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Y2 MCD Diagnostics > Bacteriology lab > Flashcards

Bacteriology lab Flashcards

1
Q

What are the common diagnostic techniques used by virology labs?

A
  1. Culture- sterile and non sterile sites - still the main way of diagnosis
  2. Serology
  3. Molecular techniques e.g. PCR but then you need to know what you are looking for
  4. Antimicrobial susceptibility testing
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2
Q

What bacterial cultures used to figure out?

A

Which antibiotic to use

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3
Q

What is the main problem with using cultures?

A

It takes 24hrs to grow the bacteria and another 24 to do the susceptibility testing

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4
Q

Why is a culture relatively easy at sterile sites?

A

There shouldn’t be any bacteria in these sites e.g. CSF/bone and blood so anything that grows is abnormal

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5
Q

What does serology look at?

A

Body’s response to infection

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6
Q

Using chicken pox as an example, what will be the difference in the blood before or at the start of the infection and when they’ve had an immunological response to chicken pox?

A

They will have gone from IgG negative to IgG positive

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7
Q

What are the pros and cons of molecular techniques?

A

Pros:
Rapid and sensitive
Useful for MRSA because resistance mechanism is encoded by MecA gene so if you do PCR for this gene you will know it’s resistant.

Cons:
Myriad of resistance genes so these aren’t good for frequent use
You need to know what you are looking for

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8
Q

How is antimicrobial susceptibility testing carried out?

A

By phenotypic methods- you impregnate agar with a microorganism and put antibiotic disc on it

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9
Q

How do blood cultures work?

A

Broth inside tube that has nutrients for bacteria and then it is incubated (around 37 degrees)- there is then an indicator at the bottom of the tube, the waste products of the bacteria will cause a change in colour of the indicator, the machine has sensors that can detect the colour change and flag it up as positive

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10
Q

What do you do once you confirm that blood cultures are positive?

A

Gram stain

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11
Q

Why do you do a gram stain?

A

It helps with selecting antibiotics because Gram positives are susceptible to certain antibiotics and Gram negatives are susceptible to others

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12
Q

Where do gram positive and negative generally tend to affect?

A

Gram positive= skin and soft tissue

Gram negative= abdomen and urinary tract

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13
Q

When you use blood cultures, what sort of agar plates do you use and why?

A

Non-Selective because there shouldn’t be any bacteria there in first place and they’re designed to grow anything

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14
Q

What is chocolate agar?

A

Cooked blood- certain bacteria will not be able to lyse blood cells so by cooking it you release some of the nutrients in the blood agar and let certain bacteria grow

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15
Q

What is the commonest bacteria that grows on chocolate agar?

A

Haemophilus influenzae

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16
Q

What is Macconkey agar designed to grow?

A

Gram-negative organisms

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17
Q

In what situation would you give antibiotics without checking cultures first?

A

Patients with meningitis or meningococcal septicaemia

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18
Q

What is the difference between Gram positive and negative bacteria?

A

Gram positive- thicker peptidoglycan cell wall which holds Gram stain and stains purple
Gram negative- outer membrane outside cell wall which stops them from taking up the stain, instead they take up the counter stain and stain pink

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19
Q

Why are many antibiotics ineffective on gram-negative?

A

They act on cell wall but outer membrane prevents them from getting there

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20
Q

What is the most common type of bacteria that you find in terms of Gram and shape?

A

Gram positive cocci

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21
Q

How do staphylococci appear?

A

They have a particular pattern of dividing where they divide in CLUMPS

22
Q

How do streptococci appear?

A

They divide end on end to form chains (divide in CHAINS)

23
Q

What is the staphylococci coagulase test used for?

A

To differentiate between the two types of staphylococci:
Coagulase positive
Coagulase negative

24
Q

What is the most important staphylococci coagulase positive bacteria?

A

Staphylococcus aureus (coagulase is a virulence factor which helps staphylococcus aureus to cause infection)

25
Q

What sort of bacteria are coagulase negative?

