Synergy between Staphylococcus aureus and Pseudomonas aeruginosa in a rat model of complex orthopaedic wounds

Background: We observed an interaction in animals inoculated concomitantly with Staphylococcus aureus and Pseudomonas aeruginosa during a study of the efficacy of surfactants for disinfection of orthopaedic wounds. This led u s to investigate whether synergy could be demonstrated between Staphylococc us aureus and Pseudomonas aeruginosa in a rat model of complex orthopaedic wounds. Methods: A wire was implanted into the spinous process of a lumbar vertebra of Sprague-Dawley rats through a dorsal incision. Animals were divided int o two groups: group one was inoculated with either Staphylococcus aureus or Pseudomonas aeruginosa, and group two received a polymicrobial inoculation with both test organisms in varying concentrations. After inoculation, the wounds were irrigated and closed. On postoperative day 14, all animals wer e killed and specimens from the wounds were cultured. The number of colony- forming units (CFU) of Staphylococcus aureus or Pseudomonas aeruginosa need ed to cause infection in 50% of the animals (ID50) was determined with use of the Reed-Muench method. The infection rate associated with each inoculum combination was calculated, and the two groups were compared. Results: The ID50 was 2.8 x 10(4) CFU for Staphylococcus aureus and 4.8 x 1 0(5) CFU for Pseudomonas aeruginosa. The combination of 10(3) CFU of Staphy lococcus aureus with low concentrations (10(2), 10(3), or 10(4) CFU) of Pse udomonas aeruginosa yielded infection rates that were higher than those fou nd with either organism alone at the same concentrations. The combination o f 10(3) CFU of Staphylococcus aureus and 10(3) CFU of Pseudomonas aeruginos a yielded a 75% infection rate, which was significantly higher (p = 0.004) than that associated with 10(3) CFU of either organism alone. As the Pseudo monas aeruginosa concentration was increased (to 10(5), 10(6), and 10(7) CF U), this trend reversed, and the infection rate decreased to 33% (p = 0.004 ). Low concentrations of Pseudomonas aeruginosa (0 to 105 CFU) combined wit h 10(6) CFU of Staphylococcus aureus yielded infection rates ranging from 8 3% to 100%. At the higher concentrations of Pseudomonas aeruginosa (106 and 10(7) CFU), however, the infection rate again decreased, to 33% (p = 0.005 ). Only Staphylococcus aureus was isolated from the cultures of the specime ns from the animals that had received a polymicrobial inoculum. Conclusions: Synergy between Staphylococcus aureus and Pseudomonas aerugino sa was demonstrated when low levels of each organism were present in the wo und. As the Pseudomonas aeruginosa concentration was increased, the infecti on rates fell well below what would be anticipated, suggesting that low con centrations of Pseudomonas aeruginosa enhance the ability of Staphylococcus aureus to cause infection in this orthopaedic wound model. At the same tim e, the presence of Staphylococcus aureus in the ratios tested decreased the rate of infection by Pseudomonas aeruginosa. Clinical Relevance: Staphylococcus aureus is a pathogen commonly seen in or thopaedic patients. The pathogenicity of Staphylococcus aureus was shown to be increased in the presence of anaerobic bacteria. This study is the firs t one that we are aware of that demonstrated synergy between Staphylococcus aureus and Pseudomonas aeruginosa, at low concentrations, in a wound model while at the same time showing that Staphylococcus aureus lowers the rate of Pseudomonas aeruginosa infection.