Internalin B promotes the replication of Listeria monocytogenes in mouse hepatocytes

Abstract

The uptake of Listeria monocytogenes by a variety of cell types in vitro is facilitated by the protein products of the in1AB (internalin) operon expressed by the organism. In the case of mouse hepatocytes, the extent to which in1AB expression influenced the uptake of Listeria in vitro was markedly dependent upon the ratio of bacteria to cells. At a ratio of 100:1, greater than 40-fold fewer transposon-induced in1AB mutant listeriae entered hepatocytes compared to the isogenic wild-type control; the difference was only fourfold, however, in cultures inoculated at a 1:1 ratio. Similarly, the uptake of in-frame in1B or in1AB deletion mutants differed only fourfold from the uptake of wild-type or in1A mutant Listeria at a 1:1 multiplicity of infection. Mutations affecting in1B or inlAB, on the other hand, resulted in a marked decrease in the capacity of Listeria to proliferate within mouse hepatocytes in vivo and in vitro. Electron micrographs of Listeria-infected hepatocytes demonstrated the impaired capacity of in1B mutants to escape from endocytic vacuoles and to enter the cytoplasm where proliferation occurs. These findings indicate that the protein product of in1B exerts a significant effect on the intracellular replication of Listeria.