Determination of the Infective Dose of Staphylococcus aureus (ATCC 29213) and Pseudomonas aeruginosa (ATCC 27853) When Injected Intraperitoneally in Sprague Dawley Rats

Objective: To determine the infective dose of Staphylococcus aureus (ATCC 29213) and Pseudomonas aeruginosa (ATCC 27853) in Sprague Dawley rats, when the inoculum is injected via the intraperitoneal route.

Design and Methods: In interest of animal welfare and to reduce the number of animals used, we utilized the “up-down procedure” for dose determination of Pseudomonas aeruginosa and Staphylococcus aureus.Animals were grouped in fours, injected IP with inoculums of known concentration of a single organism and observed for 10 days. Clinical signs such as lethargy, increased respiration, porphyrin staining was recorded. Gross necropsy was performed ten (10) days post infection. Only one infective dose was done at a time and depending on the outcome, the size of inoculum was adjusted for the next step in the experimental infectious process.

Results: Doses of inoculum was carefully titrated and the highest tolerable dose for each organism was determined. These doses allowed for survival of the animals and gave clinical signs, which mimicked the scenario in a human population. Symptoms of infection included lethargy, ruffled fur, porphyrin staining, dehydration and hunched back. At necropsy at 10 days post infection, common indicators of infection observed were ascites, abscesses on the intestinal wall, kidneys, liver and spleen. There were also fibrin tags, rounded livers, enlarged spleen and increased pericardial effusion was prominent in the S. aureusinfected group.

Conclusions: The infective doses were determined based on clinical signs, survival and post mortem changes. The doses determined for Staphylococcus aureus (ATCC 29213) was 1.75 X 1010 cfu/ml and Pseudomonas aeruginosa (ATCC 27853) was 3.0 X 108cfu/ml. These doses would be useful in infective animal studies to determine the efficacy of antimicrobial agents.