Staphylococcus Aureus and Disease

Question 1

Describe chararacteristics of Staphlococci?

Answer 1

• Gram positive spherical cells(cocci)
• Irregular grape-like clusters
• non-motile(absence of flagella)
• Resistant to dry conditions and up to 10% NaCl(= halodurent)

Question 2

Where do Staphilococcus colonise?

Answer 2

Moist skin folds, oropharynx, gatrointestinal tract and urogenital tract.

Question 3

How are S. aureus transmitted?

Answer 3

Mainly human to human(shedding human lesions, skin)

Question 4

What are some of the S.aureus strains resistant to?

Answer 4

Penicillin(B-lactamase)
Methicillin(MRSA)
Vancomycin(VRSA)

Question 5

How do you diagnose Staphylococci?

Answer 5

• 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.

Question 6

What are the virulence factors responsible for?

Answer 6

Enable the bacteria to colonist the host, damage host tissue, spread from site of infection and evade the immune response.

Question 7

What do virulence factor adhesins MSCRAMMS do?

Answer 7

=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.

Question 8

What are some virulence factors that damage the host cell?

Answer 8

> Cytolysins
Exfoliative toxins(ETA, ETB)
Spreading factors
Immunopathogenic factors- Superantigens

Question 9

What are some cytolysins called and what do they do?

Answer 9

• 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.

Question 10

What are the exfoliative toxins and what do they do?

Answer 10

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.

Question 11

What are the spreading factors and their function?

Answer 11

1. Lipases= hydrolyse lipids, for invasion of tissues
2. Nucleases- hydrolyse DNA, decrease viscosity of pus.
3. Hyaluronidase= hydrolyses hyaluronic acid in connective tissue.
4. Proteases= serine protease, aureolysin(metallo-protease)
5. Fibrinolysin(staphylokinase)= causes fibrinolysis-dissolves clots

Question 12

What are some superantigens and what do they do?

Answer 12

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

Question 13

What are the virulence immune evasion factors?

Answer 13

Capsule
Slime layer
Catalase
Clumping factor
Protein A

Question 14

What is the role of a capsule?

Answer 14

Dense polysaccharide coat that prevents opsonisation with ab and complement(inhibits phagocytosis)
-also contains water which protects bacteria against desiccation.

Question 15

What is the slime layer made of and its function?

Answer 15

Made up of water-soluble film(biofilm) from polysaccharides

Protects from immune factors and antibiotics

Question 16

What does catalase do?

Answer 16

Detoxifies peroxide(ROI) produced by macrophages and neutrophils. 2H2O2= 2H2O + O2
Used to identify staphylococcus from streptococci(don't produce a catalase)

Question 17

What does the clumping factor CIF-A(bound coagulase)do?

Answer 17

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.

Question 18

What does protein A do?

Answer 18

It binds IgG in wrong orientation. (via Fc region), thereby preventing opsonisation and phagocytosis.

Question 19

What are some of the major staphylococcus diseases?

Answer 19

1. Superficial skin disease= boils, impetigo and Folliculitis
2. Invasive diseases= deep abscess, bacteremia, endocarditis, osteomyelitis, septic arthritis, pneumonia
3. Toxigenic diseases= Food poisoning, scalded skin syndrome, toxic shock

Question 20

What is Impetigo?

Answer 20

Localised cutaneous infection wih pus-filled vesicles-affects young children

Question 21

What is Folliculitis?

Answer 21

Impetigo involving hair follicles

Question 22

What are foruncles or boils?

Answer 22

Painful-pus filled nodules

Question 23

What are carbuncles?

Answer 23

joining of foruncles together=leads to bacteremia

Question 24

What is bacteremia and endocarditis?

Answer 24

> 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

Question 25

How is osteomyelitis caused?

Answer 25

After hematogenous dissemination of bacteria to bone or by secondary infection from trauma.
-Involves metaphyseal area of long bones.

Question 26

How is septic arthritis caused?

Answer 26

In children and adults receiving intra-articular injections.

-characterised by painful erythematous joint and pus in joint space.

Question 27

How is scalded skin syndrome caused?

Answer 27

By action of exfoliative toxins

Originally described as Ritter’s disease

Question 28

How is Pneumonia caused?

Answer 28

After aspiration of oral secretion or by hematogenous spread resulting in abscess formation in lung.
Cytolytic toxins play a role.

Question 29

How is staphylococcus food poisoning caused?

Answer 29

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.

Question 30

What is the cause of toxic shock syndrome?

Answer 30

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.