Role of lipopolysaccharide in K. pneumoniae virulence. Role of lipopolysaccharide in K. pneumoniae virulence. LPS is composed of three major subunits: lipid A, an oligosaccharide core, and O antigen. Lipid A inserts into the bacterial membrane and is a potentially potent activator of inflammation. K. pneumoniae may modify its lipid A to make it less inflammatory during infection, and lipid A may also protect against the bactericidal action of cationic antimicrobial peptides. O antigen is the outermost subunit of LPS. It has important roles in protecting against complement, including preventing C1q binding to bacteria, which inhibits subsequent activation of the complement pathway, as well as binding C3b away from the outer bacterial membrane and, thus, abrogating bacterial lysis by the complement membrane attack complex. Michelle K. Paczosa, and Joan Mecsas Microbiol. Mol. Biol. Rev. 2016; doi:10.1128/MMBR.00078-15