When the opsonization of various Pseudomonas aeruginosa strains--PAC 1, its O-chain-deficient mutant PAC 605, and an intermediate strain, P14--was measured either directly by determination of the amount of C3b attached to the bacterial surface or indirectly by assessing phagocytosis by human polymorphonuclear leukocytes and the responses of chemiluminescence, it was demonstrated that PAC 1 was opsonized and phagocytized to a lower extent than P14 and PAC 605. In contrast to PAC 605, PAC 1 showed an increased consumption of complement in the fluid phase and a rapid release of lipopolysaccharide antibodies bound to the bacterial surface due to the alternative pathway of the complement system. Furthermore, it was shown that with respect to PAC 1 and PAC 605, the lack of an O-chain resulted in increased sensitivity to serum and decreased virulence. From both in vivo and in vitro experiments, we concluded that the structure of the O-antigen polysaccharide chain of lipopolysaccharide is an important virulence factor of P. aeruginosa against the defense mechanisms of the host
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