Involved in biosynthesis of legionaminic acid (5,7-diamino-3,5,7,9-tetradeoxy-D-glycero-D-galacto-non-2-ulosonic acid)(Leg), a sialic acid-like derivative that is incorporated into virulence-associated cell surface glycoconjugates such as lipopolysaccharide (LPS) which could be a key determinant in the ability of L.pneumophila to inhibit the fusion of phagosomes with lysosomes. LPS contains a majority alpha2,4-linked homomer of legionaminic acid. Catalyzes the conversion of UDP-N,N'-diacetylbacillosamine (Bac2Ac4Ac) into 2,4-diacetamido-2,4,6-trideoxymannose and UDP. H2O + UDP-N,N'-diacetylbacillosamine = 2,4-diacetamido-2,4,6-trideoxy-alpha-D-mannopyranose + H(+) + UDP kcat is 59.1 sec(-1) for UDP-N,N'-diacetylbacillosamine (at pH 7.5 and 37 degrees Celsius). Belongs to the UDP-N-acetylglucosamine 2-epimerase family.