Microbiology Lab Assignment

Think of these lab packets as homework for the lab part of the class. They are designed to help prepare you for the departmental lab exams. To answer these questions you will need to use both the lab workbook as well as your text book. In addition to these questions, you will need to turn in the signed data sheets from your workbook.

Gram Staining

1. Describe the molecular basis behind Gram staining. What part of the bacteria is stained with the primary stain, what part is stained with the counter stain? Why do we wash alcohol over the slide before the counter stain?

2. Describe in great detail the steps in performing a Gram stain. Include incubation times. De?ne primary stain and counter stain. In the Gram stain, what dye is primary stain, which is the counter stain?

Special Stains

3. What is the stain is shown in the picture below? What is stained green, what is stained red?

4. Describe in great detail the steps in performing an endospore stain. Include incubation times.

5. What does it mean to be Gram positive, Gram negative?

6. Label the different ?agellum arrangements seen below.

7. Describe the negative staining technique and determine when it is used.

Colony Morphology

8. We can learn some information about bacteria by just looking at its growth on an agar plate. What is a small isolated growth of bacteria on an agar plate called and what can it tell us about the bacteria?

Kirby-Bauer Method

9. Describe in great detail the technique used to determine bacteria sensitivity to antibiotics?

10. What is the type of media used in the Kirby-Bauer test and why is it used?

Describe what is observed for a bacteria that is sensitive to antibiotics in a Kirby-Bauer test, and to a bacteria that is not sensitive to antibiotics.

11. How does the antibiotic get from the disk into the agar?

Does the agar have an antibiotic beyond the zone of inhibition?

12. De?ne the MIC and explain how it is used to determine if a bacteria is resistant or sensitive to the antibiotic.

Is the presence of a clear zone around the disc enough evidence to determine antibiotic resistance?

Disinfectants

13. Compare your results with the class data.

Which germicide was most effective and at what concentration?
Which was the least effective? Defend your choices.

14. Which organism seemed to be most resistant to the germicides?

15. What purpose did the four controls serve? Be speci?c.