Streptococcus agalactiae, commonly recognized as Group B Streptococcus (GBS), is a bacterium that can cause infections in vulnerable populations. While often residing harmlessly in the body, its presence can lead to serious conditions under specific circumstances. Understanding its fundamental characteristics is important for recognizing its impact.
Basic Structure and Classification
Streptococcus agalactiae is a Gram-positive bacterium, appearing purple with Gram stain, a common laboratory diagnostic technique. Its morphology is cocci, a spherical shape. These cocci typically arrange themselves in chains, frequently in pairs or very long chains. Individual cells usually measure between 0.6 and 1.2 micrometers in diameter.
The bacterium is non-motile; it does not move, and it is also non-spore-forming. Its classification as Group B Streptococcus stems from the Lancefield serological grouping system, which identifies a specific carbohydrate antigen, the ‘B’ antigen, present on its cell wall. This grouping system helps microbiologists broadly categorize streptococcal species. S. agalactiae is further subclassified into ten serotypes (Ia, Ib, II-IX) based on the immunological reactivity of its polysaccharide capsule.
Growth and Metabolic Features
Streptococcus agalactiae exhibits facultative anaerobic growth, thriving in environments with or without oxygen, though it generally prefers limited oxygen. Optimal growth occurs at 35-37°C, mirroring human body temperature. When grown on blood agar, GBS is characterized by beta-hemolysis, completely breaking down red blood cells, resulting in a clear zone around its colonies. This complete lysis is due to a pore-forming toxin called beta-hemolysin.
S. agalactiae is catalase-negative, lacking the enzyme catalase. It also yields a positive result in the CAMP test, producing a factor that enhances the hemolytic activity of Staphylococcus aureus. Additionally, GBS is positive for hippurate hydrolysis, indicating its ability to break down sodium hippurate. These distinct metabolic and growth characteristics are significant for laboratory identification, allowing microbiologists to differentiate S. agalactiae from other bacterial species.
Typical Environments and Spread
Streptococcus agalactiae is a common inhabitant of the human body, frequently residing as a commensal organism without causing illness. Its primary natural habitats include the gastrointestinal and genitourinary tracts in healthy adults. The gastrointestinal tract is considered a major reservoir, potentially serving as the source for vaginal colonization. Up to 30% of healthy human adults may carry GBS asymptomatically in these areas.
GBS commonly colonizes the vaginal and rectal areas of pregnant women without symptoms. The primary mode of transmission to newborns is vertical, occurring from a colonized mother to her baby during childbirth. This occurs via exposure to amniotic fluid or mucous membranes during birth. Late-onset infections in infants may also be acquired from contact with the mother or the environment. Fecal-oral transmission and direct contact can also contribute to its spread.
Associated Health Conditions
Streptococcus agalactiae is a leading cause of severe infections in newborns, categorized as early-onset disease (EOD) and late-onset disease (LOD). EOD typically manifests within the first week of life, often within 24-48 hours of birth, leading to conditions such as sepsis, pneumonia, and meningitis. LOD occurs in infants from 7 days up to 3 months of age and is associated with bacteremia or meningitis, with potential long-term neurological consequences like hearing loss or mental impairment.
In pregnant women, S. agalactiae can cause various infections, including urinary tract infections, which may induce preterm labor. It can also lead to more severe conditions like chorioamnionitis, an infection of the placental tissues and amniotic fluid, and postpartum endometritis, an infection of the uterine lining after childbirth. These infections in mothers can be linked to prematurity and fetal death.
S. agalactiae also causes infections in non-pregnant adults, particularly those with underlying health conditions such as diabetes, liver disease, cancer, or compromised immune systems, and in the elderly. Common adult infections include skin and soft tissue infections (e.g., cellulitis or abscesses) and urinary tract infections. More severe invasive conditions include bacteremia, the presence of bacteria in the bloodstream, and endocarditis, an infection of the heart’s inner lining.