Catalyzes the transfer of sialic acid from the substrate CMP-N-acetylneuraminate to the terminal galactose residue of the lacto-N-neotetraose branch of surface lipooligosaccharide (LOS), forming an alpha-2,3-sialyl linkage. Thus, functions in the sialylation of LOS, which plays a role in the evasion of the host immune response by protecting N.meningitidis from complement-mediated serum killing and from phagocytic killing by neutrophils. In vitro, can use a number of different synthetic acceptors with lactose or galactose as the saccharide portion, but shows a strong preference for the N-acetyllactosamine containing acceptor. a beta-D-galactosyl-(1->4)-N-acetyl-beta-D-glucosaminyl derivative + CMP-N-acetyl-beta-neuraminate = an N-acetyl-alpha-neuraminyl-(2->3)-beta-D-galactosyl-(1->4)-N-acetyl-beta-D-glucosaminyl derivative + CMP + H(+) Optimum pH is 6.0. Bacterial outer membrane biogenesis; lipooligosaccharide biosynthesis. Homodimer. Is exclusively found in outer membrane preparations of the pathogen and is surface exposed. Inner membrane, periplasmic, cytoplasmic, and culture supernatant fractions are devoid of Lst. Present in outer membrane vesicle formulations which are used as vaccines in human. Belongs to the glycosyltransferase 52 family.