Learning Objective: The learner will be able to describe the role of the bacterial capsule as an antiphagocytic virulence factor, identify major encapsulated organisms, and explain their clinical and immunologic significance.
Encapsulated bacteria possess an outer polysaccharide capsule that serves as an antiphagocytic barrier, protecting them from host immune responses. These bacteria are opsonized by antibodies and complement and cleared mainly by the spleen. Asplenic individuals (e.g., after splenectomy or with sickle cell anemia) have decreased opsonization and a high risk of sepsis caused by encapsulated organisms.
Major Encapsulated Bacteria
| Bacterium | Mnemonic | Key Clinical Association |
|---|---|---|
| Pseudomonas aeruginosa | SHiNE SKiS | Opportunistic infections (burns, cystic fibrosis) |
| Streptococcus pneumoniae | SHiNE SKiS | Pneumonia, meningitis, otitis media |
| Haemophilus influenzae type b | SHiNE SKiS | Meningitis, epiglottitis, pneumonia |
| Neisseria meningitidis | SHiNE SKiS | Meningitis, meningococcemia |
| Escherichia coli (K1 strain) | SHiNE SKiS | Neonatal meningitis, UTI |
| Salmonella spp. | SHiNE SKiS | Typhoid fever, gastroenteritis |
| Klebsiella pneumoniae | SHiNE SKiS | Aspiration pneumonia, abscesses |
| Group B Streptococcus (S. agalactiae) | SHiNE SKiS | Neonatal sepsis, meningitis |
Activity:
Mnemonic
“Please SHiNE my SKiS”
Pseudomonas, Streptococcus pneumoniae, Haemophilus influenzae type b, Neisseria meningitidis, Escherichia coli, Salmonella, Klebsiella, Streptococcus (Group B).
Activity:
Key Points
- The capsule acts as an antiphagocytic virulence factor that allows bacteria to evade immune destruction.
- Capsular polysaccharides can be used as vaccine antigens.
- Polysaccharide-only antigens are T-cell independent, while protein-conjugated capsules elicit T-cell-dependent immunity.
- Asplenic patients are highly susceptible to sepsis from encapsulated organisms.
- Vaccination against S. pneumoniae, H. influenzae type b, and N. meningitidis is essential in these patients.
