O-antigen specificities of the serotype strains of Serratia marcescens.

O antigens of the 24 O-serotype strains of Serratia marcescens were investigated in dot enzyme immunoassay with whole-cell antigens and by immunoblotting with lipopolysaccharide (LPS) antigens. Three pairs of strains, O2/O3, O6/O7, and O12/O14, had indistinguishable LPS antigens, despite having distinct specificities in agglutination tests with whole-cell antigens. Strong cross-reactions were also found in LPS antigens from strains O9/O15, O17/O19, O10/O22, and O16/O20. No high-molecular-weight LPS corresponding to O-side-chain material was detected in strain O11 or O13. A panel of absorbed antisera was prepared to facilitate the detection of a reduced set of LPS antigens in a dot enzyme immunoassay. We conclude that there are discrepancies between the existing serotypes as defined by agglutination tests and the antigenic composition of LPS antigens extracted from the serotype strains and that surface antigens other than LPS make a major contribution to the definition of serotype in the species.