Immunity and intracellular infection: resistance to bacteria in mice infected with a protozoan.

Mice infected with the intracellular parasite Toxoplasma gondii for periods of as long as 7 months were resistant to challenge with numbers of Listeria monocytogenes and Salmonella typhimurium that were uniformly lethal to normal mice. This resistance did not appear to depend on the strain of toxoplasma employed or the route of inoculation of either toxoplasma or bacteria. Onset of immunity to listeria was demonstrable as early as 1 to 2 days after infection with toxoplasma. Resistance to toxoplasma was not demonstrable in mice immune to listeria. Interferon did not appear to be a mediator of the immunity observed in toxoplasma-infected mice.