Described herein are microorganisms that are modified so that they have an increased ability to be recognized by the innate immune system of a eukaryote, relative to an unmodified microorganism. A microorganism may be a gram-negative bacterium that has been modified to produce high potency lipopolysaccharide, e.g., Yersinia pestis expressing LpxL. Such modified microorganisms may be used as vaccines for protection against an infection by the unmodified microorganism. They may also be used as delivery vehicles of one or more heterologous antigens, e.g., antigens from pathogens or those associated with a hyperproliferative eukaryotic cell.