Immunochemical study of the peptidoglycan of gram-negative bacteria

The specificity of antibodies directed against the peptidoglycan of gram-negative bacteria was studied. The peptidoglycans of Proteus vulgaris, Escherichia coli, Moraxella glucidolytica, Neisseria perflava, give identical precipitin reactions. By means of inhibition studies with various peptidoglycan subunits and synthetic peptides, it was shown that the antibodies are essentially directed against the peptide moiety of the peptidoglycan: L-Ala-D-Glu (L)-mesoA2pm-(L)-D-Ala, that the peptide reacts better with antibodies when it is not cross-linked, and that the C-terminal portion-meso-A2pm-D-Ala of the peptide is immunodominant. These results explain the immunological identity of the peptidoglycans of gram-negative bacteria, which possess the same peptide subunit. Only weak cross-reactivity was observed with the peptidoglycans of gram-positive bacteria (Streptococcus faecium, Micrococcus lysodeikticus, Corynebacterium poinsettiae) where meso-diaminopimelic acid is replaced by L-lysine or L-homoserine. However, the peptidoglycan of Bacillus megaterium which possesses the same peptide subunit as gram-negative bacteria, gives only a reaction of partial identity with these bacteria. This result suggests the presence on the peptidoglycan of gram-negative bacteria, of other undefined antigenic determinants.