Group B Streptococcus: Bacterial Classification and Characteristics
Group B Streptococcus (Streptococcus agalactiae) is a gram-positive, beta-hemolytic, facultative anaerobic bacterium that can be found as a normal commensal in the human genitourinary and gastrointestinal tracts. 1
Microbiological Characteristics
- Group B Streptococcus is encapsulated with a polysaccharide capsule, which serves as a major virulence factor, with 10 known serotypes based on immunological activity 1
- It is beta-hemolytic, meaning it causes complete lysis of red blood cells when cultured on blood agar plates 1
- GBS is a facultative anaerobe, meaning it can grow in both the presence and absence of oxygen, though it prefers oxygen when available 1
- The organism is commonly identified in clinical laboratories using standard bacterial identification procedures, including those recommended by the CDC 1
Epidemiology and Colonization
- Approximately 30% of the general population are asymptomatic carriers of Group B Streptococcus 1
- Vaginal colonization is most frequently caused by serotypes Ia, III, and V globally 1
- In pregnant women, GBS colonization rates are around 20% worldwide 2
- GBS colonization is often asymptomatic but can contribute to infectious morbidity in both pregnant/postpartum individuals and newborns 3
Clinical Significance
- GBS is the leading cause of infections in neonates with potentially high fatality rates 4
- It can cause both early-onset disease (EOD) in the first week of life and late-onset disease (LOD) between 7-89 days 2, 5
- In pregnant or postpartum individuals, GBS can cause postpartum endometritis and, rarely, maternal sepsis 2
- Invasive GBS infections in neonates can manifest as sepsis, pneumonia, meningitis, osteomyelitis, and/or septic arthritis 5
- Despite standardized clinical algorithms, GBS disease remains a significant public health concern 3
Laboratory Identification
- Standard culture techniques involve using sheep blood agar plates, where GBS typically produces beta-hemolysis 1
- While anaerobic incubation has been studied for some bacterial cultures, the evidence for its benefit specifically for GBS is not clearly established in the provided literature 6
- Molecular testing methods are also available for rapid identification of GBS 3
Prevention and Treatment
- Intrapartum antibiotic prophylaxis (IAP) is effective in reducing rates of early-onset GBS disease 2
- Penicillin is considered a core antibiotic for treating early-onset GBS infections 4
- Screening pregnant women at 35-37 weeks gestation with both rectal and vaginal swabs is recommended to identify carriers 6
- Maternal vaccination is being explored as a potential strategy to prevent both early and late-onset GBS disease 2
Group B Streptococcus differs from Group A Streptococcus (Streptococcus pyogenes), which is more commonly associated with pharyngitis and skin infections 6.