• Monika A Krezalek, Sanjiv Hyoju, Alexander Zaborin, Emeka Okafor, Laxmi Chandrasekar, Vitas Bindokas, Kristina Guyton, Christopher P Montgomery, Robert S Daum, Olga Zaborina, Susan Boyle-Vavra, and John C Alverdy.
• Center for Surgical Infection Research and Therapeutics, Pritzker School of Medicine, University of Chicago, Chicago, IL.
• Ann. Surg. 2018 Apr 1; 267 (4): 749-758.

ObjectiveTo determine whether intestinal colonization with methicillin-resistant Staphylococcus aureus (MRSA) can be the source of surgical site infections (SSIs).BackgroundWe hypothesized that gut-derived MRSA may cause SSIs via mechanisms in which circulating immune cells scavenge MRSA from the gut, home to surgical wounds, and cause infection (Trojan Horse Hypothesis).MethodsMRSA gut colonization was achieved by disrupting the microbiota with antibiotics, imposing a period of starvation and introducing MRSA via gavage. Next, mice were subjected to a surgical injury (30% hepatectomy) and rectus muscle injury and ischemia before skin closure. All wounds were cultured before skin closure. To control for postoperative wound contamination, reiterative experiments were performed in mice in which the closed wound was painted with live MRSA for 2 consecutive postoperative days. To rule out extracellular bacteremia as a cause of wound infection, MRSA was injected intravenously in mice subjected to rectus muscle ischemia and injury.ResultsAll wound cultures were negative before skin closure, ruling out intraoperative contamination. Out of 40 mice, 4 (10%) developed visible abscesses. Nine mice (22.5%) had MRSA positive cultures of the rectus muscle without visible abscesses. No SSIs were observed in mice injected intravenously with MRSA. Wounds painted with MRSA after closure did not develop infections. Circulating neutrophils from mice captured by flow cytometry demonstrated MRSA in their cytoplasm.ConclusionsImmune cells as Trojan horses carrying gut-derived MRSA may be a plausible mechanism of SSIs in the absence of direct contamination.

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