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Infectious Diseases > Staphylococci > Flashcards

Staphylococci Flashcards

1
Q

Who first observed Staphylococcus?

A

Pasteur and Koch

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2
Q

What is meant by the term Staphylococus?

A 
Staphyle = bunch of grapes
Kokkus = grain or berry
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3
Q

Are Staphylococcus gram +ve or -ve?

A

+ve

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4
Q

Describe the characteristics of Staphylococcus

A

Arrange as irregular, grape-like clusters.

Approx 1um in diameter.

Non-sporing

Non-motility

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5
Q

Describe the resistant of Staphylococcus to certain conditions.

A

Resistant to dry, salty conditions.

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6
Q

What is meant by the term ‘sporing’?

A

A small, usually singled-celled reproductive body that is resistant to adverse environmental conditions and is capable of growing into a new organism.

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7
Q

Describe the oxygen tolerance of Staphylococcus.

A

Facultative anaerobes.

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8
Q

Are Staphylococcus Catalase -ve or +ve?

A

+ve

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9
Q

Describe the appearance of Staphylococcus on blood agar

A

Smooth
2-3mm in diamater
Grey/ white/ pale yellow/ golden

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10
Q

Can you distinguish between Staphylococcal colonies on a blood agar plate?

A

No

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11
Q

How are different Genus of Staphylococcus split?

A

Using the coagulase test

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12
Q

Describe how the coagulase test separates different staphylococcus species.

A

Exocellular coagulase enzyme converts fibrinogen to fibrin. The fibrin then causes plasma to clot.

A +ve result is shown by formation of a clot/ gel within the sample.

A -ve result is shown when the sample remains fluid.

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13
Q

Name a coagulase +ve staphylococcal species.

A

S. aureus

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14
Q

Name a coagulase -ve Staphylococcal species.

A

Staphylococcus epidermidis

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15
Q

Describe how Staphylococcus aureus is spread.

A

Exogenously (come from the environment) or endogenously (infection has arisen from organisms that an individual was already colonised with).

Spread mostly via direct contact - hands, clothing, dust, shedding from the skin. Or via airborne transmission.

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16
Q

Where are most Coagulase-negative Staphylococci found?

A

They are commensals of the skin.

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17
Q

Explain how the Catalase test is used on presumptive Staphylococcal species.

A

Hydrogen peroxide + Catalase enzyme - Water + Oxygen.

So if the colony has the catalase enzyme, when reacted with hydrogen peroxide, water and oxygen will be produced. The result is shown positive by the bubbling of oxygen.

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18
Q

Are Streptococci catalase -ve or +ve?

A

Negative

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19
Q

Are enterococci catalase -ve or +ve?

A

-ve

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20
Q

Are Staphylococci catalase -ve or +ve?

A

+ve

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21
Q

What tests could be used as confirmatory tests for S. aureus?

A

Selective agar (e.g. Mannitol salt agar)

DNase production

Staphylococcal latex agglutination test

API Staph test

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22
Q

Describe how selective agars such as Mannitol Salts Agar can be used to test for S.aureus.

A

S.aureus ferments mannitol forming yellow colonies.

S.aureus are also able to withstand the selective salt concentration and the differential mannitol component.

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23
Q

Why does S.aureus cause yellow colonies?

A

Ferments mannitol and produces acid. This causes a pH change which results in the agar on the plate (which contains an indicator) to change colour.

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24
Q

An 85-year-old male has presented with white patches on the roof of the mouth, inner cheek, tongue and the back of the throat. The GP thinks the patient has oral candidiasis. You have been sent an oral swab.

i) What growth media are you going to inoculate?
ii) Are you going to perform microscopy?
iii) How will you incubate the inoculated media?

A

i) Sabouraud’s dextrose agar
ii) Yes - Gram stain
iii) Incubated at 37 degrees Celsius for approx. 24 hours. Hot room.

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25
Q

A 45-year-old female has presented at her GP’s surgery with symptoms including a burning sensation when urinating and a persistent urge to urinate. A diagnosis of a urinary tract infection (UTI) is made and a mid-stream urine sample sent to the lab.

i) What growth media (other than a chromogenic UTI agar) are you going to inoculate?
ii) Are you going to perform microscopy?

A

i) CLED

ii) No

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26
Q

A 36-year-old female has presented at her GP’s surgery complaining of symptoms of diarrhoea and vomiting. It is thought that she may have a Salmonella infection. When the faecal sample arrives in the lab, how will you process it?

i) What growth media are you going to inoculate?
ii) Are you going to perform microscopy?
iii) How would you incubate the sample?

