Infection due to encapsulated bacteria 2019-04-08T18:54:35Z An infection by an encapsulated bacterial agent. Isolates which cause invasive disease are usually surrounded by a polysaccharide capsule, which is a major virulence factor and the key antigen in protective protein-polysaccharide conjugate vaccines. Encapsulated bacteria have been responsible for the majority of bacteremia and meningitis in children for many decades. In children aged over 3 months, Streptococcus pneumoniae (pneumococcus), Neisseria meningitidis (meningococcus), and Haemophilus influenzae have been the predominant pathogens, with H. influenzae being a less significant problem since the introduction of vaccines against type b organisms. Disease is most common in young children less than 5 years of age and older adults aged over 65 years. In neonates and young infants, Group B Streptococcus (GBS) is the major cause of bacteremia and meningitis. These organisms have the shared characteristic of being surrounded by a polysaccharide capsule, which is a key virulence factor because it helps the bacteria evade complement deposition and subsequent phagocytosis and killing. These polysaccharides have also been the basis for successful vaccines against all except GBS, because immune responses against the polysaccharide capsule are the primary mechanism of protection for the human host. Each species can be encapsulated by polysaccharides of different biochemical compositions, which has been used for categorization into capsular groups or serotypes. Opportunistic bacterial infection