The colonization of Peyer’s patches by a strain of Salmonella typhimurium cured of the cryptic plasmid

We cured a strain of Salmonella typhimurium of its cryptic plasmid and confirmed that orally administered cured strains lost virulence for mice. Loss of the cryptic plasmid ren-dered the S. typhimurium strain sensitive to the bactericidal action of normal human se-rum. However, loss of the plasmid did not change the ability of the strain to associate with HeLa cells in tissue culture. Furthermore, when administered orally to mice, both the plasmid-containing and plasmid-free strains invaded the Peyer's patches of the small intestine to the same extent, and both were capable of inducing resistance to oral chal-lenge with virulent S. typhimurium. When injected intraperitoneally, the cured strain was eliminated rapidly, whereas the parental strain persisted. We also showed that the cured strain did not contain a plasmid copy in the chromosome. We propose that although the plasmid-cured strain of S. typhimurium is able to colonize Peyer's patches, it cannot sur-vive when administered intraperitoneally because it is susceptible to elimination by mac-rophages. The course of an infection with Salmonella typhimu-rium in mice is well established. Following oral ad