Microbiology: A Systems Approach 4th Edition by Marjorie Kelly Cowan

Exercise 7

Referring to figure 12.1 a , determine whether the cultured bacterium is sensitive, intermediate, or resistant to each of the antibiotic drugs tested and provide an explanation in each case.
b. Referring to figure 12.3 a , determine the minimum inhibitory concentration (MIC) of the drugs being tested, providing an explanation for your answer.
Figure 12.1a Technique for preparation and interpretation of disc diffusion tests. (a) Standardized methods are used to seed a lawn of bacteria over the medium. A dispenser delivers several drug-impregnated discs onto a plate, followed by incubation. Interpretation of results: During incubation, antimicrobials become increasingly diluted as they diffuse out of the discs into the medium. If the test bacterium is sensitive to a drug, a zone of inhibition develops around its disc. Roughly speaking, the larger the size of this zone, the greater is the bacterium's sensitivity to the drug. The diameter of each zone is measured in millimeters and evaluated for susceptibility or resistance by means of a comparative standard (see table 12.3).

(a) *R and S values differ from table 12.3 due to differing concentrations of the antimicrobials.
Table 12.3 Results of a Sample Kirby-Bauer Test

Figure 12.3a: Tube dilution test for determining the minimum inhibitory concentration (MIC). (a) The antibiotic is diluted serially through tubes of liquid nutrient from right to left. All tubes are inoculated with an identical amount of a test bacterium and then incubated. The first tube on the left is a control that lacks the drug and shows maximum growth. The first tube in the series that shows no growth (no turbidity) contains the concentration of antibiotic that is the MIC.