Klebsiella pneumoniae: a warrior with a powerful shield, the capsule.

Feb ,14 2017

All my PhD research, in a way or another, is turning around the bacterial capsule…

On one side, I am trying to elucidate why a capsule mutant of Klebsiella pneumoniae is unable to produce a functional colibactin, a genotoxin that has been linked with tumour formation in colorectal cancer. On the other side, I want to explore the effect of the bacterial capsule on the pathogenesis of Klebsiella pneumoniae subsp. rhinoscleromatis, responsible of rhinoscleroma, a human disease characterized by the formation of granuloma in the airways.

However, I always receive the same question from my friends and my family. What is this capsule? Why do the bacteria build this “shield”? Is it really important to accomplish their “war” against the host?

Here is, in few words, the answer for curious minds!

Capsule: WHAT?

The capsule is a gelatinous layer covering the surface of many bacteria. It is composed of complex polysaccharides, i.e. sugars, and these polymers are composed of repeating oligosaccharide units. The sugar components of polysaccharide varies within the species of bacteria, which determines their serologic types.
You can easily recognize a capsulated bacterium like Klebsiella pneumoniae because of the slimy and mucoid aspect of the colonies.

Capsule: WHY?

The capsule is considered a virulence factor because it enhances the ability of bacteria to cause disease. Indeed the capsule protects the bacterium from phagocytosis and saves engulfed bacteria from the action of neutrophils. It is responsible for bacterial resistance to complement-mediated killing, it protects the bacteria against desiccation and it helps bacterial adhesion to surfaces and biofilm formation at the infection sites. Moreover, capsular polysaccharides are used as antigens in certain vaccines (e.g. polyvalent vaccine of Neisseria meningitidis capsule).

Capsule: WHO?

The capsule is found most commonly among Gram-negative bacteria but we can find also Gram-positive bacteria with a capsule.
I found, reading around, a really funny mnemonic used to remember some encapsulated pathogens: Some Killers Have Pretty Nice Capsule….

Streptococcus pneumoniae
Klebsiella pneumoniae
Haemophilus influenzae
Pseudomonas aeruginosa

Neisseria meningitidis
Cryptococcus neoformans

Anti-phagocytic activity of encapsulated bacteria (source of the image: microbeonline.com).