Penetration of the polysaccharide capsule of Escherichia coli (Bi161/42) by bacteriophage K29

Abstract

The interaction between the capsulated Escherichia coli strain of serotype K29 and a capsule K29-specific bacteriophage has been studied, using virus adsorption kinetics and immunological methods in combination with electron microscopy. The adsorption of the phage to capsulated wild-type (w.t.) E. coli is fast, with a rate constant “ k” of 1 to 2.5 × 10 −8 ml/min at 37°, and 4 × 10 −9 ml/min at 0°. Mutant strains with temperature-sensitive defects in production of capsule antigen showed a much reduced virus adsorption rate when grown at permissive temperatures. The virus, being capable of enzymatically hydrolyzing the receptor polysaccharide, destroyed the macromolecular structure of both the isolated polysaccharides (p.s.) and the capsule in vivo. Phage adsorption occurred without release of virus DNA. Virus adsorption carried out with w.t. cells in the presence of isolated p.s. revealed receptor competition in which the isolated p.s. from mutant cells was about 103-fold less efficient than the isolated p.s. from the wild-type cells. Electron microscopy of ultrathin sections enabled us to follow the virion in its travel through the w.t. capsule. To observe this, the capsule had to be stabilized with anti-capsule IgG after phage adsorption. In the capsule of the infected cell, a tunnel-shaped penetration path of the virus became visible; the path of the virus was often but not exclusively unidirectional toward the outer membrane (OM) of the cell. A virus particle that had reached the OM might subsequently move along the surface of the OM or might turn back into the capsule. Movement of the adsorbed virion was halted by anti-capsule IgG which caused trapping of the virus particles. The virion was eventually found to be positioned over one of the adhesion sites at which inner and outer membrane are fused. After 4 min, the virus released its DNA, as judged from a decreased state of filling of the phage heads. The data support a multiple-step adsorption model.