Bacterial capsular polysaccharides

Bacterial capsules are composed of polysaccharides (with some exceptions like Bacillus anthracis, which has a poly-D-glutamate capsule). They surround the bacterial cell wall and protect the organism from phagocytosis by preventing complement (C3b) and antibody deposition from being recognized by phagocyte receptors, effectively acting as an anti-phagocytic virulence factor. Encapsulated organisms are classically associated with infections in asplenic patients (e.g., post-splenectomy, sickle cell disease) because the spleen is critical for clearing encapsulated bacteria via IgM-mediated opsonization and complement activation—splenic macrophages are essential for removing poorly opsonized encapsulated bacteria from the blood.

Key encapsulated organisms (mnemonic: 'SHiNE SKiS'): Streptococcus pneumoniae, Haemophilus influenzae type b, Neisseria meningitidis, Escherichia coli (K1 capsule), Salmonella, Klebsiella pneumoniae, Group B Streptococcus (Streptococcus agalactiae).

Clinical relevance: Patients with asplenia or hyposplenism (sickle cell disease, surgical splenectomy) are at increased risk for fulminant infections with encapsulated organisms, particularly S. pneumoniae, H. influenzae, and N. meningitidis, necessitating prophylactic vaccination against these pathogens.

Vaccine design: Polysaccharide capsules are T-independent antigens, meaning they stimulate B cells directly without T-cell help, resulting in weaker, shorter-lived immune responses without immunologic memory, and poor efficacy in children under 2 years old (whose immune systems rely heavily on T-cell-dependent responses). To overcome this limitation, conjugate vaccines were developed by covalently linking the polysaccharide to a protein carrier (e.g., diphtheria toxoid, tetanus toxoid), converting the immune response to a T-cell-dependent one. This recruits T-helper cells, leading to improved antibody affinity maturation, isotype switching (IgG), and generation of memory B cells—critical for effective immunization in infants and young children.

Examples of conjugate vaccines: Hib conjugate vaccine, pneumococcal conjugate vaccine (PCV13/15/20), and meningococcal conjugate vaccines (MenACWY).

Laboratory identification: Capsules can be visualized using India ink staining (showing a clear halo around the organism, classically for Cryptococcus neoformans, a fungus with a polysaccharide capsule) or Quellung reaction (capsular swelling in response to specific anticapsular antibodies, historically used for pneumococcal serotyping).