Polysaccharide encapsulated bacteria

SEM micrograph of an encapsulated bacterium (Pneumococcus).

Polysaccharide encapsulated bacteria, frequently referred to simply as encapsulated bacteria and less precisely called encapsulated organisms, are a group of bacteria that have an outer covering, a bacterial capsule, made of polysaccharide.

Examples of encapsulated bacteria

• Haemophilus influenzae type b (Hib)^[1]
• Streptococcus pneumoniae (pneumococcus)^[1]
• Neisseria meningitidis (meningococcus)^[1]
• Group B streptococcus (GBS)^[1]
• Klebsiella pneumoniae
• Salmonella typhi^[2]

Role in disease

Many encapsulated bacteria are pathogens that lead to a significant amount of morbidity and mortality.^[3] Encapsulated bacteria are antiphagocytic and hence are susceptible to antibody mediated immune response (B cells) as opposed to cell mediated immune response. Patients with B cell deficiencies are highly susceptible to encapsulated bacterial infections.

Asplenia

People that lack a spleen, functionally (as in sickle cell disease^[4][5]) or anatomically (due to a splenectomy or congenital absence), have been shown to be more susceptible to these pathogens.^[6] Therefore, it is standard medical practice to recommend vaccination.

It is generally recommended that people with asplenia are vaccinated against Neisseria meningitis, Haemophilus influenzae, and Streptococcus pneumoniae.^[6] These vaccines can be remembered with the mnemonic: SHiN. Alternatively they can be called the NHS bacteria, as in National Health Service in the UK.

Children

Young children do not have the ability to make antibodies to polysaccharide and are, therefore, more susceptible to encapsulated bacteria. This is especially true for children between six months and one year old (who are typically not breast feed); in these children the maternal antibodies are depleted and endogenous synthesis is not perfected yet.

Complement deficiency

Patients with Complement deficiency are at risk of infections with encapsulated organisms. There are several types of complement deficiency, two of note are C3 and C5,6,7,8,9. A C3 deficiency allows for infection by encapsulated organisms such as; Neisseria, Group B Strep, Klebsiella, Salmonella, HIB and Pneumococci. A C5,6,7,8,9 (the MAC, membrane attack complex, or terminal complement proteins) deficiency lead to unopposed Neisserial infections.^[7] Hence the need for vaccinations in these conditions.

See also

• Conjugate vaccine

References