agalactiae

Streptococcus agalactiae, commonly known as group B Streptococcus (GBS), is a Gram-positive, beta-hemolytic coccus that grows in chains and is encapsulated. It is classified as Lancefield group B and is part of the normal microbiota of the human lower gastrointestinal and genitourinary tracts. In animals, including cattle, it is a pathogen that can cause mastitis.

In humans, GBS is a leading cause of early neonatal sepsis and meningitis and remains an important cause of perinatal morbidity. Maternal colonization occurs in about 10–30% of pregnant women and can be transmitted to the infant during birth, leading to early-onset disease within the first week of life and, less commonly, late-onset disease up to several months of age. In adults, GBS can cause cellulitis, bacteremia, pneumonia, urinary tract infection, and, in some cases, invasive disease, particularly among the elderly or immunocompromised.

Diagnosis relies on culture or molecular testing. S. agalactiae is typically beta-hemolytic on blood agar, catalase negative, and hippurate hydrolysis positive; the CAMP test is often positive. Screening of pregnant women with vaginal-rectal swabs at 35–37 weeks gestation is used in some guidelines to guide intrapartum prophylaxis.

Prevention and treatment focus on intrapartum antibiotic prophylaxis (usually penicillin G) for colonized women or those with risk factors. Alternatives exist for penicillin allergy. In newborns with suspected disease, empirical therapy is used. While penicillins remain the mainstay, resistance to macrolides and lincosamides has been reported in some GBS isolates, underscoring the role of susceptibility testing when alternatives are needed.