Antibiotic susceptibility testing

Question 1

Antibiotics

Answer 1

-metabolic by-products of certain microorganisms that can inhibit or kill other microorganisms

Question 2

Alexander Fleming

Answer 2

-was culturing staphylococcus aureus to study growth properties when he discovered an agar plate with culture was contaminated by a Penicillium fungal infection (penicillium notatum)

Question 3

Penicillium

Answer 3

crude antimicrobial extract that was found to inhibit the growth of staphylococcus aureus
-it inhibits the formation of peptidoglycan cross links by inactivating the responsible enzyme (osmotic pressure –> cell death)

Question 4

What other name is the diffusion susceptibility test called

Answer 4

Kirby Bauer Test

Question 5

Kirby Bauer Test

Answer 5

-Test if a bacteria of interest is resistant or susceptible to antibiotic
-experiment involves culturing a bacteria species on a petri dish in the presence of an antibiotic
FIRST
- petri plate will be uniformly inoculated with a standardize amount of bacteria species of interest
SECOND
- a small disk is added with standard concentration of an antibiotic
THIRD
-after incubation, a lawn of bacteria will grow everywhere on the plate except regions around the disks

Question 6

Do antibiotics with a large molecular weight diffuse faster or slower?

Answer 6

slower, travel less distance

Question 7

Zone of inhibition

Answer 7

clear area where the bacteria did not grow
-the diameter of the zone is measured and compared to a standard table

Question 8

The larger the diameter of the zone of inhibition means what?

Answer 8

-the more effective that antibiotic is against killing the bacteria species being tested

Question 9

If there is no growth on the petri plate what do you measure?

Answer 9

• you measure the diameter of the disk

Question 10

What does no zone of inhibition mean?

Answer 10

-means the bacteria is resistant to the antibiotic

Question 11

Susceptible result on Kirby Bauer means?

Answer 11

-the antibiotic is killing the bacteria

Question 12

Intermediate result on Kirby Bauer means?

Answer 12

-further testing required to determine if the antibiotic can be used as a treatment option against the bacterial species

Question 13

Minimal inhibitory concentration (MIC)

Answer 13

the smallest amount of the antibiotic that can inhibit the growth of the bacteria in question
-are recognized by the presence of growth inhibitor zones

Question 14

What is the chart used in the Kirby Bauer test called?

Answer 14

antibiogram

Question 15

List what affects the size of growth inhibition zone

Answer 15

1. type of culture
2. dept of agar plate
3. concentration of agar in plate
4. rate of diffusion of the antibiotic
5. the number of organisms on the plate
6. the rate of growth of the organisms
7. the degree of susceptibility of the organism to the drug
8. the gram reaction of the organism

Question 16

If the dept of the agar plate is too shallow what might happen?

Answer 16

-antibiotic will diffuse more laterally thereby increasing zone of inhibition

Question 17

If the dept of the agar plate is too deep what might happen?

Answer 17

-the antibiotic will sink more below the disk and diffuse less laterally- thereby decreasing the zone of inhibition

Question 18

Why does an antibiotic work better on gram positive bacteria than gram negative bacteria

Answer 18

the cell walls of gram positive bacteria are more permeable to antibiotics

Question 19

True or false… A zone of inhibition for an antibiotic is absolutely indicative that the organism is susceptible to that antibiotic

Answer 19

False
A small zone of inhibition may still be indicative of clinical resistance of the bacteria to the antibiotic. While the antibiotic may inhibit the growth of the bacteria to some degree, it is not enough to correlate to susceptibility in a clinical (patient) setting

Question 20

How does the antibiotic get from the disc to the agar?

Answer 20

the antibiotic is released from the disc into the agar, the antibiotic then diffuses into the agar (outward)

Question 21

What is the principle of the agar disk diffusion method?

Answer 21

-there will be absence of bacterial growth across the surface of an agar plate if there is the presence of an effective antibiotic. Persistence of bacterial growth indicates resistance to the antibiotic

Question 22

Susceptibility

Answer 22

a term used when microbes are unable to grow in the presence of antimicrobial drugs