Flashcards in streptococcus Deck (23):
What are streptococci?
These are a large heterogenous group of bacteria which are gram positive, facultative anaerobic, cocci that grow in chains with some strains producing capsules and all strains being apparently still susceptible to penicillin
They are commonly found in the oropharynx and skin of healthy children and young adults
What are the epidemiological features of streptococci?
There is asymptomatic colonisation of the upper respiratory tract with 20% of the healthy population being carriers
S. Pyogenes can survive on dry surfaces for long peroids with bacteria being spread from person to person by respiratory droplets with corwding increasing the risk of spreading
For unknown reasons Maori and Pacific Ilsanders have higher infection rates compared to Europeans
S. pyigese can be further divided into M serotypes based on variations of the M protein
What are the non-invasive diseases caused by S. Pyogenes?
Impetigo, Pharyngitis, Scarlet fever and cellulitis
What is impetigo?
A localized cutaneous infection with pus filled vesicles which primarily affects young children this is highly communicable and often seen in crowded places with a hot or humid climate
What is pharyngitis?
This develops in 2-4 days after exposure and symptoms include sroe throat, fever, reddened pharynx and tonsillitis
What is scarlet fever?
This is a complication of pharyngitis that can develop into a serious systemic disease where there is a yellowish coat on the tounge which is shed off to reveal a raw surface and is due to the involvement of streptococcus pyrogenic endotoxin A which is a superantigen
This often causes a characteristic rash on arms and chest
What is cellulitis?
This is an infection of the skin that involves subcutaneous tissue which is acute and rapidly spreading
What are the invasive disease caused by S. Pyogenes?
Necrotising fasciitis and streptococcal toxic shock syndrome
What is necrotising fasciitis?
This is a deep infection of the skin that involves destruction of muscles where S. pyogenes is introduced through minor cuts, trauma, burn, surgery or vesicular viral infection
This deep infection is supported by spreading factors such as DNAses, proteases and hyaluronidase it can often develop into a severe systemic disease with high mortality
What is streptococcal toxic shock syndrome?
This often follows necrotising fasciitis and sepsis with a high mortality rate
There is a massive release of proinflammatory cytokines which occurs in response to super antigen secretion leading to overstimulation of the immune response with systemic inflammation
Symptoms include fever headache, multiorgan failure and shock
What are the auto immune disease caused by S. pyogenes?
Rheumatic fever and rheumatic heart disease these develop after untreated/chronic sore throats due to S. Pyogenes, there is inflammation of the endocardium, myocardium and pericardium which results in a thickened deformed heart valves and granulomas in the myocardium
The disease often starts with inflammatory changes in joints
This disease is triggered by molecular mimicry where the M protein of S. Pyogenes generates cross reactive antibodies
What are the adhesions of S. pyogenes?
MSCRAMMS (microbial surface components recognizing adhesive matrix molecules) these are cell wall attached proteins found in gram positive bacteria which specifically bind to extracellular matrix proteins
There are also pili which have a tip protein believed to function as an adhesion
What are the factors of S. Pyogenes that damage the host cell?
Streptolysin O which is an oxygen labile cytolysin which forms pores in host cell membranes
Streptolysin S which is an oxygen stable cytolysin that lyses leucocytes, RBCs and platelets this molecule is highly antigenic and can be used intests for previous infection in the case of rheumatic fever
What are the spreading factors of S. Pyogenes?
Lipases, Nucleases, Hyaluronidase, Proteases and streptokinase (this converts serum plasminogen into plasmin which cleaves fibrin)
What are the super antigens of S. Pyogenes?
These include streptococcal pyogenic exotoxins and streptococcal mitogenic endotoxin Z
These compounds are structurally and functionall related to the staphylococcal superantigens however they do not cause food poisoning
They do however trigger a massive relase of proinflammatory cytokines leading to over stimulation of the host immune response leading to systemic inflammation playing an important role in toxic shock syndrome
What are the immune evasion factors of S. Pyogenes?
A capsule, DNAses, M-Protein C5a peptidase, streptococcal inhibitor of complement, SpyCEP, Immunoglobulin G degrading enzyme, streptococcal cysteine protease
How does a capsule function as an immune evasion factor?
The dense hyaluronic acid coat prevents opsonisation with ab and complement (this is only present in some strains)
How do DNases function as an immune evasion factor?
These cleave the backbone structure of neutrophil extracellular traps which are a net like structure consisting of DNA and attached bactericidal proteins which trap and kill bacteria
How does Mprotein function as an immune evasion factor?
This can recruit complement regulators factor H and C4BP to prevent complement opsonisation by C3b
How does C5a peptidase function as an immune evasion factor?
This is a protease which cleaves complement factor C5a preventing neutrophil migration to the site of infection
How does Streptococcal inhibitor of complement function as an immune evasion factor?
This binds to the C5b-C7 complex inhibiting complement mediated lysis
It also binds and inhibitsalpha defensin and LL37
How does SpyCEP function as an immune evasion factor?
This is a cell wall anchored protein which is shed to release an extracellular form that cleaves interleukin-8 preventing neutrophil infiltration to the site of infection