Catalyzes the formation of CMP-N-acetylneuraminic acid (CMP-NeuNAc), which is essential for the formation of the capsule. an N-acylneuraminate + CTP = a CMP-N-acyl-beta-neuraminate + diphosphate Inhibited by the CTP analogs 5-mercuri-CTP and CTP-2',3'-dialdehyde. Optimum pH is 9.0-10.0. Monomer. May form aggregates. Belongs to the CMP-NeuNAc synthase family.