Streptococcus agalactiae (also known as group B streptococcus or GBS) is a gram-positive coccus (round bacterium) with a tendency to form chains (as reflected by the genus name Streptococcus). It is a beta-hemolytic, catalase-negative, and facultative anaerobe.

S. agalactiae is the most common human pathogen of streptococci belonging to group B of the Rebecca Lancefield classification of streptococci. GBS are surrounded by a bacterial capsule composed of polysaccharides (exopolysaccharide). The species is subclassified into ten serotypes (Ia, Ib, II–IX) depending on the immunologic reactivity of their polysaccharide capsule.

The plural term group B streptococci (referring to the serotypes) and the singular term group B streptococcus (referring to the single species) are both commonly used synonymously with S. agalactiae even though S. halichoeri and S. pseudoporcinus are also group B Streptococci. These species test positive as group B, but are not frequently carried by humans, and only rarely cause disease.

In general, GBS is a harmless commensal bacterium being part of the human microbiota colonizing the gastrointestinal and genitourinary tract of up to 30% of healthy human adults (asymptomatic carriers). Nevertheless, GBS can cause severe invasive infections especially in newborns, the elderly, and people with compromised immune systems.

S. agalactiae is also a common veterinary pathogen, because it can cause bovine mastitis (inflammation of the udder) in dairy cows. The species name agalactiae meaning "of no milk", alludes to this.

GBS grows readily on blood agar plates as colonies surrounded by a narrow zone of β-hemolysis. GBS is characterized by the presence in the cell wall of the antigen group B of Lancefield classification (Lancefield grouping) that can be detected directly in intact bacteria using latex agglutination tests. The CAMP test is also another important test for identification of GBS. The CAMP factor produced by GBS acts synergistically with the staphylococcal β-hemolysin inducing enhanced hemolysis of sheep or bovine erythrocytes. GBS is also able to hydrolyze hippurate and this test can also be used to identify presumptively GBS. Hemolytic GBS strains produce an orange-brick-red non-isoprenoid polyene (ornithine rhamnolipid) pigment (granadaene) when cultivated on granada medium that allows its straightforward identification. GBS can also be identified using MALDI-TOF (Matrix Assisted Laser Desorption/Ionization-Time of Flight) instruments. 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. A summary of the laboratory techniques for GBS identification is depicted in Ref 7.

GBS is a normal component of the intestinal and vaginal microbiota in some people; GBS is an asymptomatic (presenting no symptoms) colonizer of the gastrointestinal tract and vagina in up to 30% of otherwise healthy adults, including pregnant women. GBS colonization may be permanent, intermittent or temporary. In different studies, GBS vaginal colonization rate ranges from 0% to 36%, most studies reporting colonization rates in sexually active women over 20%. It has been estimated that maternal GBS colonization worldwide is 18%, with regional variation from 11% to 35%. These variations in the reported prevalence of asymptomatic GBS colonization could be related to the detection methods used, and differences in populations sampled.

As other virulent bacteria, GBS harbors an important number of virulence factors (virulence factors are molecules produced by bacteria that enhances their capacity to infect and damage human tissues), the most important being the capsular polysaccharide (rich in sialic acid) and a pore-forming toxin, β-hemolysin. Today it is considered that GBS pigment (granadaene) and hemolysin are identical or closely related molecules.