Indigenous microorganisms prevent reduction in cecal size induced by Salmonella typhimurium in vaccinated gnotobiotic mice.

Germfree CD-1 mice challenged by the oral route with Salmonella typhimurium had ceca that were abnormal in appearance and reduced in size compared to those of germfree controls. Similarly, germfree mice injected with heat-killed S. typhimurium or gnotobiotes associated with three indigenous microbes (Lactobacillus, Bacteroides, Clostridium), and subsequently challenged with S. typhimurium also had small ceca. By contrast, gnotobiotic mice that had been both injected with the heat-killed S. typhimurium and associated with the three indigenous microbes before challenge with S. typhimurium had ceca similar in size and appearance to germfree mice. Thus, indigenous microorganisms could interfere with the mechanism by which the pathogen induced the decrease in cecal size, but could do so only in mice injected with heat-killed bacteria. This phenomenon suggests synergism between the interference effected by the indigenous bacteria and the resistance mechanisms of the animal.