3. Bacteriology lab

Question 1

What is endocarditis (SBE)?

Answer 1

• (subacute bacterial) Endocarditis
• Infection of the inner lining of the heart
• Affects the heart valves too

Question 2

What are common bacterial STIs?

Answer 2

• Gonorrhoea
• Syphilis
• Chlamydia

Question 3

What is brucellosis?

Answer 3

• Disease caused by Brucella
• Passed from animals to humans
• Also caused by ingestion of unpasteurised milk or undercooked meat

Question 4

Where is Meliodosis (gram-negative) likely to be picked up from?

Answer 4

• Far-east

* Soil and water

Question 5

What are common diagnostic techniques in the bacteriology lab?

Answer 5

• Culture of sterile sites (blood/CSF) and non-sterile sites (bowel/skin)
• Serology - immune response to infection
• Molecular techniques e.g. PCR
• Antimicrobial Susceptibility Testing

Question 6

How long do you culture blood for?

Answer 6

About 5 days

Question 7

How do you test for bacteria to perform a blood culture?

Answer 7

• Take blood from patient and put in 2 different bottles (aerobic and anaerobic)
• Blood incubated at a temperature similar to body temperature - bacteria multiplies
• Bacteria reproduces and produces CO2 - pH and colour change on disc at the bottom of the bottles

Question 8

Once tested positive in a bottle, how can blood be cultured?

Answer 8

• Put onto different agar plates
- blood agar
- chocolate agar (haemolysed blood)
- MacConkey agar
- neomycin agar
• Different bacteria use different salts and nutrients
• Most bacteria can grow on blood/chocolate agar
• Gram negative grow on MacConkey (produces a colour change)
• Plates incubated for 24 hours

Question 9

How can you distinguish between gram positive and negative bacteria?

Answer 9

• Gram stain
• Gram positive - thick peptidoglycan wall
- purple/blue
• Gram negative - thin peptidoglycan wall, sandwiched by 2 membranes
- pink/red

Question 10

What is the coagulase test - when is it used?

Answer 10

• There are different types of staphylococcus
- on the skin - coagulase negative (normally not a problem)
• Some strains may contain the enzyme coagulase
• If positive - it’s staphylococcus aureus
• Coagulase changes fibrinogen => fibrin in our blood

Question 11

What can commensals like staphylococcus infect?

Answer 11

• Low pathogenic potential
• However, can:
- infect prosthetic material
- cause (central) line infections
- pacemaker infections
- endocarditis

Question 12

How can you tell if streptococci is alpha or beta-haemolytic, and give an examples of each?

Answer 12

• Alpha - produces a green tinge to the blood agar
- e.g. streptococcal pneumonia
• Beta - use up all the blood (haemolytic)
- e.g. group A streptococcus

Question 13

Which type of bacteria should we worry about causing a septic shock?

Answer 13

• Gram negative (rod)

* Outer membrane can produce toxins and cause shock

Question 14

Which bacteria, parasites and viruses can cause diarrhoea?

Answer 14

Bacteria
• Salmonella (S. typhi)
• Shigella
• Campylobacter
• E. coli
• C. difficile
• Cholera

Parasites
• Amoeba
• Giardia
• Cryptosporidium

Viruses
• Norovirus

Question 15

Which bacteria are looked for routinely in stool samples?

Answer 15

• Salmonella
• Shigella
• Campylobacter

Cultured on agar plates

Question 16

Which bacteria from stool samples can’t be cultured, and what is done instead?

Answer 16

• Clostridium difficile

* Toxin detection or PCR for toxin gene instead

Question 17

What does sensitivity testing refer to?

Answer 17

Looking for the point at which bacteria is sensitive or resistant to a specific concentration of drug

Question 18

What is the minimum inhibitory concentration (MIC)?

Answer 18

Lowest amount of antibiotic required to inhibit the growth of bacteria in vitro

Question 19

How can you test the MIC and general sensitivity?

Answer 19

• Perform doubling dilution of antibiotics
• Look at point at which bacteria starts growing

e.g.
• Incorporate antibiotic into a strip, with different concentrations at each point
• Place on agar
• Where the bacteria stop growing to the furthest point is the MIC

• Disc diffusion
• The bigger the circle around the antibiotic soaked disc, the more sensitive the bacteria is

Question 20

What are beta-lactamases?

Answer 20

• Enzymes that bacteria have
• Allows bacteria to survive against their environmental competitors
• There are different types of beta-lactamases
• Antibiotics have helped evolve bacteria

Question 21

Does IgG or IgM respond earlier in an infection?

Answer 21

IgM

Question 22

Does IgG or IgM rise after second exposure to an antigen?

Answer 22

• Both rise

* IgG rises more

Question 23

Why may you get a negative serology result for IgG when an infection is present?

Answer 23

• Samples sent too early

* Important to repeat test 2-4 weeks later in these scenarios