Bacterial meningitis is a serious global health problem and one of the major causative organisms is Neisseria meningitidis, which is also a common commensal in the upper respiratory tract of healthy humans and the only agent able to cause epidemics of meningitis. In bacteria, numerous loci involved in biosynthesis of surface exposed antigenic structures, such as outer lipopolysaccharides, lipoproteins, pili, flagella and other secreted proteins that are involved in the interaction between bacteria and host, are frequently subjected to homologous recombination and phase variation. These mechanisms are well described in Neisseria, and phase variation provides the ability to change these structures reversibly in response to the environment.