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2.2 Microbiology > ID: Staphylococcus and Micrococcus > Flashcards

ID: Staphylococcus and Micrococcus Flashcards

1
Q

What are the basic characteristics for staphylococcus
(6)

A

No haemolysis
No odour
Catalase positive*
Oxidase negative
KoH negative

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2
Q

What Preliminary ID test results indicate Staphylococci

A

Cocci

Gram positive

Catalase positive

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3
Q

What other genus might by cocci, gram positive, catalase positive other staphylococci?

A

Micrococci

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4
Q

What are the three clinically significant staphylococci?

A

Aureus

Epidermidis

Saprophyticus

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5
Q

What five tests would you do to speciate staphylococci?

A

DN’ase test

Mannitol salt agar

SAIDE agar -> Chromid S.aureus ELITE agar

Novobiocin susceptibility test

Coagulase test -> Staphaurex P test -> protein A test

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6
Q

How is DN’ase used to speciate staphylococci?
What is the principle behind the DN’ase test?
(4)

A

S. aureus produces DN’ase

DN’ase agar contains 0.2% DNA

After growth, plate is flooded with 1M hydrochloric acid which precipitates DNA and turns the medium cloudy

S. aureus colonies will show a distinct clear zone around inoculum as they have broken down the DNA and is therefore no longer available for precipitation by HCL

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7
Q

How do you carry out a DN’ase test?
(8)

A

Ask for DN’ase agar containing 0.2% DNA

Split plate down the middle

Split one half into halves again for the positive and negative control

Spot inoculate test strain and controls

Incubate at 37 degrees Celsius

After incubation put int fume cupboard and flood plate with 1M HCL

Allow HCL to permeate for 10 minutes then carefully pour off excess

Observe plate for distinct clear zones around colonies (S. aureus) or cloudy DNA precipitates (non S. Aureus)

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8
Q

What is a positive DN’ase test

A

Zone of clearance around colonies

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9
Q

What does a positive DN’ase test indicate?

A

S. aureus

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10
Q

What is a negative DN’ase tes

A

Cloudy cultures

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11
Q

What does a negative DN’ase test indicate?

A

S. epidermidis

S. saprophyticus

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12
Q

What are your controls for the DN’ase tes

A

+ S. aureus

  • S. epidermidis or S. saprophyticus
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13
Q

What is the principle behind mannitol salt agar?
(3)

A

Selective and differential for Staphylococci

High salt concentration of salt (7.5%) selects for staphylococci

Mannitol and pH indicator phenol red is differential and will turn yellow if organism can ferment mannitol

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14
Q

What is a positive result for mannitol salt agar

A

Yellow coloured colonies

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15
Q

What is a negative result for mannitol salt agar

A

Pink coloured colonies

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16
Q

What does a yellow coloured colony on mannitol salt indicate?

A

Pathogenic Staphylococci such as S. aureus

17
Q

What does a pink coloured colony on mannitol salt indicate?

A

Non pathogenic staphylococci such as S. epidermidis (most likely) or S. saprophyticus

18
Q

What is the principle behind SAIDE (Chromid S. aureus Elite agar)?
(2)

A

SAIDE agar is a chromogenic medium designed for the selective culture of S. aureus

Chromogenic substrates in the agar target specific enzymes in S. aureus to enable the differential growth of S. aureus colonies as pink colonies

19
Q

What does pink colonies on SAIDE agar indicate?

A

S. aureus

20
Q

What is the principle behind the Staphaurex Plus test?
(4)

A

Protein A is found on about 95% of Staph aureus strains

Protein A has the ability to bind to the Fc portions of IgG

Staphaurex Plus uses latex particles coated with porcine fibrinogen and rabbit IgG including specific polyclonal antibodies raised against capsular polysaccharides of S. aureus

When the reagent is mixed on a card with colonies of S, aureus, rapid agglutination occurs through the reaction between fibrinogen and clumping factor, Fc portion of IgG and Protein A, specific IgG and capsular polysaccharide

21
Q

What is a positive Staphaurex Plus result

A

Agglutination

22
Q

What is a negative Staphaurex Plus result

A

No agglutination

23
Q

What does a positive Staphaurex result indicate

A

S. aureus

24
Q

What does a negative Staphaurex result indicate

A

Coagulase negative staph

Non S. aureus staph

S. epidermidis or S. saprophyticus

25
Q

Why should Staphaurex Plus be used instead of a coagulase test?
(3)

A

Some strains of methicillin-resistant strains of S. aureus may express undetectable levels of clumping facto and protein A

These MRSA strains possess capsular polysaccharide

The capsule can mask both Protein A and the clumping factor thereby preventing agglutination

26
Q

How do you carry out a Staphaurex Plus test
(6)

A

Ask for latex reagent, reaction card, control latex (some sticks to mix)

Shake the latex reagent to mix

Dispense one drop of test latex onto one of the circles and one drop of control reagent onto another circle

Using a loop (wooden end of swab/stick) pick up and smear 2-3mm of Staph growth onto a circle and mix this in the control latex reagent

Using a clean loop/stick proceed in the same way with the Test Latex

Pick up and rock the card for 20 seconds

27
Q

How do you differentiate S. epidermidis from S. saprophyticus

A

Test for novobiocin resistance

(Saprophyticus is resistant to novobiocin while epidermidis is not)

28
Q

How do you test for novobiocin resistance?

A

Ask for blood agar and novobiocin disc and (bijou bottle? containing nutrient broth - don’t necessarily need this you can just streak out a plate and add disk)

Lawn inoculum

29
Q

What is the principle of the tube coagulase test?
How do you carry it out in theory?
(5)

A

Designed to detect both free and bound coagulase

Done by emulsifying 1 colony of test organism in 1ml rabbit plasma and incubating at 37 degrees

Check for clot production at 4 hours and again after incubation

Positive = visible clot formation or lose web of fibrin

Negative = no clot formation, plasma flows freely

2.2 Microbiology (60 decks)

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