The species is subclassified into ten serotypes (Ia, Ib, II–IX) depending on the immunologic reactivity of their polysaccharide capsule.

agalactiae is also a common veterinary pathogen, because it can cause bovine mastitis (inflammation of the udder) in dairy cows.

The CAMP factor produced by GBS acts synergistically with the staphylococcal β-hemolysin inducing enhanced hemolysis of sheep or bovine erythrocytes.

[8] Hemolytic GBS strains produce an orange-brick-red non-isoprenoid polyene (ornithine rhamnolipid) pigment (granadaene)[10] when cultivated on granada medium that allows its straightforward identification.

[12] GBS colonies can additionally be identified tentatively after their appearance in chromogenic agar media, nevertheless GBS-like colonies that develop in chromogenic media should be confirmed as GBS using additional reliable tests (e.g.latex agglutination or the CAMP test) to avoid potential mis-identification.

[17] These variations in the reported prevalence of asymptomatic GBS colonization could be related to the detection methods used, and differences in populations sampled.

GBS is the leading cause of bacterial neonatal infection in the baby during gestation and after delivery with significant mortality rates in premature infants.

[18] In 2008, after widespread use of antenatal screening and intrapartum antibiotic prophylaxis (IAP), the CDC reported an incidence of 0.28 cases of EOD per thousand live births in the US.

It was estimated that 226 infants (49 per 100,000) in the United States had a clinically significant GBS infection, and that approximately 11 (2.4%) of those cases resulted in death.

These factors include onset of labor before 37 weeks of gestation (premature birth), prolonged rupture of membranes (≥18h before delivery), intra-partum fever (>38 °C, >100.4 °F), amniotic infections (chorioamnionitis), young maternal age, and low levels of GBS anticapsular polysaccharide antibodies in the mother.

[30][31] A previous sibling with EOD is also an important risk factor for development of the infection in subsequent deliveries, probably reflecting a lack of GBS polysaccharides protective antibodies in the mother.

[30][31] Intravenous vancomycin is recommended for IAP in women colonized with a clindamycin-resistant Group B Streptococcus strain and a severe penicillin allergy.

The risk-based strategy identifies candidates to receive IAP by the aforementioned risk factors known to increase the probability of EOD without considering if the mother is or is not a GBS carrier.

[14][29][31] The ACOG committee issued an update document on Prevention of Group B Streptococcal Early-Onset Disease in Newborns in 2019.

The key measures necessary for preventing neonatal GBS early onset disease continue to be universal prenatal screening by culture of GBS from swabs collected from the lower vagina and rectum, correct collection and microbiological processing of the samples, and proper implementation of intrapartum antibiotic prophylaxis.

This new recommendation provides a five-week window [44] for valid culture results that includes births that occur up to a gestational age of at least 41 weeks.

The culture-based screening approach is followed in most developed countries[45] such as the United States,[29][30][31] France,[46] Spain,[47] Belgium,[48] Canada, Argentina,[49] and Australia.

[9][30] GBS-like colonies that develop in chromogenic media should be confirmed as GBS using additional reliable tests to avoid mis-identification.

[9] Nucleic acid amplification tests (NAAT) such as polymerase chain reaction (PCR) and DNA hybridization probes have been developed for identifying GBS directly from recto-vaginal samples, but they has a high false negative rate, and still cannot replace antenatal culture for the most accurate detection of GBS carriers.

Serious life-threatening invasive GBS infections are increasingly recognized in the elderly and individuals compromised by underlying diseases such as diabetes, cirrhosis and cancer.

[68] GBS is a major cause of mastitis (an infection of the udder) in dairy cattle and an important source of economic loss for the industry.

[69] Outbreaks in herds are common, so this is of major importance for the dairy industry, and programs to reduce the impact of S. agalactiae disease have been enforced in many countries over the last 40 years.