Is a urine culture growing Lactobacillus considered contamination?

Lactobacillus in Urine Culture: Usually Contamination

Lactobacillus growth in urine culture is typically considered contamination rather than true infection, especially when isolated as part of mixed flora. 1 This is particularly true for female patients, where lactobacilli are normal commensals of the vaginal flora.

Assessment of Lactobacillus in Urine Culture

Factors suggesting contamination:

• Lactobacilli are normal commensals of the vaginal flora
• Mixed flora including Lactobacillus strongly suggests contamination 1
• Proper specimen collection techniques significantly reduce contamination rates (7.8-26.7% for midstream clean-catch vs. higher rates for other methods) 2, 1

Rare instances when Lactobacillus may be significant:

• Pure growth of Lactobacillus (>10^5 CFU/mL) in a properly collected specimen 3, 4
• Presence of urinary symptoms with no other identified pathogen
• Immunocompromised patients
• Patients with structural urinary tract abnormalities

Clinical Approach

For suspected contamination (most cases):

1. Evaluate specimen collection method - improper collection is the most common cause of contaminated specimens 2, 1
2. Assess for mixed flora - multiple organisms typically indicate contamination 5
3. Consider patient symptoms - asymptomatic patients with Lactobacillus growth should not receive antibiotics 1

For suspected true infection (rare):

1. Obtain a new specimen via improved collection technique:

   • Proper midstream clean-catch with periurethral cleansing 1
   • Consider straight catheterization for more reliable results 1
   • Suprapubic aspiration has lowest contamination rate (0%) but is more invasive 1

2. If repeat culture shows pure Lactobacillus growth with symptoms:

   • Consider treatment with β-lactam antibiotics (penicillin, ampicillin) 1
   • Avoid metronidazole as Lactobacillus species are typically resistant 1

Common Pitfalls

• Overtreatment of contaminated specimens: Leads to unnecessary antibiotic use and promotes antimicrobial resistance 1
• Misinterpretation of normal flora: Lactobacilli are beneficial organisms in the vaginal microbiome and help prevent UTIs by competing with pathogens 6, 7
• Failure to obtain proper specimens: Contamination rates vary significantly by collection method, with midstream clean-catch having lower rates than bag collection or diaper collection 2
• Delayed processing: Prompt processing or refrigeration of specimens prevents bacterial overgrowth that can lead to false results 1

Special Considerations

While Lactobacillus is generally a contaminant, rare cases of true Lactobacillus UTI have been reported, particularly with Lactobacillus delbrueckii 3, 4. These cases typically involve:

• Elderly patients
• Patients with urological abnormalities
• Immunocompromised individuals
• Pure growth in properly collected specimens

In these exceptional cases, treatment should be guided by susceptibility testing, with β-lactams being the typical choice for confirmed infections 1, 3.