A urine culture is a laboratory test used to identify and quantify microorganisms in a urine sample, typically ordered when a urinary tract infection (UTI) is suspected. Most UTIs are caused by Gram-negative bacteria, primarily Escherichia coli, which accounts for up to 75% of uncomplicated cases. The appearance of Streptococcus species in a urine culture is less common and requires careful interpretation. Providers must determine if the bacteria represent a true infection or contamination from surrounding external flora.
Interpreting the Finding: Contamination or True Infection
Evaluating a Streptococcus-positive urine culture requires distinguishing between true infection (bacteriuria) and contamination during sample collection. The perineum and distal urethra are normally colonized with various bacteria, including Streptococcus species, which can inadvertently be introduced into the sample, even with a midstream clean-catch.
Laboratories use quantitative analysis, measuring the concentration of bacteria in Colony Forming Units per milliliter (CFU/mL), to make this distinction. For a standard clean-catch sample, the traditional threshold for confirming a true UTI is a single type of bacteria at \(10^5\) CFU/mL or greater. Lower counts (\(10^4\) to \(10^5\) CFU/mL) may still indicate infection, especially if the patient is symptomatic.
A low bacterial count, often below \(10^4\) CFU/mL, strongly suggests sample contamination, particularly when multiple different types of bacteria are present. Growth reported as “mixed flora” or three or more different organisms typically indicates the sample was compromised. A catheter-collected sample, which minimizes contamination risk, may consider counts as low as \(10^2\) to \(10^3\) CFU/mL to be significant.
Group B Streptococcus (GBS): Unique Significance
Streptococcus agalactiae, known as Group B Streptococcus (GBS), is the most clinically significant Streptococcus species found in urine. GBS naturally colonizes the gastrointestinal and genitourinary tracts of 20% to 30% of healthy adults without causing disease. Detection of GBS in the urine often indicates heavy colonization of the individual’s rectal or vaginal area.
The finding of GBS in a urine culture is particularly important for pregnant individuals, regardless of bacterial count or symptoms. GBS bacteriuria during pregnancy is a marker for extensive colonization. It is directly associated with an increased risk of vertical transmission to the newborn during delivery, which can lead to serious infant conditions like sepsis or meningitis.
For this reason, detecting GBS in the urine at any point during pregnancy automatically qualifies the individual for prophylactic intrapartum antibiotic treatment (PIAT). This protocol involves administering intravenous antibiotics during labor to reduce the bacterial load and minimize the risk of infection transmission. Other species, such as Streptococcus viridans, are often dismissed as contaminants unless present in high counts and the patient is symptomatic.
Treatment Approaches for Urinary Tract Strep
Once a true urinary tract infection caused by Streptococcus is confirmed, clinical management involves targeted antibiotic therapy. For non-pregnant individuals with a symptomatic Streptococcus UTI, first-line treatment typically involves penicillin derivatives. Streptococcus species, including GBS, remain universally susceptible to penicillin and ampicillin, making these preferred choices due to their narrow spectrum of activity.
The specific antibiotic selected and the duration of treatment must be guided by susceptibility testing. This laboratory process, known as an antibiogram, determines which antibiotics are effective against the isolated strain. Treatment for an uncomplicated infection is commonly 7 to 10 days. While penicillin is reliable, alternatives like clindamycin may be necessary for individuals with penicillin allergies, but susceptibility testing is crucial due to increasing resistance patterns.
Asymptomatic Bacteriuria (ASB)
A distinct consideration is the management of asymptomatic bacteriuria (ASB), where Streptococcus is found in high counts but the patient has no UTI symptoms. Outside of pregnancy, ASB caused by Streptococcus generally does not require treatment in otherwise healthy adults. Treatment may be warranted if the patient has underlying conditions, such as a compromised immune system or structural abnormalities in the urinary tract. Follow-up testing is often performed after treatment to confirm the bacteriuria has cleared.