2.6 Antimicrobial susceptibility testing

Question 1

What are the two purposes of anitmicrobial testing?

Answer 1

Some bacteria isolates show high levels of antimicrobial resistance, antimicrobial susceptibility testing investigates which antibiotics these bacteria isolates are sensitive to.
Also allows for the tracking of anitbiotic resistance within a bacterial population, which is important for detecting new mechanisms of resistance.

Question 2

Name the 3 commonly used methods of antimicrobial testing?

Answer 2

1. Broth dilution method
2. Antimicrobial gradient method
3. Disk diffusion method

Question 3

What is the minimum inhibitory concentration (MIC)

Answer 3

Minimum inhibitory concentration is the lowest concentration of agent which is required to completely inhibit the growth of the test microorganism

Question 4

Describe the broth dilution method

Answer 4

1. A gradient of known antibiotic concentrations are created by a doubling dilution
2. Each tube is then innoculated with a standardised amount of the test bacteria and incubated for a defined amount of time
3. The tubes are then examined for observable bacterial growth (evidenced by turbidity). The MIC is the lowest concentration of antibiotic that has prevented bacterial growth.

Question 5

Describe the antimicrobial gradient method

Answer 5

1. A special test strip is generated which is impregnated along its underside with a gradient of dried antibiotic concentration.
2. Up to 5 or 6 strips can be placed in an agar plate which has been innoculated with the test bacteria
3. After overnight innoculation the strips are read wherein the MIC is identified as the point at which the growth touches the strip.

Question 6

Describe the disk diffusion method

Answer 6

1. A standard amount of the bacteria is spread over the entire surface of an agar plate
2. Discs, each containing a known amount of a different anitbiotic are then placed on the surface of the dish and the dish is incubated for standard time and temperature
3. As the antibiotic diffuses through the agar each disc generates its own zone of growth inhibition.
4. The zones of growth inhibition around each disc are measured to the nearest millimetre. The greater the zone of inhibition, the more sensitive the bacteria is to the antibiotic disc.

Question 7

Which antimicrobial susceptibilty tests detect the MIC?

Answer 7

Broth dilution method and antimicrobial gradient method

Question 8

What are the limitations of the disc diffusion method?

Answer 8

It relies on even spreading of the bacteria across the surface of the agar. The discs must be placed far enough apart to avoid overlapping of inhibition zones. As the test relies on the diffusion of antibiotics, direct comparisions of antibiotics are not possible.