Strains of the Gram-negative bacterium Cronobacter (Enterobacter) sakazakii have been identified as emerging opportunistic pathogens that can cause enterocolitis, bacteraemia, meningitis, and brain abscess, and have been particularly associated with meningitis in neonates where infant-milk formulae has been epidemiologically linked to the disease. A study of the lipopolysaccharides produced by clinical isolates using chemical, 2D 1H and 13C NMR, and MS methods revealed that the O-polysaccharide produced by C. sakazakii (3290), a clinical strain from the Tennessee outbreak, was a branched polymer of repeating pentasaccharide units composed of 2-acetamido-2-deoxy-D-galactose, 3-(N-acetyl-L-alanylamido)-3-deoxy-D-quinovose, D-glucuronic acid, and D-glucose present in the molar ratio 1:1:1:2 and had the structure:<div class="absequation"><img src="/cisti/journals/rp/gifs/o09-004s1h.gif" /></div>The O-PS structure provides a unique specific structurally defined marker for the clinical tracking of this pathogen.