Group B Streptococcus (GBS), also named Streptococcus agalactiae, is a Gram-positive coccus which cause human diseases. The principal reservoir for GBS is the lower gastrointestinal (GI) tract, and GBS colonization in the GI tract increases the risk for vaginal colonization. GBS colonizes the lower GI tract at rates ranging from 10% to 25% in men and colonize the genital area or low GI tract at rates ranging from 10%-40% in pregnant and non-pregnant women. GBS also colonizes the adult oropharyngeal cavity with a rate about 5%.

GBS causes a diverse range of human diseases. GBS were reported as the major pathogens causing septicemia and meningitis in neonates and young infants. Maternal-to-infant transmission may occur in neonates born to women colonized with GBS who do not receive intrapartum antibiotic prophylaxis either by the ascending route or at the time of delivery. Implementation of maternal intrapartum chemoprophylaxis program over the past two decades has led to a substantial decline in neonatal GBS infections and pregnancy-related morbidity. In contrast, invasive GBS diseases in adults, presenting as septicemia, skin and soft tissue infections, pneumonia, urinary tract infection, etc., represent an increasing burden worldwide. Medical conditions predisposing to GBS infection in adults include diabetes mellitus, malignancy, liver disease, renal disease, lung disease, heart disease, etc. In particular, the incidence of invasive GBS diseases is higher among nursing home residents than community-dwelling residents.

Penicillin is the first-line drug for the treatment of GBS infections, while erythromycin, clindamycin, vancomycin, and fluoroquinolones are important alternatives for those who are allergic to penicillin. However, high rates of resistance to clindamycin and erythromycin have been reported from different regions. Moreover, isolates with penicillin-reduced susceptibility have been reported from Japan, and fluoroquinolone-resistant GBS strains have emerged in East Asian countries, including China, Japan, and Korea.