Lesson 3: The Bacitracin Test: Which Type of Strep?

The bacitracin test involves placing a disk of bacitracin in a Petri dish with a growing culture of bacteria. Bacitracin is a bactericidal drug used to treat skin infections. A positive test results in the inhibition of bacteria growth around the disk and usually indicates the presence of a group A streptococci. Can you complete your bacteria identification for the specific species of streptococcus using the results from the bacitracin test?

Doing the science

1. Open the Bac ID simulation. Select Part II.

Sample Preparation
2. Select one of the known streptococcus bacteria samples from the top shelf and move it to the tabletop. Do not choose a staphylococcus bacteria sample.
3. Select the 0.85% saline tube from the shelf and move it to the tabletop.
4. Select the inoculating loop and move the loop to the burner flame.
5. To collect a sample, move the sterilized loop to the plate containing the known streptococcus bacteria sample on the table.
6. Move the loop from the sample plate to the 0.85% saline test tube.

Bacitracin Test
7. Select the blood agar (BA) plate from the shelf and move it to the tabletop.
8. Select the marker and label the BA plate with the name of the bacteria.
9. Select the swab on the table and move the swab to the 0.85% saline test tube
10. Move the swab to the BA plate.
11. Select a bacitracin disk from the shelf and move it to the BA plate.
12. Move the BA plate with the bacitracin disk to the Anaeropak box incubator.
13. When the incubation period is complete, select the magnifier.
14. Carefully observe the sample. Note and record your observations in the appropriate space on Table 1. Close the magnified view.
15. Repeat steps 2–15 for the remaining streptococcus bacteria samples only. Do not choose a staphylococcus bacteria sample. Make sure to record your results in Table 1.
16. Repeat steps 2–15 for one of the unknown bacteria samples on the top shelf.
17. Make sure to enter the code for the unknown sample in Table 1.
18. Using the results from your analysis, identify the bacteria type in your unknown sample. Record the name of your unknown bacteria type in Table 1.