Diagnostics 3 - Bacteriology Lab Flashcards

1
Q

What are common bacteriology diagnostic tests?

A
  1. Culture (watch cell growth at 37 degrees for 5 days) - sterile sites (e.g. Blood/CSF - expect no bacterial growth), and non sterile sites
  2. Serology
  3. Molecular techniques (PCR)
  4. Antimicrobial / AB Susceptibility testing
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2
Q

Why do blood culture discs change colour?

A

Bacteria reproduce –> due to bacteria reproducing –> produce CO2 –> alters pH

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3
Q

Which type of bacteria tend to clump?

A

Gram positive cocci - e.g. staphylococci or streptococci

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4
Q

Differentiate gram positive and gram negative bacteria

A

Gram positive = thick peptidoglycan layer - retains dye so stains purple

Gram negative = thin peptidoglycan layer - retains counterstain - stains red. In gram negative, outer cell membrane usually produces bacterial toxins

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5
Q

What is the peptidoglycan layer made up of?

A

2 different peptides:

  1. NAM - N-acetylglucosamine
  2. NAG - N-acetylmuramic acid
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6
Q

Where is gram positive bacteria generally found? What about gram negative?

A

Gram positive = skin and soft tissue

Gram negative = abdomen and urinary tract

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7
Q

What are the 3 types of agar and explain their use

A
  1. Chocolate agar - cooked blood - certain bacteria can’t lyse RBC - cooking releases nutrients in nutrient agar which allows certain bacteria to grow. e.g. Haemophilus influenzae
  2. MacConkey Agar - designed to grow gram negative organisms
  3. Neomycin agar - growth of anaerobic microorganisms
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8
Q

In which 2 scenarios must antibiotics be given ASAP?

A
  1. Meningitis

2. Meningococcal septicaemia

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9
Q

Explain how the coagulase test can be used to detect staphylococci

A

Coagulase converts fibrinogen into fibrin. (it is a virulence factor)

Staph. can either be coagulase positive or negative.

Coagulase positive = staph. aureus (potentially MRSA). Positive result = coagulation/clot

Coagulase negative = common skin commensals of low pathogenic potential

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10
Q

Staphylococcus don’t tend to cause infection unless opportunistic circumstances. They can infect prosthetic material which can cause?

A

Line, pacemaker infections

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11
Q

Streptococci are divided into 2 groups depending on blood agar. They are?

A
  1. Alpha haemolysis - incomplete haemolysis - turns agar green (e.g. S. pneumonia - causes pneumonia and meningitis)
  2. Beta haemolysis - complete haemolysis - clears agar. Subdivided into group A and B.
    - Group A (e.g. S. pyogenes). Skin and soft tissue infections, rheumatic and scarlet fever
    - Group B (e.g. S. agalactiae). Sepsis in young children. Infections in diabetics.

(3. Non-haemolytic e.g. gut enterococci)

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12
Q

What are the causes of diarrhoea?

A
  1. Bacteria (salmonella, shigella, campylobacter. Or e.coli, C. difficile, cholera
  2. Parasites - amoeba, giardiasis, cryptosporidium
  3. Viruses
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13
Q

Explain how stool sample investigations are done for bacteria and parasites

A

Bacteria - cultured on agar plates. Only salmonella, shigella and campylobacter are checked for routinely. C. difficile can’t grow - so toxin detection or PCR is done for toxin gene.

Parasites - concentrate then use special stains.

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14
Q

What type of agar plate is salmonella grown on?

A

XLD (xylose lysine deoxycholate)

Salmonella can’t ferment xylose –> causes red colour.

Most there coliforms or enteric bacteria ferment xylose causing a yellow colour

Salmonella - forms H2S which forms black colonies

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15
Q

How is campylobacter plate grown?

A

Little oxygen environment - they can survive at 42 degrees so this temperature is used for incubation - kills other bacteria in stool sample

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16
Q

What type of agar plate is v. cholerae grown on and what colour is a result?

A

TCBS agar plate and colour is green

17
Q

Infectious diarrhoea doesn’t usually persist for how long?

A

4 months

18
Q

How can it be determined if bacteria are resistant or sensitive

A

Zone diameter breakpoints

19
Q

What is MIC and how can gradient MIC strips be used?

A

Minimum amount of AB needed to inhibit bacterial growth in vitro after overnight incubation

Gradient MIC strips - varying conc of AB - read off conc at edge of no bacterial growth

20
Q

Disc diffusion can also be used to measure sensitivity

A

Use AB discs on agar plate - incubate for 24 hours —> measure how far AB spreads out (look at cleared area diameter)

21
Q

If bacteria have MIC below the breakpoint, what does this mean?

A

There is a good correlation with clinical success if you use that antibiotic

22
Q

If bacteria have MIC above the breakpoint, what does this mean

A

Reported as resistant

23
Q

What do bacteria have as natural protection?

A

Beta-lactamases

24
Q

Describe IgG and IgM relationships in 1st and 2nd infection

A

1st infection - IgG peaks later than IgM

2nd infection - IgG rises rapidly first and higher than IgM