A Klebsiella pneumoniae Strain that Shares a Type‐Specific Antigen with Shigella flexneri Serotype 6

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Abstract
A strain of Klebsiella pneumoniae was found as the only isolate with pathogenic potential from the stool of a two‐year old patient with diarrhoea. A strong serological cross‐reactivity with Shigella flexneri serotype 6 was demonstrated. The cross‐reacting antigens were shown to reside in the cell wall lipopolysaccharide. Studies of the pathogenic potential of the Klebsiella strain showed low level of invasion of HEp‐2 cells. However, tests for adherence to HEp‐2 cells as well as tests for toxin production were negative. The strain had several small plasmids and was multidrug resistant. These data do not form a sufficient basis for estimating the pathogenic potential of the organism. No K antigen was detected. The structure of the O‐antigenic polysaccharide from the K. pneumoniae strain was investigated using methylation analysis, NMR spectroscopy, and Smith degradation as the principal methods. The O‐antigenic polysaccharide has the following pentasaccharide repeating unit: This structure is not identical to any of the previously described O‐antigens of K. pneumoniae. The strong serological cross‐reactivity with the Shigella flexneri serotype 6 O‐antigen can most likely be attributed to the structural element α‐l‐Rhap ‐(1→)‐α‐l‐Rhap present in the O‐polysaccharide repeating unit of this serotype. Antiserum raised against the K. pneumoniae strain also agglutinated S. dysenteriae serotype 1 and strains of all different serotypes of S. flexneri. The Shigella strains contain the structural element α‐l‐Rhap ‐(1→3)‐α‐l‐Rhap in their O‐antigen polysaccharides which may be responsible for the observed cross‐reactivity.