grow a recognized uropathogen at appropriate colony counts to confirm infection 1
Critical Pitfall to Avoid
The most common error is treating contaminated cultures or asymptomatic bacteriuria, which leads to:
- Unnecessary antibiotic exposure and resistance development 1
- Eradication of potentially protective commensal flora 1
- Increased risk of subsequent true UTIs with resistant organisms 1
Treating non-pathogenic organisms like Micrococcus luteus provides no clinical benefit and only causes harm through antimicrobial resistance and adverse effects 1