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They're Everywhere: Bacteria

Author(s): Nancy Moreno, PhD, and Barbara Tharp, MS.

Let's Talk About It

This activity allows students to observe bacteria and to compare relative amounts of bacteria living in a variety of common places. Students will discover that bacteria grow almost everywhere.

Ask, What has changed inside your petri dishes? Bacteria will discolor the surface of the gel and form smooth, wrinkly or slimy blotches (called colonies) of different colors. Fungi, which form fuzzy colonies, also may be present. Have students decide how many different kinds of organisms might be growing on their gels, based on differences they can observe. Do not allow students to open the dishes.

Next, have students decide whether some sample sources had more bacteria than others by counting the number of colonies and/or by comparing the sizes of colonies in each petri dish. Have groups record their observations and conclusions, and then share their results with the rest of the class. Based on the results, instruct students to decide which locations have the most bacteria, and which have the least. 

Ask, If there are bacteria all around us, why aren’t we all sick? Do all bacteria make us sick? What about the gel in the petri dishes—would you want to eat it? Do you think it is good (safe? healthy?) to have bacteria growing in our food? Help students understand that contamination of food by bacteria can cause serious health problems. 

Ask students for suggestions about how to keep food clean. Possibilities include: using clean hands and utensils for food preparation, keeping food covered and refrigerated until used, and cooking food thoroughly to kill bacteria that might be present. (See “Bacteria Busters!” in the sidebar of the teacher’s guide)


Funded by the following grant(s)

National Institute of Environmental Health Sciences, NIH

National Institute of Environmental Health Sciences, NIH

My Health My World: National Dissemination
Grant Number: 5R25ES009259
The Environment as a Context for Opportunities in Schools
Grant Number: 5R25ES010698, R25ES06932