Bacterial Growth Lab

Bacterial Growth Lab

PSI Biology Name______

Purpose

How can bacterial growth be inhibited?

Materials (per group)

2 sterile nutrient agar plates

glass marking pencil

test tube rack

Bunsen burner

Flint striker or matches

2 sterile cotton swabs

beaker of water

8 sterile filter-paper disks

culture of E.coli or other bacteria sample

3 disinfectants: bleach, hydrogen peroxide, window cleaner

3 antibiotics: ampicillin, penicillin, amoxicillin

distilled water

sterile forceps

transparent tape

Safety: Class should discuss safety precautions before starting the lab. Safety goggles and aprons should be worn. Long hair should be tied back.

Note: Class should also discuss the importance of keeping the materials sterile.

Procedure

Day 1

1. Turn each of your agar plates over while keeping them closed and use a glass marking plate to divide the plate into four even quadrants like a pie. Near the edge of each quadrant, number the quadrants 1-4. Near the bottom center of each dish write your period and group number. Turn agar plates right side up.

2. Light the Bunsen burner.

3. Remove the cap from the E. coli sample culture test tube and pass the mouth of the tube back and forth through the burner flame.

4. Insert a sterile cotton swab into the bacterial culture, shake off any excess liquid into the culture tube. Remove cotton swab

5. Pass the mouth of the culture tube back and forth through the flame and replace the cap. Return the culture tube to the test tube rack.

6. Open the sterile nutrient agar plate slightly and place the tip of the cotton swab near the top center of the agar plate and streak the agar as demonstrated in the Bacteria Growth Lab notebook.

7. Place the top of the cotton swab in the flame of the Bunsen burner until it catches fire. Remove from flame and place into the beaker of water to extinguish the cotton swab.

8. Repeat steps 3-7 for the other sterile nutrient agar plate.

9. Use sterile forceps to pick up a disk of filter paper and insert the disk into the bleach. Shake of any excess liquid.

10. Open one of the inoculated agar plates just slightly and position the disk in the center of quadrant 1, pressing the disk against the agar until it sticks. Remove the forceps and close the nutrient agar plate.

11. Pass the forceps back and forth through the flame several times.

12. Repeat steps 9-11 for the same nutrient agar plate, placing the hydrogen peroxide disk in quadrant 2, the window cleaner disk in quadrant 3, and the distilled water disk in quadrant 4.

13. Repeat steps 9-11 for your other nutrient agar plate using the following disks: ampicillin in quadrant 1, penicillin in quadrant 2, amoxicillin in quadrant 3, distilled water in quadrant 4.

14. Seal each Petri dish closed with transparent tape. Innoculate the dishes upside down for 48 hours at 37oC. Do not open the Petri dishes again.

15. Record predictions in data chart

Day 2: Do not open the Petri dishes!

16. Observe the Petri dishes and note the presence or absence of a zone of inhibition around each disk on your data sheet.

Data

Disinfectant / Prediction / Zone of Inhibition
1. Bleach
2. HydrogenPeroxide
3. Window Cleaner
4. Distilled Water
Antibiotic / Prediction / Zone of Inhibition
1. Ampicillin
2. Penicillin
3. Amoxycillin
4. Distilled Water

Analysis and Conclusions

1. Which disinfectant most effectively inhibited the growth of E. coli? Provide support for your answer.

2. Which antibiotic most effectively inhibited the growth of E. coli? Provide support for your answer.

3. How do you know that any inhibition you observed is due to the disinfectants and antibiotics on the disk?

4. If the same experiment was repeated using a different strain of bacteria and a zone of inhibition was observed around the disk soaked in window cleaner, what could account for the different results?

5. You wake up one morning with a severe sore throat and notice white dots lining your tonsils. You quickly visit the doctor, who diagnoses you with a Streptococcus infection (strep throat) and prescribes a 10-day antibiotic treatment. After 3 days, you begin to feel better. Why should you continue to taking the antibiotic for the full 10 days?

6. What causes a zone of inhibition?

7. What causes a plaque?

8. Both bacteria and viruses can cause diseases. Why would it be unwise to treat a viral infection with antibiotics?