Patent No. 599598 Disclosed is a Shigella bacterium which expresses no more than 4 of Tol A, Tol B, Tol Q, Tol R and Pal proteins. Also disclosed is a Shigella bacterium which does not express a Tol R protein. Also disclosed is a Shigella bacterium which expresses Tol A, Tol B, Tol Q, Tol R and Pal proteins, wherein the Tol A, Tol Q, TolR and/or Pal protein (a) is located in the bacterium's inner or outer membrane, and (b) includes one or more amino acid sequence mutation(s) such that, compared to the same bacterium without said mutation(s), the bacterium releases greater quantities of outer membrane blebs when growing in culture medium.
