Bacterial capsules are outermost structures composed of polysaccharides or polypeptides that protect pathogenic bacteria and enable virulence. Examples include Streptococcus pneumoniae, Escherichia coli, Proteus mirabilis, and Bacillus anthracis. Capsules aid in adhesion, resistance to host defenses and antimicrobials, and intracellular survival. They enable bacterial colonies to appear smooth versus rough and are often required for full virulence. Capsule composition and genes involved in synthesis vary between species but generally help bacteria evade clearance and persist in the host.

1. BACTERIAL CAPSULES

2. TOPIC TO COVER …..
o INTRODUCTION TO CAPSULES
o COMPOSITION
o EXAMPLES OF SOME BACTERIA
 S. pneumonia
 E. coli
 P. mirabilis
 B. anthracis
o FUNCTION OF CAPSULE

3. CAPSULES
• Capsules are the outermost
structures of bacterial and
fungal cells.
• These are physically associated
with the cell surface and cannot
be readily washed off.
• Thickness ≈ 0.1 to 10 microns.
• Capsules have a sufficient size
and are readily visible, referred
as capsules .
• Whereas many microorganisms
produce a variety of
extracellular materials they may
be too thin or soluble in
medium and called as slime
layer .

4. .
 Capsulated cells of pathogenic bacteria are usually more virulent than the cells
without capsules.
 When grown on the agar media, encapsulated bacteria usually form colonies with
the distant appearance and morphology .
 In some bacteria, the colonies of encapsulated bacteria are referred as “smooth
colonies “ and those which are unencapsulated are the “rough colonies “ .
 Some strains fail to produce the polysaccharide capsular material are “ Avirulent “
and those which produce are called as “ Virulent “.
 Eg. of avirulent : S. pneumoniae
of virulent : B. anthracis

5. COMPOSITION OF CAPSULES
 Capsules is a gelatinous polymer made up of either polysaccharide or polypeptide
or both.
 Capsules are usually polymers of simple sugars : mucus polysaccharide – colanic
acid or M antigen .
exception : B. anthracis –poly glutamic acid (PGA).
 Capsules can be detected by light and electron microscopy after staining bacteria
with dye.
By Mordant – cause precipitation of capsular material by metal ions
alcohol , acetic acid .
India Ink – the capsules is displayed as clear halo between the
stained cells and the background .
 Reaction of capsule with specific antiserum is to enlarge the capsular area soit is
visualized under microscope .

6.  The polysaccharide capsular substance produced by S. pneumonia confers the
immunological specificity and are also associated with virulence.
 90 distinct capsular serotypes ,each differing in sugar composition and/or linkages have
been recognized and structures are determined.
 Type 3 pneumonococcal polyscacharide is composed of :
-- glucopyranose and glucuronic acid is alternating β-1,3 and β-1,4 linkage.
 Synthesis of Type 3 capsular polysaccharides of S. pneumonia requires UDP-glucose
(UDP-Glc) and UDP-glucuronic acid (UDP-GlcUA) for the production of 3-β-D-Glc-(1,4)-β-
D-Glc-[1]n polymer.
 The generation of UDP-Glc procced by the conversion of;
Glc-6-P to Glc-1-P to UDP-Glc and is mediated by the phosphoglucomutase ( PGM)
Glc-1-P- uridylyltransferase respectively.
S. pneumoniae CAPSULE

7. .
 Gene encoding both Glc-1-P-uridylyltranferase (cps3U) and for
PGM homolog (cps3M) are present in the Type-3 capsule locus, but
the genes are not essential for capsule production.
 Other S. pneumonia serotypes and in other streptococci,enzyme
expected for the capsule production are the present in the capsule
locus and encoded by the genes.
 Eg. Type-14 locus lacks genes for the synthesis of UDP-glucose
,UDP-galactose and UDP-N-acetylglucosamine , precursor of type
14 polysaccharide.
 In other strains of S.pneumonia and S.pyogenes the sugar or other
intermediate is important component of teichoic acid or
peptidoglycan and cells uses existing cellular pools of these for
synthesis of capsular polysaccharide.

8. E. coli CAPSULE
 A number of strains of E.coli produce capsular polysaccharide called as
K antigen .
 They are structurally diverse and give rise to serological specificity.
 There are 70 recognized K antigen in E.coli.
 the K antigen are placed in either Capsular Group I (heat stable) or
Capsular Group II (temperature stable).
 Eg. K30antigen if E.coli is member of Group I and K5 of Group II.
 The Group II K antigen are characterized by acidic components such as
2-keto-3-deoxy-D-mannooctulonic acid,N-acetylneurominic acid (sialic
acid ),N-acetylmamosaminouronic acid .

9. .
 The k5 polysaccharide is nonimmunogenic and strains of E.coli
expressing this capsule are quite virulent.
 The capsular polysaccharide 17-kb multi-gene cluster encodes the
proteins requires for the synthesis, activation, and assembly of K
antigen.
 Three distinct regions have been identified in this gene cluster.
 Central region or region 3 – neu genes (polysaccharide synthesis)
 Region 1and 3 -kps genes (polymer assembly and transport )
 E.coli K-12 and other enteric bacteria produce a slime polysaccharide ,
colonic acid , called M antigen .
 The genes rcsAk12 and rcsAK30 – expression of colanic acid synthesis .

10. P. mirabilis CAPSULES
 P. mirabilis produce an acidic capsular polysaccharide .
 It is of high MW of branched trisaccharide units composed of 2-
acetamido-2-deoxy-D-glucose(N-acetyl-D-glucosamine) , 2-
acetamido-2,6-dideoxy-L-galactose (N-acetyl-L-fucosamine) and D-
glucuronic acid .
 The acidic nature of this polysaccharide play a role in urinary calculi
(stone) formation .
 The process is initiated by the a minute focus of solid matter that
grows continually larger as chemical crystals that precipitate from urine
adhering to it.

11.  The B.anthracis capsules is a polypeptide of γ-D-glutamyl subunits
originally thought to be produce only in vivo during infection but later
in culture if access of CO2 or bicarbonate is present.
 High MW peptide capsules produced by – Yersinia pestis and Bacillus
anthracis.
 Other species of bacilli (eg. B.subtilis) produce a polypeptide capsule
containing a mixture of D- and L-glutamic acid sub units.
B. anthracis CAPSULES

13. FUNCTION OF CAPSULES
 A capsular layer of extracellular polysaccharide material can enclose many
bacteria into a biofilm and serves many function.
 The function includes : adhesion ,transmission, resistance to innate host
defenses , resistance to host’s adaptive immune response and intracellular
survival.
1. Attachment to surfaces.
2. Protection against phagocytic engulfment.
3. Protecting the cell wall against attack by various kinds of antibacterial
agents, lysozyme, bacteriophages, etc .
4. Protection against osmotic stress.
5. Protection from chemical present in environment eg. Detergents
6. Protection against harsh environmental condition.

14.  Reference : MOAT , FOSTER AND SPECTOR
 Microbial physiology 4 Edition .

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