Is a urine culture result showing 33,000 colonies of Enterobacter Aerogenes, sensitive to Amikacin, significant for a urinary tract infection?

Is 33,000 CFU/mL of Enterobacter aerogenes Clinically Significant?

Yes, 33,000 CFU/mL of a single organism (Enterobacter aerogenes) is likely clinically significant if the patient has urinary symptoms and the specimen was collected appropriately, particularly if obtained by catheterization. 1

Colony Count Interpretation

The traditional threshold of 100,000 CFU/mL is not absolute, and lower colony counts can indicate true infection in symptomatic patients:

• For catheterized specimens: Colony counts as low as 10,000 CFU/mL may be clinically significant 1
• For midstream clean-catch specimens: Colony counts between 25,000-50,000 CFU/mL may represent significant infection, especially when obtained via catheterization 1
• Current guidelines acknowledge that "lower CFU counts can still indicate significant infections in symptomatic patients" even though 100,000 CFU/mL remains the historical standard 2

Key Factors Supporting True Infection

The presence of a single organism (Enterobacter aerogenes) rather than multiple organisms strongly supports true infection rather than contamination 1. This is critical because:

• Mixed flora with multiple organisms typically indicates contamination 3
• Single-organism growth, even at lower colony counts, is more likely to represent genuine infection 1

Clinical Context Required

You must evaluate the following before treating:

• Urinary symptoms: Dysuria, urgency, frequency, flank pain, or fever 4
• Urinalysis findings: Presence of pyuria (WBCs) and/or bacteriuria 4
• Collection method: Catheterized specimens are more reliable than midstream clean-catch; bag-collected specimens have extremely high false-positive rates and should never be used for treatment decisions 4
• Patient risk factors: Diabetes, anatomic abnormalities, immunosuppression, or indwelling catheters 2

Treatment Decision Algorithm

1. If symptomatic + pyuria on urinalysis + proper collection method: Treat as significant UTI 1, 4
2. If asymptomatic: Do NOT treat—this represents asymptomatic bacteriuria, and treatment leads to inappropriate antibiotic use and resistance 4
3. If symptoms persist but unclear diagnosis: Repeat culture with proper collection technique 1

Amikacin as Treatment Option

Since your isolate is sensitive to amikacin:

• Amikacin is FDA-approved for serious Gram-negative infections including Enterobacter species 5
• Amikacin demonstrates high susceptibility rates (>90%) against urinary pathogens and is recommended for complicated UTIs in recent surveillance data 6
• Clinical success rates of 97.2% have been documented for lower UTIs caused by resistant Gram-negative organisms treated with intramuscular amikacin 15 mg/kg/day for 10 days 7
• Amikacin is particularly valuable for ESBL-producing organisms or multidrug-resistant pathogens 7, 6

Critical Pitfall to Avoid

The most common error is treating asymptomatic bacteriuria—verify that your patient has actual UTI symptoms before initiating antibiotics 4. Asymptomatic bacteriuria should not be treated except in pregnancy or before urologic procedures 2.