Q&A for How to Grow Bacteria in a Petri Dish

While it is generally recommended to use fresh dishes for each experiment, it is possible to reuse dishes if needed. Sterilization methods will vary depending on the material of your dish (glass vs. plastic). If your plastic Petri dishes were used to harvest live pathogens, it is recommended to use a new dish as the risk of cross-contamination is high.

To grow bacteria in a lab-like setting, you need agar. You can find it online or through a science supply store.

Using two sets of petri dishes, create one group of bacteria samples taken before washing your hands and one group taken after washing your hands. Compare growth of the bacteria between groups over time.

The agar is fine, as long as you add a protein agent, like blood. That will make it suitable for bacterial growth.

So that they aren't compromised by foreign or unwanted bacteria. You want to have a pure bacteria culture.

Bacteria is capable of growing at room temperature, which is around 20 degree Celsius. Therefore, although the use of an incubator is recommended, it is not required. As for safety concerns, as long as you do not collect samples from places like bathrooms which may contain pathogenic bacteria, and if you properly dispose of any bacteria grown at the end of the experiment, it should be relatively safe to do at home.

Theoretically, as long as there is enough agar and moisture, the bacteria is capable of dividing indefinitely.

Bacteria normally forms colonies and can be seen in small, smooth-like bubbles opposed to fungi which is often fuzzy and fluffy in appearance.

You would count them manually. Bacterial colonies are large clusters of microorganisms that can be seen without the aid of a microscope. The clusters seen will be the number of bacterial colonies.

Some dishes have a grid on the bottom that you can use to measure the colonies. If your dish doesn't have that, measure the end of your cotton swab and use that to get a rough estimate.

Maybe the agar you used does not support bacterial growth. For instance, Sabouraud agar is for fungal growth, so if you use such an agar, you are likely to find fungi.

Wear a glove, wipe it up, and clean the floor with sanitization wipes or alcohol wipes. If these things are not available, water should work as well. Just make sure to clear all of the agar and bacteria off of the floor. It is recommended that you do not attempt to grow bacteria collected from places like bathrooms, etc. that may be pathogenic, so if you do have a spill, it wouldn't be a huge issue since other types of bacteria do little to no harm to human bodies.

You may have used too much water or too little agar. Make sure the agar is completely dissolved and the liquid is clear.

It depends on the type of bacteria they consumed, but it could make them very ill.

Yes, but you have to add a protein for the bacteria to feed on.

No. Incubators are for keeping bacteria at a constant, tolerable temperature in order to encourage growth over a period of time (like at body temperature to imitate conditions in the human body). Microwaves will simply kill your bacteria and make your microwave potentially unusable for food (and will make it smell horrible!).

You can just throw it away like anything else. (Recycling it would also be an option.)

It depends. You can just use your hand, if it is safe to do so, or if you want to collect samples from different surfaces, use a slightly moisturized Q-tip or cotton swab.

Create a petri dish with multiple bacteria, then harvest the specific bacteria you want and place it in another petri dish.

Then something is in there other than bacteria, which means the things that appear to look like worms are not the bacteria you are trying to grow. You must have put something in there than just the sterilized cotton swab.

There are alternatives such as gelatine or guar gum. None of these, including agar, are that expensive and should be easy to find either in the supermarket, a health food store or online.