Streptococcus agalactiae (also known as group B streptococcus or GBS) is a Gram-positive bacterium.
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 “no milk”, alludes to this.