Nosocomial infection of Serratia marcescens may induce a protective effect in monkeys exposed to Bacillus anthracis.

This study was originally designed to collect data on the natural history of inhalational anthrax in a new nonhuman primate model. An uncontrollable event created a new experimental condition which allowed us to retrospectively evaluate the power of the innate immune system to protect from an aerosol exposure of B. anthracis. Five African green monkeys (AGMs) had intravenous catheters implanted. One catheter was accidentally pulled out, leaving four AGMs with catheters and one without. All were exposed, to multiple lethal doses of B. anthracis Ames strain. Blood was collected twice daily to evaluate bacteremia. The AGM with no catheter had blood drawn from a femoral vein and became bacteremic on Day 9; succumbed to inhalational anthrax on Day 10. The other four AGMs had S. marcescens contamination in the catheter; indicated by pure colonies grown from the blood. None of these AGMs showed clinical signs of illness, had B. anthracis or a detectable level of protective antigen in the bloodstream. It appears that the presence of S. marcescens may have induced a "Coley's toxin" effect in this experiment. The innate immune response may have protected the AGMs from a lethal inhalational dose of B. anthracis spores.