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

Abstract

The presence of the outer membrane protein TraT, encoded by plasmid R6-5, reduces the sensitivity of Escherichia coli cells to phagocytosis by macrophages. This effect is independent of the bacterial capsule and is more evident in the presence of adsorbed normal human serum. The property of inhibiting phagocytosis is specifically abolished by anti-TraT protein antiserum and anti-TraT immunoglobulin G but not by Fab fragments. These results indicate that the TraT protein is a passive inhibitor of phagocytosis. Inhibition of phagocytosis is produced because the TraT protein antagonizes opsonization by complement, such that C3 deposition is reduced and altered in distribution.Phagocytosis and the bacteriostatic and bactericidal properties of the serum constitute the first line of defense against invasive bacterial infections. The antibacterial activity of blood is effective in eliminating most normally encountered transient bacteremias and is the result of a constellation of specific (e.g., antibodies) and nonspecific (e.g., complement, iron-binding proteins) factors (7,16).The ability of bacteria to avoid host defenses and become invasive rests largely in cell surface components which are important in the preliminary steps of the infective process and crucial in determining its outcome (35). The role of surface components such as capsules, peptidoglycan, proteins, pili, and 0-antigens in increasing the virulence of bacteria is well documented (34). Although most of these are encoded by genetic determinants located on the chromosome, recent reports testify to the importance of plasmidencoded gene product modifications to the cell surface, which alter bacterial virulence (13-15, 31, 33, 34).The Escherichia coli plasmid R6-5 carries a gene specifying resistance to the antibacterial activity of serum (31). This determinant, which has been identified as traT, is one of the genes of the tra operon, the genetic unit conferring conjugative ability upon the bacterial cell (1, 31). The traT gene directs the synthesis of a highly exposed outer membrane protein, which mediates resistance to serum. Since phagocytosis, the other main line of defense against invasive bacterial infections, involves complement, we explored the possibility that the TraT protein may confer upon bacteria the ability to resist phagocytosis. In this communication, we show that this is the case and that the protein interferes mainly with opsonization by the alternative pathway of complement by restricting and altering the pattern of C3 deposition. MATERIALS AND METHODSBacterial strains and plasmids. Bacterial strains and plasmids are listed in Table 1. Plasmid DNA, isolated by standard procedures, was introduced into bacterial strains by transformation or conjugation (45). Strains were stored at -20°C in 60% glycerol. Molecular cloning procedures. Restriction endonuclease * Corresponding author.cleavage of plasmid DNA, ligation reactions, transformations, screening, and analysis by agarose gel electrophoresis were performed as described (45,46). Media. ...

Plasmid-mediated resistance to phagocytosis in Yersinia enterocolitica

“…This protein has been shown to mediate resistance to both serum-and phagocyte-dependent killing. The antiphagocytic function of these surface structures appears to be related to their interference with opsonization by NHS (1,12,36).…”

Interaction of complement with serum-sensitive and serum-resistant strains of Pseudomonas aeruginosa

“…Moreover, Fries et al (4) have demonstrated that C3b complexed with IgG is significantly more effective in alternative pathway consumption of C3 than is fluid-phase C3b. By using an E. coli strain carrying the plasmid-encoded outer membrane protein, traT, a protein conferring resistance to the antibacterial activity of serum (20), Aguero et al (1) found that this protein also reduces the deposition of C3 on the bacterial surface and alters its distribution, thus antagonizing the complement-mediated opsonization of this strain. Further studies are in progress to explain the basis for this discrepancy between C3 consumption and binding observed on these two P. aeruginosa strains.…”

Bacterial self-defence: howEscherichia colievades serum killing

“…Studies have suggested that TraT inhibits later stages of MAC activity (Binns et al, 1982) and the formation or structure of the C5b6 complex (Pramoonjago et al, 1992). TraT may also alter C3 deposition on the bacterial surface and affect outer membrane permeability (Aguero et al, 1984;Sukupolvi & O'Connor, 1987).…”