Activity 5: Bacterial Growth

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Objective

Students will grow bacterial colonies and then determine the effectiveness of disinfectants in killing the bacteria.

Background Information

Most bacteria can proliferate rapidly through binary fission. Under optimal conditions, some bacteria can divide as rapidly as once every 20 minutes, a trait that has proved invaluable to geneticists. However, until the 1940s, scientists were not sure that bacteria would be useful subjects for genetics research because it seemed that their method of reproduction was too simple. At that time, researchers found that although bacteria do not reproduce sexually in the same manner as plants and animals, they do have mechanisms for transferring genes from one individual organism to another. These mechanisms allow bacterial strains to become resistant to antibiotics.

Many disease-causing bacteria respond to treatment by antibiotics, which are used to kill or slow the growth of bacteria. Like antibiotics, disinfectants can also inhibit the growth of certain types of bacteria. In this activity, students will study the effectiveness of common disinfectants.

Materials

For each group:

• Agar-filled Petri dish
• Wax pencil
• Cotton swab
• Filter paper
• Hole punch
• Forceps
• 3 disinfectant solutions
• Distilled water
• Tape
• Bleach (for cleaning)

Preparation

1. Use a hole punch to cut filter paper discs. These will be used to apply disinfectants. Keep the disks in a covered container until time for use.
2. Prepare nutrient agar and pour into sterile, disposable plastic petri dishes. Allow to set.
3. Bring in three household disinfectants to use for the activity. Each should be clearly labeled with a numeral 1-3. You may want to write the name of each disinfectant on the board.

Instructions

1. Have students turn the petri dishes upside down and use wax pencils to mark four sections on the outside of the dish. The sections should be labeled 1, 2, 3, and 4.
2. Ask students to lightly moisten a cotton swab with distilled water. They should then rub the cotton swab lightly across a surface in the classroom, such as a desk, doorknob, or window. Tell them to roll the swab as they move it over the surface to collect the maximum number of bacteria.
3. Next, have students lift the top of the Petri dish just enough to get the swab inside. They should quickly streak the swab in a zigzag motion all over the agar surface, and then replace the cover.
   
   
4. Tell students to use their forceps to dip one of the filter-paper disks in a disinfectant solution. Students should reopen the Petri dish, and place the disk in the section labeled 1. Have students repeat the procedure to place a disk dipped in the second disinfectant in section 2, and a disk dipped in the third disinfectant in section 3. For the fourth section, students should place a filter disk that has been dipped in distilled water.
5. Have students tape the Petri dishes shut and incubate them upside-down for 48 hours in a warm, dark place. (Incubating the plates upsidedown prevents condensation from falling on the bacterial colonies.)
6. After incubation, have students compare the diameters of the clear areas around each filterpaper disk. Remind students not to remove the cover of the disk while observing the bacterial colonies, and to wash their hands well after the investigation.
7. After the activity has been completed dispose of the dishes properly by flooding them with a 10% bleach solution for 30 minutes.

Discussion Questions

1. The agar and Petri dishes were sterilized before this investigation. Why do you think this was necessary? (To prevent the growth of bacteria that might have already been present in the agar or on the Petri dishes.)
2. Which disinfectant appears to be the most effective? (Answers will vary.) How can you tell? (The paper disk with the largest clear area around it (the area of inhibition) was saturated with the most effective disinfectant.)
3. How can you tell if there is more than one type of bacteria growing on the agar dish? (Different types of bacteria will produce different types of colonies.)
4. If this experiment were performed with antibiotics rather than disinfectants, how could you tell if the bacteria was resistant to the antibiotic? (Answers will vary but should include the idea that more bacteria will grow if the bacteria are resistant to the antibiotic.)

Extension

Have students repeat the activity using over-the-counter antibiotics such as polymyxin B sulfate, bacitracin zinc, and neomycin sulfate.

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