Resistance to Infection with Salmonella paratyphi C in Mice parasitized with a Relatively Avirulent Strain of Salmonella typhimurium

Abstract

ALTHOUGH previous specific active or passive immunization may prolong slightly the survival-time of mice infected with mouse-pathogenic Solmonellæ (for example, Salm. typhimurium, Salm. enteritidis), the number of long-term survivors is but little improved^1–3. Similarly, attempts to enhance resistance against mouse typhoid non-specifically, either by previous infection with Mycobacterium tuberculosis (strain BCG)⁴ or by injections of lipid A from Gramnegative bacteria⁵, have also met with only limited success. On the other hand, Hobson³ showed that mice were highly resistant to virulent Salm. typhimurium, in terms of survival, if they had been infected previously with an avirulent variant of the same species. As Salm. typhimurium is a reticulo-endothelial parasite in mice, like Myco. tuberculosis and Brucella abortus, the possibility arises that this greatly increased resistance might resemble the cellular antibacterial immunity mechanism inferred from investigations with these latter microorganisms^6–8. Evidence has been obtained, however, both from in vitro macrophage studies and mouse infection experiments, which suggests that ‘cellular immunity’ following infection with Myco. tuberculosis cross-reacts against Brucellæ and vice versa^9–12. The present communication illustrates that there is also a considerable non-specific element in the resistance produced by avirulent Salm. typhimurium, for this has been found to be effective against a heterologous Salmonella infection. Salm. enteritidis was considered unsuitable for use in this role as it shares somatic antigen components with Salm. typhimurium. Instead, Salm. paratyphi C (6,7,Vi : c : 1,5) was selected as it shares only one second-phase flagellar antigen with Salm. typhimurium (1,4,5,12 : i : 1,2) and, although not a natural mouse pathogen, produces an experimental infection similar to mouse typhoid.