NeuA O-acetylesterase activity is specific for CMP-activated O-acetyl sialic acid in Streptococcus suis serotype 2

► The Streptococcus suis NeuA is characterized for the first time. ► The CMP-Neu5Ac synthetase domain alone was inactive, while esterase domain alone was active. ► S.suis NeuA strictly de-O-acetylated CMP-O-acetyl-Neu5Ac, whereas Escherichia coli NeuA preferentially de-O-acetylated CMP-O-acetyl-Neu5Ac. ► The NeuA O-acetylesterase is essential for synthesis of capsular Neu5Ac in E. coli.