Infection with a reduced virulence Listeria monocytogenes strain increases resistance to salmonellosis in mice.

A model to study the non-specific resistance of OF1 mice was standardized using a Listeria monocytogenes strain of reduced virulence to induce this resistance, and a fully virulent Salmonella typhimurium strain (as challenge) to measure it. Dose, route of inoculation and timing were optimized. Kinetics of infection were carried out using Listeria and Salmonella strains respectively, inoculated intravenously and subcutaneously in the hind footpad, in order to minimize the number of dead and uninfected mice. An intravenous inoculation of 1 x 10(4) Listeria colony forming unit (0.25% of the lethal dose 50%) followed up 3 days later by an intravenous challenge of Salmonella (maximum used: 0.15% of the lethal dose 50%) was the optimum way of ensuring a mean infection level of the group stimulated with Listeria that was significantly lower than mean infection level of the controls (P less than 0.01). Under similar conditions, using a subcutaneous challenge inoculation, this period was of 5 days. An increased resistance against Salmonella was found between days 3 and 9 using an intravenous challenge and between days 3 and 5 using a subcutaneous challenge. Utilization of this non-specific resistance to infection seems to be limited to prevention of expected pathological risks over a relative short period of time.