Bacteria are single-celled organisms. They are very tiny—tiny enough that you cannot see one with the naked eye (although you can see a whole bunch). Bacteria serve all sorts of functions in the environment and your body, such as helping nutrients in the soil, and helping you digest your lunch. They are also harmful kinds of bacteria, which is why people always stress the importance of hand-washing. There is no escape from bacteria…they are everywhere!
Bacteria are asexual, meaning they reproduce on their own. They reproduce by binary fission, which divides on cell in to two, two cells into four, and four cells into eight! This is how you will grow enough bacteria to see with the naked eye.
Discover which public places have the most bacteria.
- 5 or more petri dishes
- Small glass dish (Pyrex)
- Zip-top plastic bags
- Agar powder
- Cotton swabs
- Labeling tape
- Nitrile disposable gloves
- Use cotton swabs to collect your bacteria samples. All you have to do is wipe the swab on a surface. Some good locations to find a lot of bacteria are door handles, bus or train seats, desks, and faucet handles. Use only one cotton swab per location. It’s a good idea to wear gloves during this experiment so that you don’t get sick from any of the germs. Make sure to wash your hands during and after the procedure.
- Store each cotton swab in its own labeled zip-top bag.
- Label your petri dishes with the locations where you took each sample.
- Mix the agar powder with water, following the directions from the manufacturer.
- Pour a small amount of the liquid agar in to the small glass dish.
- Take a clean, brand new cotton swab and wipe a clean petri dish.
- Dip the swab into the dish of agar. Then wipe again on the petri dish and label it “control.” Why should you always have a control? What purpose does the control serve?
- Clean out the small dish. You do not want to contaminate the agar between samples.
- Take each of your collected samples, dip it in agar, and swab the appropriately labeled petri dish.
- Keep the dishes at room temperature and record your observations over few days. Which locations seem to have the most bacteria?
The agar plates with more bacteria come from places that harbor more bacteria. Results will vary depending on which locations you chose to test.
Agar is used as the surface for the bacteria to grow. Agar itself cannot be digested by most organisms, but agar packets for growing cells often have additives that are bacteria eat and use to grow. Bacteria also grow rapidly with moisture and relative warmth.
The control group is important because it gives you a basis to compare the other bacteria growth. Hopefully, if the petri dishes are clean, you will have no growth. If you do have bacteria growth in the control petri dish, it was somehow contaminated with bacteria, and you will have to consider this when looking at the results of your experiment.
Locations where many different people are touching a given surface are most likely to have a large amount of bacteria (for example, the hand rail on an escalator). Bacteria can spread through bodily fluids, leaving the body when someone coughs or sneezes and making a temporary new home in the surrounding area. Because people often sneeze or cough into their hands, surfaces that come into contact with hands, like door handles, tend to have a lot of bacteria. This is why it is better to cough and sneeze into the crook of your arm, because then you can prevent bacteria from spreading to others by your hands.