A

Common skin microbes e.g.staphylococcus epidermidis - don’t cause infection unless opportunistic circumstances
Can infect prosthetic material causing line/pacemaker infections or endocarditis.

26
Q

What are the two groups of streptococci and what are the groups based on?

A

Based on how they look on blood agar:
Alpha haemolysis
Beta haemolysis

27
Q

What is alpha haemolysis?

A

Incomplete haemolysis- turns agar a green colour

28
Q

What is beta haemolysis?

A

Complete haemolysis- clears the agar

29
Q

What is an example of alpha haemolytic streptococci?

A

Streptococcus pneumoniae–> pneumonia and meningitis

30
Q

What is an example of beta haemolytic group A streptococci?

A

Streptococcus pyogenes–> skin and soft tissue infections and rheumatic fever

31
Q

What is an example of beta haemolytic group B streptococci?

A

Streptococcus agalactaie–> sepsis in young children and involved in infections in diabetes

32
Q

What are possible causes of diarrhoea?

A

Bacteria- Salmonella, Shigella, Campylobacter, E coli O157, C difficile and Cholera

Parasites- Amoeba, Giardia and cryptosporidium

Viruses e.g. norovirus

33
Q

Who is affected by E. coli O157?

A

Young children and elderly people

34
Q

What is the prevalence of c. difficile?

A

5% of population but more common in children

35
Q

What is generally associated with Diarrhoea and Vomiting?

A

Viruses

36
Q

What are the three main bacteria that stools are routinely tested for?

A

Salmonella
Shigella
Campylobacter
- these often cause food poisoning.

37
Q

Why is testing for C. difficile difficult?

A

It got its name because it is difficult to grow so labs tend to use detection of toxin or PCR for the toxin gene

38
Q

What happens to salmonella when put on XLD (xylose lysine deoxycholate) agar?

A

Salmonella can’t ferment xylose –>red

whereas most other coliforms or enteric bacteria can ferment xylose –> yellow colour

39
Q

What does salmonella also form and what is it responsible for?

A

Hydrogen sulphide which forms black colonies

40
Q

How do you test for campylobacter?

A

It takes a long time to grow (48hr) but it’s able to survive at 42 degrees so you incubate the sample at 42 degrees to kill other bacteria in stool sample then put the remaining campylobacter on selective agar

41
Q

How do you test for vibrio cholerae?

A

On TCBS agar, cholera makes it go green

42
Q

What is the minimum inhibitory concentration (MIC)?

A

Lowest amount of antibiotic required to inhibit the growth of bacteria in vitro - varying concentrations need to be given to see which allows bacterial growth.

43
Q

What are breakpoints?

A

These are values set by people which correlate the MIC with clinical success when using the antibiotis

If bacteria has a MIC below the breakpoint then there is good correlation with clinical success

44
Q

What does it mean if the bacteria has an MIC below the breakpoint?

A

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

45
Q

What does it mean if the bacteria has an MIC above the breakpoint?

A

It is resistant

46
Q

How do most labs measure MIC?

A

Using graded MIC strips - these have antibiotic on them. Where the bacteria crosses the strip number is the MIC.

47
Q

What is the traditional way of testing antibiotic sensitivity?

A

Disc diffusion-
Set conc of antibiotic on each disc which is then placed on agar and incubated for 24 hours, zone size is interpreted using breakpoints and zone diameter can be used to determine whether it is sensitive or resistant

Highly effective antibiotics (disk C) will produce a wide ring of no bacterial growth.

48
Q

What is special about carbapenamase?

A

It is a type of beta lactamase which breaks down all classes of beta lactam antibiotics so they are resistant to pretty much everything and untreatable

49
Q

Describe the appearance of gram stained gram+ and gram- bacteria.

A

Gram +- have a thick peptidoglycan cell wall so stain purple
Gram - - thin peptidoglycan cell wall with a cell membrane on top so stain pink

50
Q

Describe the appearance of gram negative bacilli and give an example.

A

Do not take up gram stain- take up counter stain –> PINK

E.g. E.coli - short and fat rods.

Y2 MCD Diagnostics (5 decks)

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