Is Streptococcus (Strep) mitis considered a cause of Urinary Tract Infection (UTI)?

Is Streptococcus mitis Considered a UTI Pathogen?

Streptococcus mitis is not a recognized uropathogen and should generally be considered a contaminant when isolated from urine cultures, though rare cases of true infection have been documented.

Standard UTI Pathogens

The established uropathogens do not include S. mitis:

• E. coli, Proteus spp., Klebsiella spp., Pseudomonas spp., Serratia spp., and Enterococcus spp. are the most common organisms causing complicated UTIs 1
• E. coli remains the leading cause of bacteremia (60%), urinary tract infection (87.4%), and bacterial meningitis (43.7%) in febrile infants 1
• The typical spectrum includes E. coli, Klebsiella pneumoniae, Proteus mirabilis, Enterococcus faecalis, and Staphylococcus saprophyticus 2

S. mitis as an Atypical Finding

S. mitis is primarily an oral and upper respiratory tract commensal:

• S. mitis presence in urine is generally considered a contaminant rather than a true pathogen 3
• The organism is a member of the mitis group of streptococci, which are commensal species of the oral cavity and upper respiratory tract 4
• S. mitis has been previously reported in isolated cases of bacterial endocarditis and liver abscesses, but not typically in UTIs 3

When S. mitis May Represent True Infection

In exceptional circumstances, S. mitis can cause genuine UTI:

• Polymorphonuclear leukocyte phagocytosis on urine microscopy supports true infection rather than contamination 3
• Repeated isolation from properly collected urine specimens with consistent clinical symptoms (dysuria, urgency, frequency) suggests pathogenicity 3
• A documented case involved a 66-year-old male with recurrent positive cultures showing multidrug-resistant S. mitis/oralis that responded to vancomycin therapy 3

Clinical Approach

When S. mitis is isolated from urine:

• First assumption should be contamination, particularly with single isolation or low colony counts 3
• Repeat culture with careful collection technique (midstream clean-catch or catheterized specimen) to confirm 3
• Look for phagocytosed bacteria on urine microscopy as evidence of true infection versus colonization 3
• Consider alternative diagnoses and evaluate for more common uropathogens 1, 2
• If confirmed as true pathogen with clinical correlation, be aware of potential multidrug resistance including to penicillins, cephalosporins, and fluoroquinolones, with sensitivity typically to vancomycin and linezolid 3