Bacterial capsules

Question 1

whats the Structure of bacterial capsules?

Answer 1

• network of acidic polysaccharide
- anchored to OM by covalently to phospholipids or lipid A
• Each capsule contains one type of polysaccharide- oligosaccharide units joined by glycosidic linkages
• Polysaccharide chains are diverse, differing in constituent sugars, branching, linkages, group substitutions
• Antigenic diversity

Question 2

how diverse are Ag?

Answer 2

– P. aeruginosa, 80 serotypes
– E. coli, ~80 serotypes
– S. pneumoniae, ~90 serotypes

Question 3

what the diversity of the E. coli capsule?

Answer 3

• K antigens ~80 different serotypes
• Four capsular types with two distinct assembly systems (groups 1 and 4 vs 2 and 3)
• Assembly systems for groups 2 and 3 are encoded by single chromosomal locus

Question 4

whats the genetic structure of the e.coli capsule?

Answer 4

• Regions 1 assembly & translocation 6-8kb
• region 2 is serotype specific synthesis/polymerisation 6kb
• region 3 export 3 kb
• 1+3 conserved

Question 5

what are the components of Capsules of E. coli assembly

Answer 5

KfiABCD - polymerisation 
KpsCMST – translocation & export
KpsU – polymer growth
KpsD – OM transport
KpsE – membrane fusion

Question 6

give an Overview of functions of bacterial capsules

Answer 6

1. Prevent Desiccation
2. Serum resistance
3. ADHERENCE (+/-)
4. IMMUNE EVASION
5. Weak immunogens
6. Molecular mimicry
7. Antigenic/Phase Variation

Question 7

what are the Functions of bacterial capsules?

Answer 7

• regulate access of molecules and ions
• retain nutrients for bacterial growth
• prevent desiccation during transmission from host to host
• promote adherence to surfaces and other bacteria
• inhibit adherence e.g. in Streptococcus pneumoniae; proposed that amounts of capsular material vary at different stages of pathogenesis

Question 8

when is the capsule required?

Answer 8

• not required for normal growth in vitro
• required for survival in host body
• isolates from invasive infection are encapsulated, but lose capsule when sub-cultured in laboratory conditions

Question 9

how does the capsule preform its Major role in evasion of the host immune system

Answer 9

• weakly immunogenic, poor activators of complement
• inhibit opsonisation
• negative charge repels phagocytes (sialic acid)
• shedding removes bound antibodies and complement components

Question 10

describe the Capsules of E. coli’s antigen diversity

Answer 10

• Different components:
– K1: sialic acid
– K20: ribose+KDO
• Different structures:
– some straight chain molecules, others branched
– side group substitutions
• K18 and K22 both ribose + ribitol phosphate
• only difference - in K18 ribose is O-acetylated
• difference due to altered expression of a trans-
acetylase – enzyme inactive or repressed in K22
• serotypes unstable
– pure population can revert to K18/K22 mix

Question 11

what is Molecular mimicry?

Answer 11

• E. coli K1 - a-2,8 linked sialic acid identical to capsules of:
– Neisseria meningitides group B (meningitis in adults)
– Pasteurella haemolytica (pasteurellosis in lambs)
• silica acid residues with a a-2,8 linkage found on surfaces of eukaryotic cells - glycoconjugates on the neural cell
adhesion molecule (NCAM)
• immunologically recognised as “self” molecular mimicry

Question 12

why are K antigens less virulent?

Answer 12

slightly different sialic acid not recognised as self - more immunogenic, less virulent:
– K1+ : SA is O-acetylated
– N. meningitidis group C: SA is a-2,9 linked
– K92: SA is a-2,9 and a-2,8 linked

Question 13

give 3 examples of molecular mimicry

Answer 13

1. E. coli K5: glucuronic acid and N-acetylglucosamine
   mimics an intermediate in heparin biosynthesis
2. E. coli K4: identical with chondroitin, a constituent
   of extracellular matrix
3. Streptococcus pyogenes capsule: hyaluronic acid

Question 14

what is Capsules of E. coli’s role in disease

Answer 14

• small fraction of ~80 K antigens associated with disease

- >90% of E. coli from neonatal meningitis are K1

Question 15

describe neonatal meningitis

Answer 15

– affects 1 in 2-4000 infants
– initial infection of blood invasion from GI tract or
nasopharynx but tropism for meninges (in brain)
– pathology inflammation of meninges
– sudden onset, kill within 24h, 40% mortality rate
– survivors may have irreversible neurological damage

Question 16

why is K1 a successful virulence factor?

Answer 16

• K1 sialic acid
• Required for evasion of host defence, and tropism and invasion of brain endothelium
• successful as very weak immunogen

Question 17

Capsules of meningococci and H. influenzae role in disease

Answer 17

• High frequency of asymptomatic carriage in upper respiratory tract
• Meningococci frequently cause septicemia by high levels of bacteria in the blood
• Invasion of meninges leads to meningitis
• Incidence of bacterial meningitis in the UK is ~1/100,000 people
• Death occurs in 5-10% of people with serious sequelae in 20% of survivors

Question 18

name the Disease-causing serogroups of H. influenzae

Answer 18

b (H. influenzae, Hib)

• Insertion of capsule of Hib into an acapsulate mutant of strain Rd restored bacteremia in animals

Question 19

how do Capsular Vaccines work?

Answer 19

Capsular polysaccharide is conjugated to protein (diptheria toxin) to stimulate a T-cell dependent immune response

Question 20

explain the flu vaccine

Answer 20

Haemophilus influenzae serogroup b vaccine introduced into UK in 1992 resulting in major drop in cases of meningitis due to this bacteria

Question 21

what is the MenC vaccine?

Answer 21

Neisseria meningitidis serogroup C vaccine introduced in 1999 almost eradicated disease due to this serogroup

Question 22

name 5 Modes of transmission

Answer 22

– Fecal-oral route
– Oral route – droplets/sputum - direct
– Fomite (infectious environmental surfaces)
– Sexually transmitted
– Vector borne

Question 23

what are the Requirements/Implications of Transmission

Answer 23

• Frequent release of large numbers of organisms
• Environmental survival – Desiccation – Spores
• Bottlenecks– Small populations impose restrictions on genetic variation

Question 24

how does Virulence Correlate with Transmission

Answer 24

• Model using Citrobacter rodentium in mice mimics enterpathogenic E. coli infections of humans
• Type III secretion system is required for virulence so investigated mutants of this system
• Mice are coprophagic resulting in efficient fecal- oral transmission
• Infected mice are co-housed with uninfected mouse and examined for signs of morbidity/mortality

Question 25

Bacterial capsules - Summary

Answer 25

-  Capsules are polymers of acidic subunits attached to outer membrane -  Multiple genes present in single genetic locus are responsible for biosynthesis, assembly and export of capsules -  Capsules have multiple functions with effects on adherence, immune evasion and transmission -  Capsule are a major determinants of virulence in E. coli and for meningitis -  Transmission requires resistance to desiccation for fecal and oral pathogens -  Virulence enhances transmission