Gram-negative bacteria remain the leading cause of sepsis, a disease that is consistently in the top 10 causes of death internationally. Curing bacteremia alone does not necessarily end the disease process as other factors may cause inflammatory damage. Bacterial outer membrane vesicles (OMVs) are naturally produced blebs from the outer membrane of gram-negative bacteria, which contain various proteins and lipopolysaccharide (LPS). We hypothesize that these vesicles initiate an inflammatory response independent of the parent bacteria. Outer membrane vesicles were isolated from cultures of Escherichia coli, and the concentration of LPS in the OMVs was measured. Adult male Sprague-Dawley rats were separated into five treatment groups: OMV, 2xOMV, LPS, lactated Ringer's, and sham. Our findings show that infused OMVs elicit physiological, histological, and molecular changes in rats that are consistent with sepsis. Hyperdynamic changes in heart rates and mean arterial pressures are observed as well as the elevation of the proinflammatory cytokines tumor necrosis factor α and interleukin 6. Downstream events such as the recruitment of neutrophils into tissues due to the presentation of vascular adhesion molecules also occur in OMV-treated animals. Although soluble LPS elicits stronger responses than did OMVs, responses to the latter consistently exceeded those associated with lactated Ringer's infusion. These results indicate OMVs, independent of the parent bacteria, do initiate an inflammatory response; however, further studies are required to better characterize the temporal biomolecular interactions involved.

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Bacterial outer membrane vesicles (OMVs) can initiate the inflammatory response in the transition of an infection to sepsis, and sepsis is a prothrombotic state. The role of OMVs in the development of prothrombotic tendency is unknown. The objective: to determine whether OMVs and endothelial microparticles can induce formation of prothrombotic monocyte‐platelet aggregates (MPA). Donor blood was incubated either with OMVs from clinical isolate of an enterotoxigenic strain of E. coli or with supernatant from HUVEC (Human Umbilical Endothelial Cells) cultured in the presence of OMV for 24 hrs. MPA were measured in whole blood by flow cytometry as CD61+CD14+ cells. In whole blood from healthy human volunteers, MPA level is 19±2% in unstimulated state and 37±8% in ADP‐stimulated state. Incubation of blood with OMVs did not significantly increase MPA neither in unstimulated blood (23±2%), nor in ADP‐stimulated blood (24±7%). However, supernatant from HUVEC cultured with OMVs increased level of MPA in ADP‐stimulated blood (54±9%). Incubation with microparticles‐free HUVEC supernatant decreases MPA in ADP‐stimulated blood to a control level (35±4%). Thus, it is likely that microparticles derived from OMV‐treated HUVEC increase MPA. Therefore OMV may contribute to a prothrombotic state by stimulating microparticles release from activated endothelium and increasing prothrombotic MPA formation.