Data are presented, demonstrating that infection with Rickettsia typhi brings about a reduction of the death rates and a prolongation of the time of survival in animals subsequently challenged with Pasteurella pestis or Pasteurella tularensis.

This interference with bacterial infection by previous rickettsial infection does not appear immediately after injection of the rickettsiae; it begins to appear around 16 hours after this injection and becomes more marked during the first 96 hours; later it decreases.

The phenomenon is essentially a local tissue reaction with weaker systemic effects.

The phenomenon of interference can be overcome by challenge with too large a number or too virulent a strain of bacteria.

A strain of P. pestis subjected to 10 passages through mice infected with Rickettsia typhi failed to develop resistance to the interfering activity of the latter microbial species. This does not rule out, but might be evidence against, an assumption that the interfering action is due to antibiotic effects from the rickettsiae.

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