Bacterial Capsule and Flagella
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Direct answer
A bacterial capsule is a gelatinous polysaccharide layer outside the cell wall that masks surface antigens and resists phagocytosis, whereas flagella are protein appendages that drive motility and carry the H antigen. The number and position of flagella, whether monotrichous, lophotrichous or peritrichous, are read in motility tests. Capsule and flagellar antigens are therefore separate markers in identifying and typing pathogens.
What you must remember
- Capsule composition: usually polysaccharide, though Bacillus anthracis makes a capsule of poly-D-glutamic acid rather than a sugar polymer.
- Antiphagocytic role: the capsule hides complement and surface antigens, so phagocytosis works only after specific antibody opsonises the bacterium.
- Mucoid colonies: heavy capsule production, as in Klebsiella pneumoniae, gives large, sticky colonies on agar.
- India ink: a negative stain that shows an unstained halo around capsulated organisms.
- Quellung reaction: type-specific antisera make the capsule appear swollen under the microscope, used for typing pneumococci and Klebsiella.
- Flagellin and H antigen: flagella are made of flagellin, and H antigens drive serotyping in Salmonella and Escherichia coli.
- Peritrichous flagella: spread over the whole surface, as in Salmonella and Proteus, with Proteus showing swarming growth on agar.
- Non-motile pathogens: Shigella, Klebsiella and Yersinia pestis are non-motile, whereas Yersinia enterocolitica is motile at 22 to 25 °C but not at 37 °C.
How flagellar arrangement is read and tested
Flagellar arrangement is a fixed feature of each species, so it is useful for identification at the bench. The table lists the three arrangements most often asked in examinations, with a representative organism for each.
| Arrangement | Flagella | Example |
|---|---|---|
| Monotrichous | One polar flagellum | Vibrio cholerae |
| Lophotrichous | Tuft at one or both poles | Helicobacter pylori |
| Peritrichous | Spread over the whole surface | Salmonella enterica |
Motility is confirmed by a hanging drop preparation or by stab inoculation into semisolid medium. Motile organisms spread away from the stab line and make the medium diffusely turbid, whereas non-motile organisms grow only along the line. Capsules are not shown by the same test, so they are looked for separately with India ink or the Quellung reaction.
The H antigen and the K (capsular) antigen are separate. H antigens are flagellar and are typed by agglutination with H antisera, while the K antigen can mask the O antigen of lipopolysaccharide during serotyping.
Where the exam tries to catch you
Most questions on this topic test whether the candidate can separate the capsule from the flagellum.
- Trap: linking the H antigen with the capsule Correct: H antigen is flagellar, the K antigen is capsular, and the O antigen belongs to lipopolysaccharide.
- Trap: assuming a Gram-negative bacillus is non-motile Correct: Salmonella is usually motile with peritrichous flagella, while Shigella is the classic non-motile genus.
- Trap: choosing Gram stain to demonstrate a capsule Correct: capsules are poorly retained by routine stains, so India ink or the Quellung reaction is used.
Frequently asked questions
Why does a bacterial capsule make an organism resistant to phagocytosis?
A bacterial capsule makes the organism resistant to phagocytosis because its polysaccharide layer masks surface antigens and blocks complement deposition. Phagocytes can then engulf the bacterium only after specific antibody opsonises the capsule.
What is the Quellung reaction used for in encapsulated bacteria?
The Quellung reaction is used to type encapsulated bacteria such as Streptococcus pneumoniae and Klebsiella pneumoniae. Type-specific antisera bind the capsule, which then appears swollen and refractile under the microscope.
Why is the capsule of Streptococcus pneumoniae important for vaccine design?
The capsule of Streptococcus pneumoniae is the main virulence factor, so it is the target of pneumococcal vaccines. Conjugate vaccines link capsular polysaccharides to a carrier protein, which gives a stronger response in infants than plain polysaccharide vaccines do.
Which flagellar arrangement does Vibrio cholerae show?
Vibrio cholerae has a single polar flagellum, which is called a monotrichous arrangement. This sheathed flagellum drives the rapid darting motility seen in a hanging drop preparation.
Why are Shigella species considered non-motile?
Shigella species are considered non-motile because they lack flagella, so they cannot swim through the intestinal mucus. They spread between epithelial cells by actin-based movement instead.