What are the possible reasons for the presence of Streptococcus (a type of bacteria) in a urine culture?

Reasons for Streptococcus Presence in Urine Culture

Streptococcus in urine culture most commonly represents contamination from periurethral, vaginal, or perineal flora, but can occasionally indicate true urinary tract infection, particularly in patients with underlying urologic abnormalities or when specific species like Streptococcus agalactiae (Group B Streptococcus) or Streptococcus gallolyticus subspecies pasteurianus are isolated. 1, 2

Primary Causes of Streptococcus in Urine

Contamination (Most Common)

• Specimen contamination during collection is the predominant reason for Streptococcus detection, especially when found as part of mixed flora with multiple organisms 1, 3
• Contamination rates vary dramatically by collection method: clean-catch midstream (7.8-27%), sterile bag collection (43.9-67.6%), and diaper collection (29-60.7%) 3
• The presence of nonhemolytic streptococci (viridans group) alongside epithelial cells strongly suggests contamination from normal skin and mucosal flora 3, 1
• Improper perineal cleansing increases contamination rates from 7.8% to 23.9% 4

True Urinary Tract Infection (Less Common)

When Streptococcus represents genuine infection, specific patterns emerge:

Species-Specific Considerations:

• Streptococcus agalactiae (Group B Streptococcus) can cause true UTI, with 87 documented cases showing single-organism growth and clinical significance 5
• Streptococcus gallolyticus subspecies pasteurianus is the most clinically significant urinary pathogen within the S. bovis group, accounting for 72% of S. bovis bacteriuria cases 2
• Streptococcus pneumoniae UTI is rare (<1% prevalence) but occurs predominantly in children with underlying urinary tract abnormalities such as ectopic kidney 6

Clinical Context Favoring True Infection:

• Underlying urologic disease is present in 37% of patients with S. bovis bacteriuria, making it more likely to represent true infection 2
• Patients with long-term catheterization frequently develop polymicrobial bacteriuria that includes streptococci and represents true mixed infection rather than contamination 7
• When the same Streptococcus species is isolated from both blood and urine cultures, this confirms true infection (documented in 9% of S. bovis bacteriuria cases) 2

Algorithmic Approach to Interpretation

Step 1: Assess Collection Method

• If bag-collected specimen in children: 60-67% contamination rate—never use to confirm UTI 1, 4
• If clean-catch without cleansing: high contamination likelihood 4
• If catheterized or suprapubic aspiration: contamination unlikely, consider true infection 4

Step 2: Evaluate Growth Pattern

• Mixed flora (≥2 organisms): contamination until proven otherwise 1, 8
• Single organism with significant colony count: proceed to Step 3 1
• Presence of epithelial cells: strongly suggests contamination 8

Step 3: Identify Streptococcus Species

• Viridans streptococci or nonhemolytic streptococci: likely contaminants 3
• S. agalactiae, S. gallolyticus subspecies pasteurianus, or S. pneumoniae: consider true infection 9, 5, 2

Step 4: Correlate with Clinical Findings

• Symptomatic patient (dysuria, frequency, fever) + pyuria (≥10 WBC/mm³): true infection more likely 2, 8
• Asymptomatic patient without pyuria: contamination or asymptomatic bacteriuria 8
• Underlying urologic disease, diabetes, or neurologic disease: increases likelihood of true infection 2

Step 5: Determine Action

• If contamination suspected: recollect using proper midstream clean-catch with cleansing or catheterization 1, 4
• If true infection likely: treat based on species identification and susceptibility testing 2
• If asymptomatic bacteriuria: no treatment unless pregnant or pre-urologic procedure 1

Critical Pitfalls to Avoid

• Never treat mixed flora from bag specimens without confirmation by catheterization or suprapubic aspiration, as 85% are false positives 1, 4
• Do not dismiss S. gallolyticus subspecies pasteurianus as a contaminant—it may indicate true UTI and warrants evaluation for underlying colorectal pathology 2
• Avoid processing specimens held at room temperature >1 hour or refrigerated >4 hours, as bacterial overgrowth can produce falsely elevated counts 1
• Do not treat asymptomatic patients based on contaminated cultures, as this leads to unnecessary antibiotic exposure 1
• When S. gallolyticus (not pasteurianus) is isolated, consider screening for endocarditis and colorectal cancer 2

Special Population Considerations

Catheterized Patients:

• Polymicrobial bacteriuria including streptococci is common and often represents true mixed infection rather than contamination 7
• Complete species identification and susceptibility testing of each isolate is warranted in symptomatic catheterized patients 7

Pediatric Patients:

• S. pneumoniae UTI should prompt urinary tract imaging to evaluate for anatomic abnormalities 6
• Uncircumcised male infants and female infants have higher UTI risk and require careful interpretation 8