Streptococcus Pneumoniae

Question 1

Describe the bacteriology of S.penumoniae?

Answer 1

• Gram positive bacteria - Forms lancet cocci (elongated) usually in pairs - diplococci, - Facultative anerobes - capable of switching to fermentation if no oxygen present - 90 different capsular serotypes

Question 2

Describe lab identification methods

Answer 2

Gram stain - positive Catalase test - negative Exhibits alpha hemolytic activity on blood agar under aerobic conditions (i.e. oxidises the iron in Hb producing dark green) Optichin sensitive - its growth will be inhibited around the discs. Bile soluble - unlike the rest of alpha hemolytic streptococci.

Question 3

What are draughstman colonies?

Answer 3

Referred to as the flattened shape with a depressed central part and raised edges seen in older S.pneumoniae colonies.

Question 4

Describe the genetics of S.pneumoniae?

Answer 4

They can take up naked DNA from their environment when in a physiologically defined state known as competence. this is called transformation. Transformation is responsible for antibiotic resistance and can occur between strains of pneumococci that co-habitate the upper respiratory tract as they compete for dominance.

Question 5

What are the different diseases caused by S.pneumoniae?

Answer 5

1. Non-invasive - otitis media, sinusitis, bronchitis. 2. Invasive pneumococcal disease - occurs when bacteria enter a normally sterile site: - lung -pneumonia - bloodstream - septicemia, bacteremia - CNS - meningitis pneumoccocal meningitis is leading cause of meningitis WW and significant cause of its mortality.

Question 6

How is IPD transmitted?

Answer 6

Occurs person to person by spread of droplets which colonise the nasopharynx.

Question 7

List the different virulence factors of S.pneumoniae and their functions.

Answer 7

1. Capsule - interferes with phagocytosis by inhibiting opsonisation by complement. - it’s negatively charged so it repels sialic acid of mucus. - phase variation of the capsule from thick to thin exposes the choline binding proteins which will facilitate adherence to ECs and invasion. 2. Exoglycosidases - deglycosylate the mucus glyconjugates which decreases viscosity and prevents entrapment. 3. Pneumolysin - forms transmembrane pores that lead to cell lysis mainly in ciliated cells, preventing removal. - inhibits phagocyte respiratory burst. - activates complement via classical pathway. - activates cd4 t cells via inflammatory mediators. 4. Lysozyme resistance - PdgA will deacetylate peptidoglycan molecules making them resistant to lysozyme in airway secretions. 5. IgA protease - cleaves secretory IgA preventing opsonisation. 6. Pneumococcal surface proteins - bind to GalNac on ECs promoting adherence. - also inhibit complement activation facilitating survival in the blood stream. 7. Choline binding proteins - bind to pIgR on ECs and induce transcytosis of epithelial cells.

Question 8

Describe method of CNS invasion

Answer 8

Adherence to ECs and transcytosis via choline binding proteins and entry into CSF. Once in CSF, the bacteria release LTA and Ply which are recognised by TLRs (TLr2 and TLR4 respectively) on macrophages which will induce production and release of inflammatory cytokines via the NFkB signalling pathway. These chemokines will upregulate adhesion molecules on ECs which will recruit neutrophils to CSF and release more cytokines - inflammation. the chronic inflammation will lead to tissue damage of brain. cytotoxic agents are also released - MMPs which will degrade ECM components and thus disrupt the BBB and peoxynitrite which will destroy fatty acids and cause loss of membrane integrity. also Microglia are a type of glial cell that are the resident macrophages of the brain and spinal cord - they are activated which can cause neuronal damage

Question 9

How is immunity to s.pneumoniae possible

Answer 9

colonisation will induce serotype specific antibodies to be released. cd4 t cells need to be activated for immune memory and B cell class switching - their activation is dependent on Ply

Question 10

What are the two vaccines available for IPD

Answer 10

1. Polysaccharide vaccine - contains purified polysaccharide from 23 serotypes. - safe and immunogenic in adults. - not immunogenic in children as it’s T cell independent and only produces IgM which isnt effective in kids. 2. Conjugate vaccine - protein-polysaccharide - will be presented by APCs and B cells to Th2 cells which allows class switching to IgG - opsonisation. - some of the plasma cells will go onto memory cells.

Question 11

Describe future vaccine strategies

Answer 11

the main issue with vaccines is serotype replacement - the fact that serotypes not included in a vaccine will steadily become more common than the ones in the vaccine. therefore we have to develop vaccines that will contain virulence factors/surface proteins that are conserved in all serotypes. examples include Ply, choline binding proteins and neuraminidase. expressing the antigenic proteins in recombinant bacteria will allow vaccine production at low cost - good for developing countries.

we also need to make sure these vaccines are immunogenic and will elicit a T cell dependent response in infants.

Question 12

What are methods of IPD treatment

Answer 12

• penicillin was previously effective but resistance has emerged. - macrolides, though resistance is now also widespread. - 2nd/3rd gen cephalosporins/fluorquinolones are used. adjunctive treatment - strategies to treat inflammation and harmful sequalae cauded by bacteria. - corticosteroids - reduce inflammation inhibitors of pro-inflammatory cytokines, MMPs and leukocyte migration to brain.