Antimicrobial Drug Sensitivity Testing

Antimicrobial Sensitivity Testing

Introduction

Antimicrobial sensitivity testing is important clinically because the proper selection of an antimicrobial drug in the treatment of a bacterial infection is ideally based on the knowledge of the sensitivities of the infecting organism. In this laboratory exercise you will be working within a group performing a commonly used test that is designed to determine whether or not an isolated organism is able to be treated using a specific antimicrobial drug. The procedure is called sensitivity testing. This testing method allows clinicians to obtain information needed in order to make an informed and concise decision in reference to the antimicrobial drug usage.

Methods and Materials

Materials being used in this test include:

• 6 Mueller-Hinton agar plates

• 14 cartridges of antimicrobial drugs

• three automatic dispensers

• two 1mL pipettes and pipette pump

• broth culture of Staphylococcus aureus and Escherichia coli

• spreading rod soaking in ethanol

• two forceps soaking in ethanol

• marking pen

• ruler

• antimicrobial sensitivity chart

To start off this lab you will

1. Label the plates with the name of the organisms (three plates per organism)

2. Create a bacterial lawn by

a. Inoculate each dish with 0.2mL of the appropriate bacteria. Deposit the inoculum in the center of the plate.

b. Remove the spreading rod from the ethanol and ignite the ethanol (Do not leave the rod in the Bunsen burner flame longer than is needed to ignite the ethanol.) Allow the ethanol to burn off. Spread the culture over the entire surface of the plate using the sterilized spreading rod. Before continuing wait five minutes to allow the culture to be absorbed by the nutrient agar.

3. Apply antimicrobial disks to the plate using the dispensers as demonstrated by your instructor. Gently press the disks to the surface of the agar with a sterile set of forceps.

4. Incubate the Petri dishes for at least 24 hours at 37 degrees Celsius.

5. Measure the diameter (in mm’s) of region around the disks where growth was inhibited. This region will appear as