Microbiology - Virulence

Question 1

what is a virulence factor?

Answer 1

a characteristic of a pathogen that contributes to its effectiveness in causing a pathology

Question 2

give examples of types of virulence factors and what they do

Answer 2

adhesin - allows organism to bind to host tissue
invasion - enables organism to invade host tissue
impeding - enables organism to avoid host defence
aggressin - direct damage to host
modulin - indirect damage to host

Question 3

what type of organism is most likely to cause an infection in the skin?

Answer 3

part of natural skin flora

i.e - gram +ve (eg - staph or strep)

Question 4

where are infections likely to occur in the skin and how can microbes invade?

Answer 4

moist areas (axilla, groin etc)
can enter through hair follicle, sweat pore or sebaceous glands

Question 5

how are staphylococci distinguished? what does this mean?

Answer 5

coagulase test
coagulase +ve = aureus
coagulase is a bacterial enzyme that gives staph aureus the ability to coagulate blood, hence acting as a virulence factor

Question 6

how can staph aureus infection arise?

Answer 6

skin colonised with staph aureus in some parts like nostrils and perineum
can be hospital or community acquired
MRSA usually hospital acquired and common in immunocompromised/unwell patients

Question 7

how can strep epidermidis infection arise?

Answer 7

colonizes 100% of skin surface and mucous membranes so can arise from break in skin
or hospital acquired via catheters etc

Question 8

describe the pathogenicity of staph aureus

Answer 8

very versatile and can adapt to many different environments so can infect many sites and cause a wide range of symptoms

Question 9

give examples of symptoms caused by different staph aureus infections

Answer 9

superifical skin leasions like boils or abscesses
toxinoses like toxic shock or scalded skin syndrome
systemic life threatening effects

Question 10

what gives staph aureus its versatility?

Answer 10

different strains carry different virulence factors (all staph aureus have coagulase)

Question 11

Give 3 examples of toxinoses caused by staph aureus

Answer 11

Toxic shock (TSST-1)
staph food poisoning
Scalded skin syndrome

Question 12

give an example of a superantigen and what it doesq

Answer 12

TSST-1 (toxic shock syndrome)
toxin from staph aureus causes huge activation of T cells (1 in 5 compared to normal 1 in 10,000) resulting in a cytokine storm and overstimulation of immune response

Question 13

what are the diagnostic criteria for toxic shock syndrome?

Answer 13

fever >39
diffuse macular rash (red and bumpy)
desquamation (skin peeling)
hypotension <90
>3 organ systems involved (including skin/mucous membranes)

Question 14

what are adhesins and what are the 2 types?

Answer 14

components of bacteria that increase coagulation of blood
also present on epithelial and endothelial surfaces
2 types = fibrinogen binding an =d collagen binding

Question 15

name 5 aspects of staph aureus that help it to evade the immune system

Answer 15

protein A (binds IgG in wrong position)
superantigens (TSST-1)
coagulase (plasma clotting)
capsule (protects from phagocytosis)
production of PVL and alpha toxin

Question 16

what 2 types of capsule can exist on staph aureus?

Answer 16

mucoid and microcapsule

Question 17

what is PVL?

Answer 17

Panton-Valentine Leukocidin
toxin produced by staph aureus that attacks leukocytes
present in most community acquired skin infections

Question 18

what is PVL associated with?

Answer 18

CA-MRSA which causes severe skin infections like sepsis/necrotising fasciitis/pneumonia and recurrent furunculosis

Question 19

what is necrotizing pneumonia and why cant it be treated with antibiotics?

Answer 19

rapidly progressing flu like syndrome of necrotizing haemorrhagic pneumonia (necrosis of lung parenchyma)
knocks out immune system so antibiotics not enough

Question 20

what are the symptoms of necrotizing pneumonia?

Answer 20

preceeding flu like symptoms > acute respiratory distress > deterioration in pulmonary function > hypoxaemia > multi organ failure

Question 21

how does understanding of virulence factors have a clinical benefit?

Answer 21

if you know the virulence factor you know the risk and allows you to think of a strategy for treatment/isolate patients etc and develop treatments

Question 22

describe strep pyogenes

Answer 22

gram +ve cocci in chains
Beta haemolysis
catalase negative
type of group A strep

Question 23

what diseases is strep pyogenes associated with?

Answer 23

impetigo
cellulitis
necrotising fasciitis
Toxic Shock Like Syndrome (TSLS)

Question 24

what is the lancefield system?

Answer 24

system of classifying catalase negative strep into groups (strep A, strep B etc) using cell wall carbohydrates
groups then further divided due to M protein antigens in cell wall

Question 25

what is impetigo?

Answer 25

contagious infection of strep pyogenes (usually in face) immediately below the surface

Question 26

what diseases can be caused by invasive group A strep (iGAS)?

Answer 26

cellulitis

necrotizing fascitis

Question 27

what is necrotizing fasciitis?

Answer 27

invasive strains of strep A penetrate mucous membrane and develop in the lesion, rapidly destroying connective tissue

Question 28

how can strep A cause tissue/cell distruction?

Answer 28

hemolysins eg - streptolysin S (SLS)

= pore forming cytolysin which is toxic to organelles, platelets and PMN

Question 29

how can the importance of SLS be shown?

Answer 29

if you knock out the gene for SLS in the bacteria, the infection is far less aggressive and doesn’t cause the damage

Question 30

what causes toxic shock like syndrome?

Answer 30

exotoxins of strep pyogenes

complication of invasive infection (e.g pharyngitis)

Question 31

what causes the variation in virulence between bacteria?

Answer 31

strain evolution through passing of genes encoding for virulence factors via transformation, transduction or conjugation

Question 32

how is the incidence of GAS and iGAS infections changing?

Answer 32

increasing

Question 33

where is strep pyogenes typically found?

Answer 33

pharynx

Question 34

how do both staph aureus and strep pyogenes cause toxic shock?

Answer 34

production of similar exotoxins, common virulence factors and disease mechanisms