This trial is investigating the effect of using mesenchymal stem cells in the treatment of septic shock (or blood poisoning). The trial is being conducted by the Erasmus Medical Centre in Rotterdam and proposes to enrol 30 patients in both groups by September 2015. Participants with severe septic shock will be randomly divided into two groups. One group will receive conventional standard of care (control group). The other group will receive an intravenous injection of donated mesenchymal stem cells within six hours of their diagnosis, in addition to conventional standard of care treatment (experimental group). The experimental group will receive a further two injections of stem cells at 24 hour intervals. Participants will be monitored for complications and changes in clinical condition.