Summary: Strains of isolated from patients with cystic fibrosis (CF) never spread systemically. This may be due to serum sensitivity since these strains are very sensitive to complement-mediated bactericidal activity. A serum-resistant mutant, TUM3 HSR, was obtained from serum-sensitive strain TUM3 from a CF patient in order to clarify the mechanism of failure of systemic spread. LPS profiles on silver-stained gels and immunological analysis revealed that a long O-polysaccharide side chain was overproduced on the LPS molecules of TUM3 HSR as compared with the LPS of TUM3. The clearance rate from the bloodstream in mice was compared in the two strains. The number of TUM3 bacteria in 1 ml of blood, 10 min after injection into the tail vein, significantly decreased from 1.7 × 10 to 3.7 × 10 c.f.u. ml. In contrast, TUM3 HSR was not eliminated during the same period (decrease from 1.9 × 10 to 3.4 × 10c.f.u. ml). Interestingly, these isogenic strains were not killed by 40% murine serum, probably reflecting immaturity of the complement-mediated killing system in mice. These results pointed to a correlation between LPS structure and blood clearance rate in mice. This was confirmed by examining blood clearance kinetics using the smooth-LPS strain LT2 and LPS-deficient mutants derived from it. S. typhimurium LT2 resisted blood clearance while the LPS-deficient mutants were cleared rapidly. None of the strains were killed by murine serum. The number of TUM3 and LPS-deficient mutants trapped in the liver following injection into the peripheral circulation was greater than that of their counterparts. These results indicate that the long O-polysaccharide side chain of LPS may play a crucial role in evading phagocytosis by the reticuloendothelial system (RES), and therefore, may control the establishment of systemic infection by Gram-negative bacteria. The interaction between complement C3 on bacteria and C3 receptors on macrophages may also be involved in the trapping mechanism by the RES. It was expected that the level of C3 bound on the cell surface would be higher in TUM3 or the LPS-deficient mutants derived from LT2. However, our flow-cytometric results demonstrated that the level of C3 was almost identical in isogenic strains.


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