week 3 - staphylococci

Question 1

What is the shape of staphylococcus aureus?

Answer 1

Gram positive (purple) cocci arranged in irregular grape like clusters

Non sporing/motile

Resistant to dry salty conditions

Question 2

What do the colonies of staph aureus look like on blood agar?

Answer 2

circular, smooth, 2-3mm, grey/wjite/pale yellow/golden.

Facultative anaerobe
Catalase positive

Question 3

Where is staph aureus and staph epidermis found?

Answer 3

Staph aureus - only 30% of ppl are colonised with it, can be found in the nose, hand, pharynx etc

Staph epidermis - 100% of people are colonised with it as it is a commensal of the skin

Question 3

What is the difference between staph aureus and staph epidermis?

Answer 3

Use the coagulase test =
Coagulase positive - staph aureus
Coagulase negative - 40 species like staph epidermis

Question 4

What are the results of a catalase test for gram positive cocci?

Answer 4

Catalase positive, observation - bubbles are formed

Question 5

How can you use mannitol salt agar to confirm the presence of staph aureus?

Answer 5

Grow the bacteria on mannitol salt agar

Staph aureus (coagulase positive) will ferment the mannitol so colonies appear yellow

Coagulase negative staph does not ferment mannitol so colonies are pink

Question 6

How can you use DNAse agar to confirm the presence of staph aureus?

Answer 6

Grow the bacteria on DNAse agar

Staph aureus (coagulase positive) will have yellow colonies

Coagulase negative staph won’t have any colour colonies

Question 7

How can you use the staph latex agglutination test to confirm the presence of staph aureus?

Answer 7

Staph aureus (coagulase positive) has a positive result as the clumping factor is detected

Coagulase negative staph has a negative result asa there is no clumping factor detected

Question 8

What are the virulence factors found on staph aureus?

Answer 8

Toxins
Secreted enzymes
Cell surface proteins

Question 9

Describe staph aureus cell surface proteins

Answer 9

Protein A - Inhibits phagocytosis, interferes with humeral response

Clumping factor - he=ides cell from immune components

Question 10

List staph aureus toxins

Answer 10

Cytolytic toxins
Leukocidin toxin
Superantigens

They disrupt the membrane

Question 11

Describe staph aureus enzymes

Answer 11

Hyaluronidase - tissue invasion through ECM

Collagenase, elastase - Tissue destruction

Question 12

What are the different type of staph aureus infections?

Answer 12

Superficial Infections - boils, sties, impetigo

Serious infections - endocarditis, septicaemia etc

Toxin mediated - food intoxication, TSS, scalded skin syndrome

Question 13

What is impetigo and how is it caused by staph aureus?

Answer 13

Staph aureus enters through a minor skin abrasion.

Is a localised infection

Forms small, honey coloured crusty lesions and very infectious

Question 14

How does staph aureus cause furuncles/abscesses

Answer 14

A furuncle is a boil that is tender, swollen and pus filled on the skin and tissues.
It is an infection around a hair follicle where staph aureus cells multiply and spread producing an inflammatory response
Treat via drainage and antibiotics

Question 15

How is food intoxication caused by staph aureus?

Answer 15

Staph aureus lies in contaminated foods and produces enterotoxins causing diarrhoea, vomiting, nausea

Question 16

What is toxic shock syndrome and how is it caused by staph aureus?

Question 17

How is scalded skin syndrome caused by staph aureus?

Answer 17

Skin infection
Caused by exfoliative toxins A, B and D
Causes the skin cells to become unstuck, blisters start to form and epidermis starts to peel.

Question 18

Describe staphylococcal endocarditis

Answer 18

Infection of the endothelial lining of the heart.
Manifests as clots (vegetations) on heart valves

Question 19

How are cardiac vegetations (clots) formed?

Answer 19

1. endothelial layer surface becomes damaged
2. Platelets/fibrin deposited
3. Bacteria attaches
4. Vegetation (clot) increases in size
5. Continuous bacteraemia (continuous presence of bacteria in the bloodstream)
6. Possible embolisation

Question 20

How are blood stream infections diagnosed?

Answer 20

Via a BacT/alert 3D blood culture:

Microorganisms are allowed to multiply in the media, generating CO2
As CO2 increases, sensor in the bottle turns from grey to yellow
Reflected light is measured and positive sample is detected.

Question 21

What happens once a positive sample is obtained?

Answer 21

1. Gram stain using a venting needle
2. Culture
3. Direct sensitivity test using blood to inoculate the plate

Question 22

What antibiotics are used to treat S. aureus?

Answer 22

Most s. aureus are resistant to penicillin

Severe infections caused by methicillin SENSITIVE S. aureus treated via flucloxacillin, erythromycin, vancomycin

Severe infections caused by methicillin RESISTANT S. aureus treated via vancomycin

Question 23

What are the virulence factors found on staph epidermis?

Answer 23

1. polysaccharide slime layer
2. biofilm associated cells are tolerant to antibiotic treatment

Question 24

How are biofilms formed on medical devices?

Answer 24

1. Cells attach to the surface
2. accumulation and production of exopolysaccharide matrix
3. early development of biofilm
4. maturation of biofilm
5. bacterial cells from the biofilm are dispersed to form more biofilms

Question 25

What are central venous catheters?

Answer 25

They are used to administer drugs and IV solutions.
Used to take blood samples
Used to monitor blood pressure

Question 26

What are the sources of medical device related infections?

Answer 26

patients microflora
hand of medical personnel
contaminated disinfectants

Question 27

What are the two types pf catheter infection?

Answer 27

1. Exit site infection: inflammation at the exit site
2. Catheter related blood stream infection: bacteraemia, sepsis, prosthetic valve endocarditis

Question 28

How is a catheter infection diagnosed?

Answer 28

vIa a roll plate
1. a 4-5cm section of catheter is rolled over the plate on blood agar
2. the blood agar plate is then incubated in air at 37 degrees for 1-2 days.

Question 29

What re the results of a catheter infection diagnosis test?

Answer 29

Positive/significant infection: 15 colonies of an organism.