The role of surface polysaccharide in determining the resistance of Serratia marcescens to serum killing.

Two O14:H12 strains of Serratia marcescens with different sensitivities to killing by normal pooled human serum were investigated. Complement binding, studied by measuring hydrophobicity and using rocket immunoelectrophoresis with anti-human C3, showed the sensitive cells (S1220) rapidly bound and fixed complement whereas the resistant cells (4444-60) bound less C3b. The strains had identical membrane protein composition. Crossed immunoelectrophoresis suggested that in S1220 cells the polysaccharide material including LPS was less antigenic and present in smaller amounts than in 4444-60 cells. This was confirmed by examining extracted polysaccharide material chemically and by SDS-PAGE. The resistant strain had 33% more phenol-extractable polysaccharide material than the sensitive strain, possibly comprising LPS with longer O antigen chain lengths, or a microcapsule of O antigen polysaccharide. Extra polysaccharide material on the surface of the resistant strain prevents complement components binding and reaching the hydrophobic membrane where lytic lesions occur.