The present invention provides novel, recombinant Gram-negative bacteria. In particular, the invention provides recombinant Gram-negative bacteria (e.g., E. coli) lacking genes involved in lipopolysaccharide (LPS, endotoxin) biosynthesis (e.g., lacking genes required for core oligosaccharide biosynthesis) and also provides recombinant Gram-negative bacteria lacking genes involved in LPS biosynthesis that contain one or more exogenous KDO transferases and/or one or more exogenous heptosyltransferases (e.g., from one or more types and/or strains of bacteria). The invention further provides methods of generating and utilizing (e.g., as or in an immunogenic composition (e.g., as or in an adjuvant and/or vaccine)) the recombinant Gram-negative bacteria therapeutic, preventative, and/or research applications.