Streptococcus agalactiae

Streptococcus agalactiae, also known as group B streptococcus, is a gram-positive, chain-forming bacterium that is beta-hemolytic, catalase-negative, and a facultative anaerobe. It is the most common human pathogen among the streptococci placed in Lancefield group B, and is subclassified into ten serotypes by the immunologic reactivity of its polysaccharide capsule. In most healthy adults it lives as a harmless commensal in the gastrointestinal and genitourinary tract, but it can cause severe invasive infection in newborns, the elderly, and people with weakened immune systems. It is also a common veterinary pathogen, causing bovine mastitis in dairy cows, the source of its species name agalactiae, meaning of no milk.