Construction and characterization of a hypervesiculation strain of Escherichia coli Nissle 1917

Outer membrane vesicles (OMVs) are produced by Gram-negative bacteria and deliver microbial molecules to distant target cells in a host. OMVs secreted by probiotic probiotic strain Escherichia coli Nissle 1917 (EcN) have been reported to induce an immune response. In this study, we aimed to increase the OMV production of EcN. The double gene knockout of mlaE and nlpI was conducted in EcN because the Delta mlaE Delta nlpI of experimental strain E. coli K12 showed the highest OMV production in our previous report. The Delta mlaE Delta nlpI of EcN showed approximately 8 times higher OMV production compared with the parental (wild-type) strain. Quick-freeze, deep-etch replica electron microscopy revealed that plasmolysis occurred in the elongated Delta mlaE Delta nlpI cells and the peptidoglycan (PG) had numerous holes. While these phenomena are similar to the findings for the Delta mlaE Delta nlpI of K12, there were more PG holes in the Delta mlaE Delta nlpI of EcN than the K12 strain, which were observed not only at the tip of the long axis but also in the whole PG structure. Further analysis clarified that the viability of Delta mlaE Delta nlpI of EcN decreased compared with that of the wild-type. Although the amount of PG in Delta mlaE Delta nlpI cells was about half of that in wild-type, the components of amino acids in PG did not change in Delta mlaE Delta nlpI. Although the viability decreased compared to the wild-type, the Delta mlaE Delta nlpI grew in normal culture conditions. The hypervesiculation strain constructed here is expected to be used as an enhanced probiotic strain.