A urinary tract infection (UTI) is typically caused by the bacterium Escherichia coli. However, some infections are caused by less common organisms, such as the group known as Streptococcus viridans (or S. viridans). This group of bacteria is typically associated with infections in other parts of the body, most notably the heart. Understanding this atypical pathogen is important for proper diagnosis and successful treatment.
Characteristics of the Viridans Group Streptococci
The Viridans Group Streptococci (VGS) is a heterogeneous collection of Gram-positive bacteria, including species like Streptococcus mitis and Streptococcus sanguinis. They are part of the normal, commensal flora found predominantly in the mouth, upper respiratory tract, gastrointestinal, and genitourinary tracts.
On a laboratory test called a blood agar plate, these organisms partially break down red blood cells, which creates a greenish discoloration—hence the name “viridans.” The VGS generally lack specific components that contribute to high virulence. This low pathogenic potential explains why they rarely cause infection in a healthy individual with an intact immune system.
Mechanisms of Urinary Tract Entry and Infection
For S. viridans to cause a UTI, the bacteria must move from their typical location into the urinary tract, often requiring a disruption of the body’s natural defenses. The presence of S. viridans in a urine sample frequently represents contamination during collection, rather than a true infection. Genuine infections are often tied to predisposing conditions that compromise the urinary system’s integrity.
A significant route of entry is through instrumentation, such as the insertion of urinary catheters, stents, or recent urological procedures. These devices create a direct pathway for the bacteria to ascend the urethra and colonize the bladder or upper tract. Once the bacteria adhere to the urinary lining, they can establish an infection, especially in compromised tissue.
Underlying medical conditions greatly increase the risk of a true S. viridans UTI. These factors include structural abnormalities of the urinary tract and a weakened immune system. The infection may also originate from a distant site, such as the mouth or gut, entering the bloodstream and seeding the urinary tract (hematogenous spread). This systemic spread is a serious scenario often correlating with severe underlying disease or prior antibiotic use.
Identifying and Treating the Infection
Identifying a S. viridans UTI begins with a proper urine sample and subsequent culture to confirm the presence of the organism. A significant colony count combined with a urinalysis showing pyuria is necessary to confirm an active infection versus simple colonization. Because this group of bacteria is an atypical urinary pathogen, antibiotic susceptibility testing is essential for guiding treatment.
The VGS group exhibits variable resistance patterns, meaning that a drug effective against one species may not work against another, requiring specific laboratory analysis. For uncomplicated infections, first-line treatments often include beta-lactam antibiotics like amoxicillin-clavulanate or penicillin/ampicillin. Alternatives such as nitrofurantoin or trimethoprim-sulfamethoxazole may be used, depending on the susceptibility profile.
Complicated infections, such as those involving structural issues or severe symptoms, may require broader-spectrum antibiotics initially, sometimes in combination with other agents like gentamicin. Management involves addressing the underlying cause, such as removing or replacing an infected catheter or managing structural abnormalities. Treatment typically ranges from seven to 14 days, with longer courses necessary for complicated or deep-seated infections.
Strategies for Minimizing Recurrence
Minimizing the risk of recurrent S. viridans UTI requires managing predisposing factors. Patients with structural abnormalities, such as urinary stones or blockages, should have these issues corrected or managed to prevent bacterial pooling and adherence.
Strategies for prevention include:
- For patients requiring urinary catheters, strict adherence to procedural hygiene during insertion and meticulous ongoing care is necessary.
- Controlling systemic conditions like diabetes, as poor glucose control can impair immune function and create a favorable environment for infection.
- Maintaining good oral hygiene and treating existing dental issues to reduce the bacterial reservoir, since VGS commonly reside in the mouth.
- Screening and treating existing bacteriuria before individuals undergo invasive urologic procedures to prevent the introduction of the organism into deeper tissue.