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Lab 3 - Antibiotics
Purpose: The purpose of this lab is to see how antibiotics affect bacteria. In this activity you will observe the effects of antibiotics on bacteria that were cultured in a petri dish.
Worksheet
Download the copy of the lab sheet for this activity. Fill out the information as needed.
| lab_3_-_antibiotics_lab.pdf |
Part One - Background
For decades medical doctors have been giving patients antibiotics to help fight infections. An antibiotic is a substance that slows down or stops bacterial growth. There are a variety of antibiotics. Not all antibiotics work on every bacteria. Therefore it is important to know which antibiotics are most effective against bacteria. How do you determine that?
This lab shows you one method of measuring the effectiveness of an antimicrobial agent against bacteria grown in culture. This is called the Kirby-Bauer disk-diffusion method, and here is how it works. The bacteria of interest is swabbed uniformly across a culture plate. Then a filter-paper disk, covered with the antibiotic to be tested, is placed on the surface of the agar. The compound diffuses out from the filter paper into the agar. The concentration of the compound will be higher next to the disk, and will decrease gradually as distance from the disk increases. If the compound is effective against bacteria at a certain concentration, no colonies will grow wherever the concentration in the agar is greater than or equal to that effective concentration. This region where no colonies of bacteria grow is called the "zone of inhibition." Thus, the size of the zone of inhibition is a measure of the compound's effectiveness: the larger the clear area around the filter disk, the more effective the compound.
(Second paragraph taken from Sciencebuddies.org)
This lab shows you one method of measuring the effectiveness of an antimicrobial agent against bacteria grown in culture. This is called the Kirby-Bauer disk-diffusion method, and here is how it works. The bacteria of interest is swabbed uniformly across a culture plate. Then a filter-paper disk, covered with the antibiotic to be tested, is placed on the surface of the agar. The compound diffuses out from the filter paper into the agar. The concentration of the compound will be higher next to the disk, and will decrease gradually as distance from the disk increases. If the compound is effective against bacteria at a certain concentration, no colonies will grow wherever the concentration in the agar is greater than or equal to that effective concentration. This region where no colonies of bacteria grow is called the "zone of inhibition." Thus, the size of the zone of inhibition is a measure of the compound's effectiveness: the larger the clear area around the filter disk, the more effective the compound.
(Second paragraph taken from Sciencebuddies.org)
Part Two - Observations and Data
In class we prepared petri dishes following the Kirby-Bauer disk-diffusion method. On each dish we added the bacteria Bacillus subtillus and three antibiotic discs: penicillin, streptomycin, and tetracycline. We also added a blank disc to act as the control. The purpose of the control is two-fold. First, it shows us what normal growth of bacteria looks like. Second, it shows us that the antibiotics, and not the paper, are affecting the bacteria. We left the petri dishes in an incubator over night so they could grow. The next day we observed the affects of the antibiotics on the bacteria.
Below you fill find a sample image of what a similar lab looks like. The person who conducted this lab used many more antibiotics. They did use a control but it is not visible in this petri dish.
Procedure
1. Measure the zone of inhibition around each antibiotic disk. To do this, measure the diameter of the zone of inhibition.
2. Record this data in the appropriate space on your data table.
Picture guide:
A. Erythromycin
B. Chloramphenicol
C. Cephalothin
D. Tetracycline
E. Vancomycin
F. Novobiocin
G. Streptomycin
H. Penicillin
Source: Microbe Library
Below you fill find a sample image of what a similar lab looks like. The person who conducted this lab used many more antibiotics. They did use a control but it is not visible in this petri dish.
Procedure
1. Measure the zone of inhibition around each antibiotic disk. To do this, measure the diameter of the zone of inhibition.
2. Record this data in the appropriate space on your data table.
Picture guide:
A. Erythromycin
B. Chloramphenicol
C. Cephalothin
D. Tetracycline
E. Vancomycin
F. Novobiocin
G. Streptomycin
H. Penicillin
Source: Microbe Library
Measure the zones of inhibition for each of the antibiotis. See procedure for names of each antibiotic.
Part Three - Analysis
Answer the following questions. Some of the answers may be found in your background or your data.
1. What is an antibiotic?
2. What is the zone of inhibition?
3. Using the Kirby-Bauer disk-diffusion method, how can you tell which antibiotic is most effective?
4. Which antibiotic was least effective against the bacteria in the picture? How do you know?
5. Which antibiotic was most effective against the bacteria in the picture? How do you know?
6. What is the purpose of putting a control disk, or blank disk, into the petri dish?
1. What is an antibiotic?
2. What is the zone of inhibition?
3. Using the Kirby-Bauer disk-diffusion method, how can you tell which antibiotic is most effective?
4. Which antibiotic was least effective against the bacteria in the picture? How do you know?
5. Which antibiotic was most effective against the bacteria in the picture? How do you know?
6. What is the purpose of putting a control disk, or blank disk, into the petri dish?
