Diagnostics - Bacteriology lab

Question 1

List 4 common diagnostic techniques

Answer 1

• Culture
• Serology
• Molecular Techniques
• Antimicrobial susceptibility testing

Question 2

What are the pros and cons of bacterial cultures

Answer 2

Pros
- Can be used to determine the correct antibiotic
Cons
- It takes up to 24 hours to grow the bacteria and another 24 hours to do the susceptibility testing

Question 3

Whats the difference between a sterile site and a non sterile site

Answer 3

A sterile site (e.g. CSF and blood) should not have any bacteria in them so anything that grows there is abnormal.
A non sterile site has loads of bacterial that could have colonised but may not be causing any problems.

Question 4

What is serology

Answer 4

Used to look at the body’s response to infection

e.g. the presence of antibodies after the infection

Question 5

Pros and cons of molecular testing

Answer 5

Pros
- Rapid
- Sensitive
Cons
- Resistance genes can interfere with results
However in some circumstances such as MRSA where the resistance gene is known (MecA) you can check for this gene to know if it is resistant

Question 6

What is antimicrobial susceptibility testing

Answer 6

You impregnate agar with a microorganism and put antibiotic discs on it

Question 7

Why is gram staining done in an infection

Answer 7

Gram negative and positive respond to different antibiotics and they tend to cause different types of infection
Gram positive = skin and soft tissue
Gram negative = abdomen and urinary tract

Question 8

What are chocolate agar and macconkey agar

Answer 8

Chocolate agar is cooked blood - cooking it releases some of the nutrients allowing certain bacteria to grow (e.g. haemophilus influenza)
Macconkey agar is agar designed to grow gram negative organisms

Question 9

When would you give antibiotics before sending cultures?

Answer 9

Meningitis or miningococcal septicaemia

Question 10

Whats the difference between gram negative and gram positive bacteria

Answer 10

Gram positive have a thicker peptidoglycan cell wall which holds the gram stain and stains purple
Gram negative has as outer membrane outside the cell wall which stops them form taking up the stain and they instead take up the counter stain turning pink

Question 11

What makes some antibiotics ineffective against gram negative bacteria

Answer 11

They may target the cell wall but be unable to pass through the outer membrane e.g. vancomycin

Question 12

Describe clumping of gram negative cocci

Answer 12

Most bacteria you find are gram positive cocci
When staphylococci divide they clump looking like bunches of grapes
Streptococci divide end on end to form chains

Question 13

Explain the staphylococci coagulase test

Answer 13

Staphylococci can also be coagulase positive or negative. Coagulase is a virulence factor which aids in causing infection. The most important coagulase positive staphylococcus is staphylococcus aureus. Coagulase negative staphylococci are common skin microbes which infect opportunistically

Question 14

What are the two groups of haemolytic streptococci

Answer 14

Alpha haemolysis - incomplete haemolysis turns the agar green (e.g. streptococcus pneumoniae)
Beta haemolysis - complete haemolysis clears the agar (e.g. group A = streptococcus pyogenes, Group B = Streptococcus agalactiae)

Question 15

Bacterial causes of diarrhoea

Answer 15

• Salmonella
• Shigella
• Campylobacter
• E. coli
• C. difficile
• Cholera

Question 16

What three bacteria are stools commonly tested for

Answer 16

• Salmonella
• Shigella
• Campylobacter
  An unusual history or travel history could mean you look for other things

Question 17

Where did C. difficile get its name

Answer 17

It got its name because it is difficult to grow in labs so detection of their toxin is used as the marker for it

Question 18

What agar specifically tests for salmonella

Answer 18

Xylose lysine deoxycholate (XLD) agar

• Most coliforms or enteric bacteria can ferment xylose turning it yellow but salmonella cant
• Salmonella also produces hydrogen sulphide turning the agar black

Question 19

How do you test for campylobacter

Answer 19

Campylobacter takes 48 hours to grow and can survive at up to 42 degrees so the sample is incubated at 42 degrees to kill any other bacteria

Question 20

What colour does vibrio cholerae turn TCBS agar

Answer 20

Green

Question 21

What is positive predictive value

Answer 21

The positive predictive value is that the more likely a patient is to have a disease the more likely a positive test result is a true positive

Question 22

What is minimum inhibitory concentration

Answer 22

The lowest amount of antibiotic required to inhibit the growth of bacteria in vitro. If the MIC is above the breakpoint then the strain should be reported as resistant