Sources of Infection for Group B Streptococcus (GBS) in Blood Cultures
The most common sources of infection for a patient with one of two positive blood cultures for Group B Streptococcus (GBS) include the urinary tract, gastrointestinal tract, skin/soft tissue, and indwelling vascular catheters. 1, 2
Primary Sources of GBS Infection
Urinary Tract
- GBS is a common cause of urinary tract infections, with uropathogenic strains showing specific binding to bladder uroepithelium 3
- Urinary tract infections caused by GBS complicate 2%-4% of pregnancies, but can also occur in non-pregnant adults 4
- GBS in urine is considered a surrogate for heavy colonization and is associated with higher risk for invasive disease 4
Gastrointestinal Tract
- The gastrointestinal tract serves as the natural reservoir for GBS and is the likely source of colonization at other body sites 4, 1
- GBS can be a gut-resident pathobiont that may predispose to colonization at other body sites before causing invasive disease 1
- Colonization can be transient, chronic, or intermittent, with approximately 10%-30% of adults colonized with GBS in the gastrointestinal tract 4
Skin and Soft Tissue Infections
- GBS can cause skin and soft tissue infections that may lead to bacteremia 2
- These infections may serve as portals of entry for GBS to enter the bloodstream 2
Vascular Catheters
- Intravascular devices can be sources of GBS bloodstream infections 4
- According to CDC guidelines, catheter-related bloodstream infections can be caused by GBS, though they are more commonly caused by other organisms like coagulase-negative staphylococci 4
- For catheter-related infections, diagnostic criteria include positive blood cultures with the same organism from both the catheter and peripheral blood 4
Less Common Sources
Bone and Joint Infections
- GBS can cause spondylodiscitis (vertebral infections) in adults, particularly in those with chronic underlying diseases 2
- These infections can lead to secondary bacteremia 2
Meningitis
- Though rare in adults, GBS can cause meningitis that may be associated with bacteremia 5
- GBS meningitis in adults tends to occur in patients with severe underlying conditions and is associated with a high case-fatality rate 5
Risk Factors for GBS Invasive Disease
- Chronic underlying diseases increase risk for invasive GBS infections 2, 5
- Advanced age is associated with poorer outcomes in GBS infections 5
- Heavy colonization with GBS (defined as culture of GBS from direct plating rather than from selective broth only) is associated with higher risk for invasive disease 4
Diagnostic Considerations
- A single positive blood culture for GBS may represent true infection or contamination 4, 6
- According to CDC laboratory criteria, a recognized pathogen cultured from one or more blood cultures meets the definition of laboratory-confirmed bloodstream infection 4
- When evaluating a patient with GBS bacteremia, it's important to search for the primary source of infection through appropriate imaging and cultures 6
Clinical Approach to GBS Bacteremia
- Conduct thorough evaluation for potential sources including urinary tract, gastrointestinal tract, skin/soft tissue, and indwelling catheters 2, 6
- Consider echocardiography to rule out endocarditis in patients with persistent bacteremia 2
- Early adequate antimicrobial therapy is key to improving patient outcomes, especially in those with criteria for sepsis or septic shock 6
- For non-pregnant adults with GBS in urine, treatment is recommended only if they are symptomatic or have underlying urinary tract abnormalities 7
Remember that while a single positive blood culture for GBS may represent true infection, the clinical context and patient's symptoms should guide management decisions. Identifying and addressing the source of infection is crucial for effective treatment.