Streptococcus agalactiae
species of bacterium
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 Streptococcus. 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 public source identifies “Streptococcus agalactiae” as species of bacterium. This brief keeps that definition visible, then builds a research path around Streptococcus, agalactiae and species.
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Datasets, specimens, observations and peer-reviewed methods provide the appropriate test for the technical claims summarized here. The source revision retrieved here is dated Aug 16, 2026. The linked authority identifier is Q132949. None of the 0 selected statements returned an explicit reference.
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This entry incorporates text from “Streptococcus agalactiae” on English Wikipedia. Contributors are listed in the page history. Text is available under the Creative Commons Attribution-ShareAlike 4.0 License. Selected authority identifiers and statements are retrieved from Wikidata under CC0; their references and qualifiers remain part of the verification path.