A

i) XLD
ii) No
iii) 37 degrees Celsius

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27
Q

A 19-year-old female has presented at A&E with the symptoms of severe headache, pyrexia, neck stiffness and photophobia. A diagnosis of suspected meningitis is made. (Note in addition to culture the sample is subjected to additional tests such as a cell count. Don’t worry about these other tests at this stage, just consider culture and microscopy requirements).

i) What growth media are you going to inoculate?
ii) Are you going to perform microscopy?
iii) How will you incubate the inoculated media?

A

i) Chocolate agar
ii) It is likely that microscopy will be required, however, this depends on the sample type taken. It is likely that a CSF sample will have been taken and so microscopy can be used.
iii) 35-37 degrees Celsius for 24 hours with approximately 5% carbon dioxide.

28
Q

A 79-year old male has been diagnosed with suspected prosthetic valve endocarditis. The prosthetic valve has been removed during surgery and sent to the lab for investigation.

i) What growth media are you going to inoculate?
ii) Are you going to perform microscopy?
iii) How will you incubate the inoculated media?

A

i) Broth for 24-48 hours and then Blood agar or Mannitol Salts Agar
ii) No
iii) 35-37 degrees Celsius for 24 hours

29
Q

Describe how DNase can be used to confirm the presence of S.aureus.

A

A positive reactions will be shown by a clear region of agar on the DNase.

30
Q

Describe how the Staphylococcal latex agglutination test can confirm the presence of S.aureus.

A

 A reagent contains antibody which is able to detect clumping factor and another antibody which is able to detect Protein A. These antibodies are bound to coloured latex beads.
 If S.aureus antibody present, it will bind to the Staphylococcal cells and cross-react. This creates a mesh-like network.
 The latex beading helps visualise this mass formation – causes a grainy agglutination.
A Negative result= milky appearance.

S.aureus will show a positive reaction with grainy agglutination.

31
Q

Describe how the API staph test can be used to confirm the presence of S.aureus.

A

Grainy agglutination will occur.

32
Q 
Describe the results of S.aureus with each of the following tests:
Coagulase
Clumping factor
Results on Mannitol Salts agar
DNase
A

Coagulase +ve

Clumping factor +ve

Results on Mannitol Salts agar = Yellow colonies

DNase +ve

33
Q 
Describe the results of S.epidermidis with each of the following tests:
Coagulase
Clumping factor
Results on Mannitol Salts agar
DNase
A

Coagulase -ve

Clumping factor -ve

Results on Mannitol Salts agar = Pink colonies

DNase -ve

34
Q

What are virulence factors?

A

A characteristic that an organism has which allows it to cause disease. The more virulence factors, the more infections the an organism is likely to cause.

35
Q

Name some of the virulence factors that S.aureus has.

A

Toxins

Leukocidin toxin

Superantigens

Cell surface proteins

Secreted enzymes

Capsule or exocellular polysaccharide

36
Q

How do the virulence toxins of S.aureus work?

A

Membrane disrupting exotoxins
Alpha, beta, gamma haemolysins
Attack membranes of Eukaryotic cells and red blood cells
Causes clear zones around organisms growing on blood agar plates

37
Q

How do the virulence Leukocidin toxins of S.aureus work?

A

Attacks white blood cells (Leukocytes)

Avoids the immune response

38
Q

How do the super antigen virulence factors of S.aureus work?

A

Trigger an abnormal immune response (e.g. toxic shock syndrome toxin) by causing T-helper to recognise all antigen types. This removes the specificity of reactions. T-helper cells are massively over stimulated and there is also an excessive release of inflammatory mediators such as IL-2.

39
Q

What different cell surface proteins does S.aureus have that act as virulence factors and how does each work?

A 
Protein A
Binds Fc regions on IgG. This interferes with humoral immune responses by inhibiting phagocytosis and complement. 
	 Clumping factor
o	Fibrinogen converted to fibrin causes a clot
o	Hides the cells from immune components
	Fibronectin binding protein
o	Binds fibronectin
	Collagen binding protein
o	Binds collagen
40
Q

What secreted enzymes does S.aureus have that act as virulence factors and how does each work?

