Background: The susceptibility of bacteria to the bactericidal activity of serum depends on the structure and organization of the bacterial outer membrane. It is known that the structure of the O-specific polysaccharide chain of lipopolysaccharide (LPS) plays an important role in the resistance of bacteria to host immune defenses.
Objectives: The susceptibility of rods belonging to Enterobacteriaceae family to the bactericidal activity of the normal human serum (NHS) was examined. The mechanisms of complement activation were also investigated.
Material and methods: The study was carried out on 15 strains containing LPSs with O-specific polysaccharides composed of mannan, belonging to the following species: Citrobacter freundii, C. werkmanii, C. braakii, C. youngae, Hafnia alvei, Escherichia coli and Klebsiella pneumoniae. The levels of C3 and C4 complement components, IgG and IgM immunoglobulin in NHS were examined using specific antibodies. The bactericidal activity of NHS and its preparations (HS50/20, HSMgEGTA) was determined. LPSs from E. coli 08 strains were analyzed by polyacrylamide gel electrophoresis (PAGE) in the presence of sodium dodecyl sulphate (SDS).
Results: Eleven strains were sensitive to NHS bactericidal activity, and four were resistant. Only one group of strains was fully susceptible to NHS action. In three other groups, both sensitive and resistant strains were found. The majority of the strains remained susceptible to NHS activity irrespective of which pathway of serum activity was blocked. All E. coli 08 strains contained smooth-type LPSs.
Conclusions: Strains belonging to the same serotype showed variable susceptibility to the bactericidal action of normal human serum. Two mechanisms of the bactericidal activity of NHS have been identified.