Humoral antibody response of rabbits against experimental Serratia marcescens septicemia

Rabbits reconvalescent from experimental septicemia due to serologically defined strains of Serratia marcescens were examined for the diversity of their humoral antibody response with traditional serological procedures and the Western blot (immunoblotting) technique. Trichloracetic acid (TCA)-whole cell extracts of the homologous and heterologous O-antigen reference strains served as the antigen for the latter procedure. Reconvalescent rabbit sera contained antibodies against the homologous lipopolysaccharide (LPS) moiety (molecular weight (MW) range = 45-31 kilodaltons (= k] and antibodies against numerous heat-modifiable, cross-reactive proteins, in particular 7 proteins characterized by MWs of 117 k, 95 k, 91 k, 71 k, 68 k, 38 k, and 33 k in TCA-whole cell extracts from the homologous as well as from 11 heterologous S. marcescens O-antigen reference strains. Rabbits, which had been actively immunized with TCA-whole cell extracts from representative S. marcescens strains, mounted a humoral antibody response remarkably similar to that of rabbits which had recovered from septicemia, except that the sera from the actively immunized animals interacted somewhat more strongly with an additional cross-reactive protein (MW = 47 k). Conversely, conventional anti-O and anti-H rabbit immune sera revealed antibodies directed predominantly against the homologous LPS moiety (MW range = greater than or equal to 200 k - less than or equal to 15 k). It was concluded that numerous proteinaceous cellular constituents of S. marcescens accounted for immunoblot cross-reactivity.