A

Hyaluronidase = tissue invasion through ECM

- Collagenase = tissue destruction - Elastase = Tissue destruction - Lipase
41
Q

Name some skin and soft tissue infections caused by S.aureus.

A

Impetigo
Caruncles
Abscesses
Furuncles

42
Q

How does S.aureus cause Impetigo?

A

It enters through minor skin abrasions and causes localised infections in the superficial, keritanised layers of the skin.

43
Q

What are Furuncles?

A

Tender, swollen, pus-filled areas of the skin and soft tissue.

44
Q

What are benefits of Furuncles?

A

They protect from host defences.

45
Q

What are the issues with Furuncles?

A

They generate an intense inflammatory response.

They may also rupture outwards or inwards and cause sepsis.

46
Q

What are the symptoms associated with Furuncles?

A

Area becomes swollen and hot due to the infiltration of white cells.

47
Q

What is the treatment for Furuncles?

A

Drainage and antibiotics.

48
Q

What is a carbuncle?

A

A cluster of furuncles.

49
Q

What are Abscesses?

A

Larger, raised areas on the skin, tender to touch, filled with pus, in the deeper tissue.

50
Q

What symptoms do food intoxications cause?

A

Diarrhoea, vomiting and nausea

51
Q

What occurs during toxic shock syndrome?

A

Massive activation of T-cells leads to high levels of cytokines

52
Q

What are the symptoms of Toxic shock syndrome?

A

Flu-like symptoms, temperature, diarrhoea, rash, dizziness, confusion.

53
Q

How is scalded skin syndrome caused?

A

Exfoliate epidermolytic toxins bind and hydrolyse Desmoglein 1 within the desmosome. The skin cells then become unstuck, blisters form and layers of the epidermis peel off.

54
Q

Who does scalded skin syndrome affect?

A

Mainly neonates and young children.

Older children are rarely affected but have a higher mortality level.

55
Q

What is Staphylococcal endocarditis?

A

A microbial infection of the endothelial lining of the heart. Usually manifests as vegetations (clots) within heart valves.

56
Q

In which side of the heart is Staphylococcal endocarditis most common and why?

A

o Most commonly in the left side because the left side of the heart is under the greatest pressures

57
Q

In what type of patients is there likely to be Staphylococcal endocarditis affecting the right side of the heart?

A

In Intravenous drug abusers

58
Q

How do cardiac vegetations occur?

A
  • Damage to the surface endothelial layer
  • Deposition of platelets and fibrin
  • Bacterial attachment
  • Vegetation increases in size
  • Continual bacteriaemia
  • Possible embolization
59
Q

What is the scenario in which vegetations within the heart are most likely to occur?

A

If there is any damage to a heart valve, turbulent blood flow may occur on the underside of the valves. This could lead to the formation of vegetations.

60
Q

Describe the treatments used for most S.aureus infections.

A
  • Most S.aureus are resistant to penicillin
  • Severe infection caused by meticllin sensitive S.aureus (MSSA)
    o Flucloxacillin (beta-lactam)
    o Erythromycin, clindamycin or vancomycin if the patient has a beta-lactam allergy
  • Severe infection caused by meticillin resistant S/aureus (MRSA)
    o Vancomycin (glycopeptide)
    o Newer agent
61
Q

What is a CSF Shunt?

A

A catheter under the skin allows excess fluid from the brain to be drained into the abdomen to release pressure on the brain.

62
Q

What causes most CNS infections?

A

Skin commensals and opportunistic pathogens typically associated with prosthetic medical devices such as artificial heart valves and hip joints.

63
Q

Describe the virulence factors which S.epidermidis has.

A

o Principle polysaccharide slime layer and biofilm (‘bacteria adherent to a solid surface embedded within a self-produce matrix’) formation
 Biofilm associated cells are tolerant to antibiotic treatment

64
Q

Outline the process of biofilm formation by S.epidermidis.

A

1) Initial attachment of cells to the surface
2) Accumulation and production of exopolysaccharide matrix
3) Early development of biofilm architecture
4) Maturation of biofilm architecture
5) Dispersion of bacterial cells from the biofilm

65
Q

What are the signs of infection being caused by a catheter?

A

a. Inflammation at exit site of catheter
i. Tenderness, swelling, pus formation, cellulitis, purulent exudate, redness of the skin
ii. Localised infection that can lead to further infections if not treated
b. Catheter related blood stream infection
i. Bacteraemia, sepsis, prosthetic valve endocarditis

Infectious Diseases (9 decks)

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