Lesson 4: The SXT Test: Which Type of Strep?

SXT stands for sulfamethoxazole and trimethoprim. The SXT test is used to distinguish between groups A and B and other groups of streptococcus. Group A and B bacteria are usually resistant to SXT, while other groups, such as D bacteria, generally show sensitivity to SXT. Can you complete your bacteria identification for the specific species of streptococcus using the results from the SXT 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.

SXT 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 an SXT disk from the shelf and move it to the BA plate.
12. Move the BA plate with the SXT 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.