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The Journal of Bone and Joint Surgery (American) 83:855-861 (2001)
© 2001 The Journal of Bone and Joint Surgery, Inc.

Synergy Between Staphylococcus aureus and Pseudomonas aeruginosa in a Rat Model of Complex Orthopaedic Wounds

Kelly J. Hendricks, MD, Tim A. Burd, MD, Jeffrey O. Anglen, MD, Andrew W. Simpson, PhD, Gordon D. Christensen, MD and Barry J. Gainor, MD

Investigation performed at the Department of Orthopaedic Surgery, University of Missouri-Columbia, and the Research Service, Harry S. Truman Memorial Veterans Hospital, Columbia, Missouri
Kelly J. Hendricks, MD
Tim A. Burd, MD
Jeffrey O. Anglen, MD
Barry J. Gainor, MD
Department of Orthopaedic Surgery, University of Missouri-Columbia, One Hospital Drive, 213 McHaney Hall, Columbia, MO 65212

Andrew W. Simpson, PhD
Gordon D. Christensen, MD
Research Service, Harry S. Truman Memorial Veterans Hospital, 800 Hospital Drive, Columbia, MO 65201

In support of their research or preparation of this manuscript, one or more of the authors received grants or outside funding from a Veterans Affairs grant. None of the authors received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, educational institution, or other charitable or nonprofit organization with which the authors are affiliated or associated.

Read in part at the Annual Meeting of the Association of Bone and Joint Surgeons, Amelia Island, Florida, May 16, 1999, and at the Annual Meeting of the Mid-America Orthopaedic Association, Scottsdale, Arizona, March 27, 2000.

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 us to investigate whether synergy could be demonstrated between Staphylococcus 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 into 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 were killed and specimens from the wounds were cultured. The number of colony-forming units (CFU) of Staphylococcus aureus or Pseudomonas aeruginosa needed 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 ¥ 104 CFU for Staphylococcus aureus and 4.8 ¥ 105 CFU for Pseudomonas aeruginosa. The combination of 103 CFU of Staphylococcus aureus with low concentrations (102, 103, or 104 CFU) of Pseudomonas aeruginosa yielded infection rates that were higher than those found with either organism alone at the same concentrations. The combination of 103 CFU of Staphylococcus aureus and 103 CFU of Pseudomonas aeruginosa yielded a 75% infection rate, which was significantly higher (p = 0.004) than that associated with 103 CFU of either organism alone. As the Pseudomonas aeruginosa concentration was increased (to 105, 106, and 107 CFU), this trend reversed, and the infection rate decreased to 33% (p = 0.004). Low concentrations of Pseudomonas aeruginosa (0 to 105 CFU) combined with 106 CFU of Staphylococcus aureus yielded infection rates ranging from 83% to 100%. At the higher concentrations of Pseudomonas aeruginosa (106 and 107 CFU), however, the infection rate again decreased, to 33% (p = 0.005). Only Staphylococcus aureus was isolated from the cultures of the specimens from the animals that had received a polymicrobial inoculum.

Conclusions: Synergy between Staphylococcus aureus and Pseudomonas aeruginosa was demonstrated when low levels of each organism were present in the wound. As the Pseudomonas aeruginosa concentration was increased, the infection rates fell well below what would be anticipated, suggesting that low concentrations of Pseudomonas aeruginosa enhance the ability of Staphylococcus aureus to cause infection in this orthopaedic wound model. At the same time, 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 orthopaedic patients. The pathogenicity of Staphylococcus aureus was shown to be increased in the presence of anaerobic bacteria. This study is the first 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.


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