The Pathobyte Series: Streptococcus agalactiae-From Maternal Carriage to Invasive Disease
What Is Group B Streptococcus (Streptococcus agalactiae)?
Group B Streptococcus (GBS) is a Gram-positive bacterium that commonly colonizes the human gastrointestinal and genitourinary tracts and can cause serious infections in newborns, pregnant women, older adults, and people with certain underlying medical conditions.
Although many individuals carry Streptococcus agalactiae without symptoms, the bacterium can become invasive under specific circumstances and cause diseases ranging from urinary tract infections to bloodstream infections, meningitis, and pneumonia. Group B Streptococcus remains one of the most important causes of neonatal bacterial disease worldwide and continues to be an important public health concern despite major advances in prevention strategies.
Understanding its biology, transmission routes, disease mechanisms, and prevention measures helps improve awareness of this medically significant pathogen.
What Are the Key Characteristics of Group B Streptococcus?
Microbe Profile
How Was Group B Streptococcus Discovered and Named?
Group B Streptococcus received its name through a bacterial classification system developed in the early twentieth century.
Streptococcus agalactiae was initially recognized as a cause of bovine mastitis, an infection of the mammary glands in cattle. During the 1930s, microbiologist Rebecca Lancefield developed a method for classifying streptococci according to carbohydrate antigens found in their cell walls. Based on this classification system, S. agalactiae was assigned to Lancefield Group B, leading to the commonly used name Group B Streptococcus.
Interest in the organism increased significantly during the 1960s and 1970s when researchers identified it as a leading cause of neonatal sepsis and meningitis. Since then, scientific research has expanded knowledge of its epidemiology, virulence factors, host interactions, and prevention strategies.
What Diseases Does Group B Streptococcus Cause?
Group B Streptococcus can cause invasive infections in newborns, pregnant women, older adults, and immunocompromised individuals.
The severity and type of disease vary according to age, immune status, and underlying health conditions.
Major Diseases Associated with Group B Streptococcus
Symptoms in Newborns
Newborns with invasive GBS disease may develop:
Difficulty feeding
Breathing problems
Fever
Low body temperature
Irritability
Reduced responsiveness
Seizures in severe cases
Symptoms in Adults
Symptoms depend on the site of infection and may include:
Fever
Chills
Fatigue
Shortness of breath
Painful urination
Redness and swelling of affected tissues
Joint pain
Early-Onset vs Late-Onset Disease
How Does Group B Streptococcus Cause Disease?
Group B Streptococcus causes disease through colonization, immune evasion, tissue invasion, and induction of inflammatory responses.
Many individuals carry the bacterium without developing symptoms. Disease occurs when the organism gains access to normally sterile sites such as the bloodstream, lungs, or central nervous system.
Key Steps in Pathogenesis
1. Colonization
GBS establishes itself on mucosal surfaces, particularly within the gastrointestinal and genitourinary tracts.
2. Adherence to Host Cells
Specialized surface proteins allow the bacterium to attach to epithelial tissues.
3. Immune Evasion
A polysaccharide capsule protects the organism from immune recognition and destruction.
4. Tissue Invasion
The bacterium can cross epithelial barriers and enter deeper tissues or the bloodstream.
5. Inflammation
Activation of the host immune response contributes to tissue injury and disease symptoms.
6. Central Nervous System Invasion
Certain strains possess mechanisms that enable them to cross the blood-brain barrier and cause meningitis.
Important Virulence Factors
Polysaccharide capsule
Surface adhesion proteins
Pili
Hemolysins
Enzymes involved in tissue invasion
How Is Group B Streptococcus Transmitted?
Group B Streptococcus is most commonly transmitted from colonized mothers to newborns during labor and delivery.
The bacterium frequently exists as part of the normal microbial community in healthy adults and is not generally considered highly contagious.
Major Transmission Routes
Risk Factors for Neonatal Transmission
Maternal GBS colonization
Premature birth
Prolonged rupture of membranes
Maternal fever during labor
Previous infant affected by GBS disease
How Is Group B Streptococcus Diagnosed?
Diagnosis relies on laboratory methods that identify the bacterium in clinical specimens.
Accurate diagnosis is important for both disease management and prevention of neonatal infections.
Common Diagnostic Methods
Culture Testing
Bacterial culture remains the reference standard for identification.
Maternal Screening
Rectovaginal swab cultures are commonly performed during late pregnancy.
Blood Culture
Used to identify bloodstream infections.
Cerebrospinal Fluid Analysis
Essential for diagnosing meningitis.
Molecular Testing
Polymerase chain reaction (PCR) assays can provide rapid detection.
Diagnostic Methods Comparison
How Is Group B Streptococcus Treated?
Antibiotics are the primary treatment for invasive Group B Streptococcus infections.
Most clinical isolates remain susceptible to beta-lactam antibiotics, making these medications the preferred treatment option.
Commonly Used Antibiotics
Penicillin
Ampicillin
Ceftriaxone (selected cases)
Factors Influencing Treatment Decisions
Age of the patient
Infection severity
Site of infection
Pregnancy status
Antibiotic susceptibility profile
Severe infections often require hospitalization and intravenous antibiotic therapy.
Treatment Goals
Eliminate infection
Prevent complications
Reduce mortality risk
Limit disease progression
How Can Group B Streptococcus Infections Be Prevented?
Prevention focuses on identifying maternal colonization and reducing transmission to newborns during childbirth.
Prevention programs have substantially reduced the incidence of early-onset neonatal GBS disease in many regions.
Prevention Strategies
During Pregnancy
Routine screening between 35 and 37 weeks of gestation
Risk assessment during prenatal care
During Labor
Administration of recommended antibiotics to colonized mothers
General Prevention Measures
Management of chronic illnesses
Timely treatment of infections
Appropriate healthcare infection-control practices
Vaccine Development
Researchers continue to develop vaccines targeting major GBS serotypes. Several vaccine candidates have demonstrated promising results in clinical studies, although no licensed vaccine is currently available.
What Are the Most Important Facts About Group B Streptococcus?
Key Takeaways
Group B Streptococcus is commonly carried by healthy individuals.
It is a leading cause of neonatal bacterial infections worldwide.
Disease occurs when the bacterium invades normally sterile tissues.
Mother-to-infant transmission during childbirth is the primary route of neonatal infection.
Screening and preventive antibiotic use have significantly reduced early-onset disease.
Antibiotics remain the cornerstone of treatment.
Vaccines are currently under development.
Reference
Raabe VN, Shane AL. Group B Streptococcus (Streptococcus agalactiae). Microbiology Spectrum. 2019.
Armistead B, Oler E, Adams Waldorf KM, Rajagopal L. The Double Life of Group B Streptococcus: Asymptomatic Colonizer and Potent Pathogen. Journal of Molecular Biology. 2019.
Vornhagen J, Adams Waldorf KM, Rajagopal L. Perinatal Group B Streptococcal Infections. Microbiology and Molecular Biology Reviews. 2017.
Patras KA, Nizet V. Group B Streptococcal Maternal Colonization and Neonatal Disease. Molecular Human Reproduction. 2018.
Shabayek S, Spellerberg B. Group B Streptococcal Colonization, Molecular Characteristics and Epidemiology. Frontiers in Microbiology. 2018.
Johri AK, Paoletti LC, Glaser P, et al. Group B Streptococcus: Global Incidence and Vaccine Development. Nature Reviews Microbiology. 2006.
Le Doare K, Heath PT. An Overview of Global GBS Epidemiology. Vaccine. 2013.