Study of Bacterial Growth and Resistance Level to Certain Antibiotics
Escherichia coli—better known as E. coli—is a gram negative, rod shaped bacteria. It is relatively harmless, but can occasionally cause food poisoning. It can also provide Vitamin K2. It prevents the establishment of pathogenic bacteria, and is associated with or found in the intestinal organ. The antibiotic that E. coli is resistant to is Penicillin. Bacillus subtilis—better known as B. subtilis—is known as the hay bacillus or the grass bacillus. It is gram and catalase positive. It is rod-shaped, with the ability to form a tough, protective, shield around itself. It can tolerate extreme environmental conditions, and is an obligate aerob. The antibiotic that B. subtilis is resistant to is Tetracycline. When a bacteria is resistant to an antibiotic, it means that the antibiotic will have no effect in treating the bacteria. For example, E. coli is resistant to Penicillin, which means that if you were to try and treat E. coli with Penicillin, the treatment would fail and the E. coli bacteria would continue to grow. But, if you were to treat E. coli with Tetracycline, the antibiotic it is NOT resistant to, the E. coli would not be able to put up a fight, and would stop its growth. HA- If we add E. coli and B. subtilis to agar, and add Penicillin and Tetracycline to the agar, then the E. coli will grow more around the Penicillin and the B. subtilis will grow more around the Tetracycline, because E. coli is resistant to Penicillin and B. subtilis is resistant to Tetracycline. HO- If we add E. coli and B. subtilis to agar, and add Penicillin and Tetracycline to the agar, then the E. coli will grow more around the Tetracycline and the B. subtilis will grow more around the Penicillin, because E. coli is resistant to Tetracycline and B. subtilis is resistant to Penicillin.
MATERIALS (PER PAIR OF STUDENTS)
• 2 agar plates with nutrient sugar
• 2 disks of Penicillin
• 2 disks of Tetracycline
• 1 glass marking pencil
• 2 sterile swabs
• 2 pairs of forceps
• 1 roll of masking tape
• 1 metric ruler
MATERIALS (TEACHER STOCK)
• Culture of harmless strain of E. coli
• Culture of harmless strain of B. subtilis
• Spray bottles with bleach
• Biohazard bag
PART A: SETTING UP THE EXPERIMENT
1) Put on goggles and plastic gloves.
2) Obtain two agar plates.
3) Using a sharpie, write your last name and bacteria type to label the petri dish lid.
4) Use the marking pencil to divide the bottom of each plate into thirds. In two of the sections, write an abbreviation for the antibiotic (P for Penicillin and T for Tetracycline) you will use. Label the third section as control (C for control).
5) Obtain the first bacterial culture from your teacher.
6) Transfer the culture to the agar plate based on your teacher’s directions using an incubator loop.
7) Repeat step 6 with the second bacterial culture and your second agar plate.
8) Open the lid of the agar plate in a “clam shell” method.
9) Using forceps, place the first antibiotic disk on the agar in the appropriate sections of the plates. Press the disks flat so that they stick to the surface of the agar. Don’t move the disks once they’re in place.
CAUTION: Do not touch the disk or the agar with your hands.
10) Use new forceps to place the second antibiotic disks on the agar plates in the proper sections. Don’t move the disk once it is in place.
11) Turn the plates upside down and incubate the culture overnight at 37 degrees Celsius, or for 2-3 days at room temperature.
PART B: OBSERVING THE PLATES
1) Examine each plate. Look in particular at the areas immediately around the three disks.
CAUTION: Do not uncover the plates.
2) Measure the diameter of any clear zones around the three...
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