Staphylococcus Aureus and Disease Flashcards
Describe chararacteristics of Staphlococci?
- Gram positive spherical cells(cocci)
- Irregular grape-like clusters
- non-motile(absence of flagella)
- Resistant to dry conditions and up to 10% NaCl(= halodurent)
Where do Staphilococcus colonise?
Moist skin folds, oropharynx, gatrointestinal tract and urogenital tract.
How are S. aureus transmitted?
Mainly human to human(shedding human lesions, skin)
What are some of the S.aureus strains resistant to?
Penicillin(B-lactamase)
Methicillin(MRSA)
Vancomycin(VRSA)
How do you diagnose Staphylococci?
- Catalase test- If the peroxidase bubbles, then its is staphyloccus +
- Coagulase test- mannitol salt sugar- staphylococcus aureus are the only bacteria that can ferment mannitol.
What are the virulence factors responsible for?
Enable the bacteria to colonist the host, damage host tissue, spread from site of infection and evade the immune response.
What do virulence factor adhesins MSCRAMMS do?
=cell wall attached proteins found in gram positive bacteria.
= bind to host extracellular matrix proteins like fibronectin, elastin, laminin, vitronectin and collagen.
=tissue colonisation eg fibronectin binding protein, collagen binding protein.
What are some virulence factors that damage the host cell?
> Cytolysins
Exfoliative toxins(ETA, ETB)
Spreading factors
Immunopathogenic factors- Superantigens
What are some cytolysins called and what do they do?
- Alpha toxin= forms membrane pores resulting in leakage and dstruction of cells.(due to large intake of water)
- Beta toxin= sphinogomyelinase C hydrolyses membrane phospholipids - sphingomyelins
- Delta toxin= is a surfactant with detergent like action
- Gamma toxin= Panton valentine leukocidin(PVL) are pore forming toxins. They are bi-component(2 components)- monomers that oligomerize to form a pore.
- Cytotoxins= toxic for leukocytes, erythrocytes, and tissue cells.
What are the exfoliative toxins and what do they do?
ETA, ETB
- serine proteases that split the intracellular bridges ie desmosomes in stratum granulosum epidermis= exfoliative dermatitis- peeling of skin
- no cytolysis and no inflammation
- Only produced by S. aureus strains of bacteria.
What are the spreading factors and their function?
- Lipases= hydrolyse lipids, for invasion of tissues
- Nucleases- hydrolyse DNA, decrease viscosity of pus.
- Hyaluronidase= hydrolyses hyaluronic acid in connective tissue.
- Proteases= serine protease, aureolysin(metallo-protease)
- Fibrinolysin(staphylokinase)= causes fibrinolysis-dissolves clots
What are some superantigens and what do they do?
Toxic shock syndrome toxin(TSST)
Staphylococcus enterotoxins(SEA,SEB)
They release pro-inflammatory cytokines such as TNF-a and IFN-y
-overstimulation of the host immune response= systemic inflammation
-also cause food poisoning
What are the virulence immune evasion factors?
Capsule Slime layer Catalase Clumping factor Protein A
What is the role of a capsule?
Dense polysaccharide coat that prevents opsonisation with ab and complement(inhibits phagocytosis)
-also contains water which protects bacteria against desiccation.
What is the slime layer made of and its function?
Made up of water-soluble film(biofilm) from polysaccharides
Protects from immune factors and antibiotics
What does catalase do?
Detoxifies peroxide(ROI) produced by macrophages and neutrophils. 2H2O2= 2H2O + O2 Used to identify staphylococcus from streptococci(don't produce a catalase)
What does the clumping factor CIF-A(bound coagulase)do?
Binds fibrinogen and converts it into fibrin.
Fibrin deposition on cell surface(camouflage as fibrin is itself a host protein) prevents opsonisation and phagocytosis.
Coagulase test used to identify staphylococci from other strains.
What does protein A do?
It binds IgG in wrong orientation. (via Fc region), thereby preventing opsonisation and phagocytosis.
What are some of the major staphylococcus diseases?
- Superficial skin disease= boils, impetigo and Folliculitis
- Invasive diseases= deep abscess, bacteremia, endocarditis, osteomyelitis, septic arthritis, pneumonia
- Toxigenic diseases= Food poisoning, scalded skin syndrome, toxic shock
What is Impetigo?
Localised cutaneous infection wih pus-filled vesicles-affects young children
What is Folliculitis?
Impetigo involving hair follicles
What are foruncles or boils?
Painful-pus filled nodules
What are carbuncles?
joining of foruncles together=leads to bacteremia
What is bacteremia and endocarditis?
> Spread of bacteria into the blood from site of infection.
Bacteria then spread to other organs particularly the heart leading to endocarditis= damge to endothelial lining of heart. Infected heart valve is coated with bacteria, platelets and cellular debris=Perforation of heart valve= flushing of debris into blood= septic emboli
How is osteomyelitis caused?
After hematogenous dissemination of bacteria to bone or by secondary infection from trauma.
-Involves metaphyseal area of long bones.
How is septic arthritis caused?
In children and adults receiving intra-articular injections.
-characterised by painful erythematous joint and pus in joint space.
How is scalded skin syndrome caused?
By action of exfoliative toxins
Originally described as Ritter’s disease
How is Pneumonia caused?
After aspiration of oral secretion or by hematogenous spread resulting in abscess formation in lung.
Cytolytic toxins play a role.
How is staphylococcus food poisoning caused?
From contamination of food by human carrier and improper food handling.
-The disease is mediated by enterotoxins(SEA,SEB) that cause severe vomiting, diarrhoea, abdominal pain, nausea and sweating headache.
What is the cause of toxic shock syndrome?
Caused by superantigen-producing strains
> Menstrual TSS- TSST producing strain in vagina, caused by prolonged use of expandable tampon
> Non-menstrual TSS- superantigen producing strain in wound. Toxins are released into blood and cause systemic disease with fever, hypotension, erythematous rash and multiple organ failure.
