Antibiotic susceptibility

Question 1

MIC

Answer 1

minimal inhibitory concentration - the minimal concentration of an antibiotic that inhibited growth. (Reveals inhibition of bacteria and NOT killing).
Corresponds to the last clear tube in a broth dilution test.

Question 2

MBC

Answer 2

minimal bactericidal concentration - the minimal concentration of an antibiotic that kills all bacteria in the sample. Take the clear tubes from the dilution test and inoculate them on agar plates, incubate overnight. If no growth = MBC.

Question 3

4 ways to determine antibiotic susceptibility

Answer 3

1. broth dilution (find MIC and MBC - now automated)
2. agar dilution (research only)
3. agar diffusion
4. epsilometer testing (E test)

Question 4

agar dilution

Answer 4

Research test, not clinical.
Test antibiotic of known concentration is added to heated liquid agar and allowed to cool into semisolid agar in petri dish. The test organism is streaked on the dish and plate is incubated overnight. No growth = organism is susceptible to concentration of antibiotic on agar.

Question 5

agar diffusion

Answer 5

Research test, less expensive, used less.
Large agar is used to test several antibiotics against a test organism simultaneously. Antibiotics are impregnated on filter paper disks and placed on the agar that has been streaked with test organism. Overnight incubation - effective antibiotics will have zones of growth inhibition around the filter paper. Zones are standardized and measured via vernier caliper.

Question 6

epsiometer test (E test)

Answer 6

popular method for smaller labs, employs agar diffusion along with MIC. Uses calibrated test strips with decreasing concentrations of various test antibiotics which are placed on the large agar plate streaked with the bacteria. Oval zones of inhibition will be present corresponding to the decreasing conc of antibiotic. The intersection of growth-no growth at the concentration indicated on the strip is defined as the MIC.