12.2: Bacteria in the Environment

Bacteria are the most prolific living things on Earth. There are thousands of known bacterial species with many more still undiscovered. Bacteria are one of the two prokaryotic domains -- the largest categories for classifying living organisms. Some bacteria prefer environments that are oxygen-rich while others prefer oxygen-poor environments. Some are free-living while others can cause disease. Bacteria can vary in their cell and colony shapes as well. We have already reviewed the general structure of prokaryotic cells in Lab 4: Cell’s Kitchen, but before getting started on this exercise, you may want to review 4.1 to refresh your memory.

For this exercise, we will explore the microscopic world all around us by taking bacterial samples from everyday objects. These samples will then be streaked onto nutrient-containing plates and then left to incubate for several days. After allowing the plates to incubate, we will observe the plates for any growth.

Before getting started, let’s look at proper techniques for collecting environmental bacterial samples and streaking plates.

When taking a sample, you will use a sterile swab. Keep the swab in the packet until you are ready to use it. When you are ready, remove the swab from its packaging. If you are swabbing a dry area, such as a table top, you will need to wet the swab with sterile water first. Make sure that if you use water, that you keep it covered and it is cool before using. If you use very hot water, the heat may kill any potential bacteria on your swab. If you are swabbing a wet area, you do not need to preemptively wet the swab. Then when you are ready to take a sample, simply roll the swab over the area.

For streaking plates, you will use a zig-zag pattern over the top of the gel, starting from the outer edge of the plate and working your way towards the middle. Since we will be taking four samples in this lab, your plate will be divided into fourths. Make sure not to overlap your samples. Below is an image demonstrating the nutrient plate set-up and streaking pattern.

When streaking your plate, it is important not to puncture or tear the gel on the bottom surface of the plate when rolling the swab over it. This gel is a nutrient-containing agar which will serve as a home and food source for any bacteria from your samples. While you want to make sure your swab comes into contact with the gel, you may not get clear results if you damage it.

"Bacterial Streaking Pattern" by Karen Marks, Reedley College is licensed under CC BY 4.0

Materials:

• 4 sterile cotton tipped swabs
• 1 nutrient agar plate
• Sterile water
• Permanent marker

Procedure:

1. Use a permanent marker to divide the bottom side of the agar plate into fourths.
2. Number each quadrant 1 through 4.
3. Along the bottom side’s margin, write at least one lab partner’s name. (Write small! You don’t want to cover the entire bottom.)
4. Open one of the sterile swabs. If sampling a dry surface, wet the swab in the sterile water.
5. Roll the swab over the surface of the object being sampled.
6. Then, open the lid on the plate just enough to get the swab inside and roll the swab on the gel in the zig-zag pattern seen above on the first quadrant on the plate.
7. Repeat steps 3-6 for each sample, using a new swab each time.

When deciding on where to take your samples from, be creative as long as you do not enter any unauthorized areas or access someone’s personal belongings without permission. Many students choose to sample areas they suspect are likely to be dirty such as cell phones and headphones, the inside of sink faucets, the bottoms of shoes, or the inside of toilets. These are just a few ideas to help you decide on where to sample. Once you’ve decided, write the sample area/object in the table on the next page.

Table 12.1 Sampled Areas and Objects

Sample #	Sampled Area/Object
1
2
3
4

Now that we’ve identified four areas to take samples from, let’s hypothesize how much growth and how many different types of bacteria we will see for each sample.

Table 12.2 Hypotheses on Growth and Variety

Sample #	Amount of Growth (Low/Med/High)	# of Bacterial Species
1
2
3
4

After plating your samples, they must incubate until the next lab period. Make sure your name is on the plate and that the lid is taped shut. Then, give your plate to your instructor, who will then store it in the incubator.

All used swabs need to go into the biohazardous waste container. Other waste, such as swab wrappers, can go into the regular trash.

After your plate has incubated, you can now look for growth and how many bacterial species were on your sample. To determine how many different types of bacteria are on your plate, use Appendix A at the end of this manual. Appendix A will help you differentiate bacterial species based on shape, margin, color, and more.

Table 12.3 Results on Growth and Variety

Sample #	Amount of Growth (Low/Med/High)	# of Bacterial Species
1
2
3
4

How did your results compare to your hypotheses?

Were there any sample results that surprised you? Why did they surprise you?

All used agar plates must be disposed of in the biohazardous waste. Do not throw any used swabs or plates into the regular trash.