Streptococcus pneumoniae Flashcards
steps of invasion of pneumococci
- transmission via water droplets
- colonization of mucosal surface of the nasopharynx (carriage)
- dissemination to lower respiratory tract > alveoli fill with serous fluid that contain bacteria, which can travel to other parts of the lung via small airways
- early consolidation: chemotactic signals produced by pneumococci and alternative complement pathway recruits neutrophils and red blood cells to alveoli
- late consolidation: bacteria are mostly cleared by neutrophils
- resolution: neutrophils are replaced by scavenging macrophages who clear the debris of the infection
How does pneumoniae damage
LytA: enzyme that causes lysis > cell wall
components trigger C3B complement cascade > inflammatory damage to host
Pneumolysin: pore forming toxin that:
- is cytotoxic for alveolar and endothelial cells
- triggers complement and inflammation
H2O2 (hydrogen peroxide): local damage
How does pneumoniae make contact with epithelial cells?
CbpA interacts with receptors (factor F) to promote adherence
How does pneumoniae interrupt immune system?
- blocks complement system by binding (CbpA) or degrading complement proteins (PspA)
- degrades/avoids mucous (capsule w negative charge repels mucus) and IgA (ZmpA) , pneumolysin that reduces ciliary beating
- impair neutrophil activity: CbpE degrades platelet activating factor (activates neutrophils)
Testing for meningitis
- lumbar puncture for cerebral spinal fluid
- turbid liquid indicates bacterial growth
- elevated white blood cell count
- decrease glucose used by bacteria as carbon source = sign of bacterial meningitis
+ increased proteins from cytokine production
Test to diagnose pneumoniae
HBA alpha haemolysis (greening) > positive cocci > catalase test negative > optochin sensitive > bile salt soluble
Catalase test
spot bacteria with hydrogen peroxide and see if they produce bubbles -> oxygen is by-product of catalase breaking down H2O2
Bile solubility tests
to test non-susceptible pneumoniae, make suspension of bacteria and split it into a saline solution (control) and a solution with bile salts
bile salts activate LytA, causing lysis, making the suspension clear = bile soluble
Serotyping pneumoniae
Quellung reaction: mix colonies with antisera to see if they get larger = positive
Latex agglutination: antisera is bound to latex beads, clumping = positive
Culture negative cases
When the bacterial load is too small
Identification
- PCR or quantitative PCR targeting LytA
Serotyping
- PCR, DNA microarray (tool to find particular mutation in gene)
Pneumonia from pneumococci diagnosis
- getting samples from lower RT is hard
radiological signs = one lobe effected - pneumococcal antigen tests
control with visualising antibodies
sample line with antibodies against pneumococcal antigen
immune deference’s against pneumococci
- capsular antibodies (preferably IgM or IgG1) that will activate complement
- neutrophil extracellular traps: DNA + enzyme web like structures that trap micro-organisms
- spleen: removes S.pneumoniae + antibody + complement complexes from red blood cells
Pneumococcal polysaccharide vaccine
- selected for most common serotypes
- polysaccharide does not activate T cells, which do not provide help to B cells, who cannot isotype switch or produce memory cells = short lived vaccine
- not good for infants, immunocompromised or elderly
Conjugate S. pneumoniae vaccine
- polysaccharide + highly immunogenic protein
-> often a tetanus toxoid - provides peptide for T cell activation
- 13 serotypes
- used for paediatric populations who failed to response to other vaccine
innate immunity in alveoli
- surfactant: bind to MO and activate complement
- SIgA
- alveolar macrophages
