Relative Contribution of ColV Plasmid and K1 Antigen to the Pathogenicity of Escherichia coli

Abstract

To study the relevance of the ColV plasmid and the capsular K1 antigen in the pathogenicity of Escherichia coli , isogenic strains that differ only in these characteristics were constructed. Studies with these variants demonstrated that the presence of the ColV plasmid increased the serum resistance of E. coli. This increase did not depend on the expression of the K1 antigen. This work also demonstrated that the presence of the K1 antigen protects E. coli from the bactericidal activity of serum. Studies using mouse peritoneal macrophages in the presence of normal serum indicated that the presence of K1 antigen protects E. coli from phagocytosis. Similar experiments with the K1 + strains performed in the presence of anti-K1 antibodies demonstrated that these antibodies opsonized these bacteria very efficiently in the absence of complement. The K1 − E. coli variants were efficiently phagocytized in the presence of normal human serum and absorbed human serum, indicating that they are able to be opsonized by complement deposited by activation of the alternative pathway of complement. Work using fluorescence microscopy confirmed that the K1 − strains are able to fix complement in the absence of antibody. It was also found that the presence of the ColV plasmid may interfere with phagocytosis of the E. coli K1 strains and deposition of complement on these cells. To test the relevance of the results of the in vitro experiments for disease, the pathogenicity of the strains was tested in mice. The results showed that the K1 antigen is the main determinant of pathogenicity of these strains and that the presence of ColV can modify the pathogenic potential of the E. coli K1 strains through a mechanism that does not depend on the production of colicin V.

Immune response to the iron-deprivation-induced proteins of Salmonella typhi in typhoid fever

“…Adsorbed sera. Portions of the pooled sera were adsorbed with S. typhi Ty2 grown under iron starvation and ironsufficient conditions, respectively, as previously described (2).…”

A plasmid-encoded outer membrane protein, TraT, enhances resistance of Escherichia coli to phagocytosis

“…Phagocytosis assays. Phagocytosis was assayed by using monolayers of mouse peritoneal macrophages prepared as previously described (2,4,21). Briefly, 5 x 106 E. coli cells were opsonized with different concentrations of the serum preparations at 37°C for 15 min.…”

“…Ability of the TraT protein to interfere with phagocytosis is independent of the bacterial capsule. Since the bacterial capsule is a surface structure capable of increasing bacterial resistance to serum (2,27,43,47), it was of interest to determine the possible influence of the presence of a bacterial capsule on the inhibition of phagocytosis mediated by the TraT protein. Plasmid pKT107 was therefore introduced by transformation into the capsuleless strain E. coli FC004.…”

“…Since preliminary results suggested that the capsule present in E. coli 59 rif interfered with the detection of small differences in deposited complement, the capsuleless E. coli strain FC004 containing pKT107 was used for these studies, in which cells exposed to concentrations of normal and absorbed human serum ranging from 2.5 to 50% were assayed for complement deposition. This strain has a smooth 018ab antigen and is relatively resistant to serum (2). These characteristics rule out the possibility of a nonimmune activation of the classical pathway of complement (3,39).…”

“…The inability of the anti-TraT protein Fab fragments to block the inhibition of phagocytosis and the fact that antibodies present in the normal human serum directed to other bacterial structures can opsonize TraT' E. coli cells indicate that TraT protein is a passive inhibitor of phagocytosis. In this respect the TraT protein exerts a shielding effect similar to the effect of the capsules of some bacteria and fungae (2,29).…”