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

They're Everywhere: Bacteria

Bacteria are the most numerous of all things living on our planet. However, they are so tiny that it is not possible to see one without the aid of a microscope. Most bacteria must be magnified at least 400 times before they can be observed. Each bacterium (a single bacteria) consists of one cell capable of reproducing very rapidly. In fact, one bacterium cell can produce millions of others in just one day.

Bacteria are essential in many ways. They are important decomposers in almost all ecosystems. Photosynthetic bacteria (also known as blue-green algae) are vital producers in aquatic ecosystems. Bacteria in the intestines of animals help break down some large food molecules during digestion.

Bacteria also can cause serious problems with food. Since bacteria are everywhere, it is easy for food to become contaminated by bacteria and begin to spoil. The slime you see on food that has been in the refrigerator too long is made of clumps of bacteria and, sometimes, fungi as well. Eating spoiled food can make humans and other animals sick.

Bacteria can be transferred to food when people do not wash their hands after using the bathroom, changing a diaper or playing with pets. Some foods, especially meats, can have bacteria on their outside surfaces. These bacteria can be transferred to other foods if knives, spoons and cutting boards used in preparing them are not washed with soap and water.

This activity allows students to observe bacteria and to compare relative amounts of bacteria living in different parts of the home, classroom or school.

Complete instructions for conducting activities in this slide set, including materials needed, setup instructions, student sheets (in English and in Spanish), answer keys and extensions, can be found in The Science of Food Teacher’s Guide, which is available free-of-charge at

Session 1: Setting up

1. Tell students that they will be learning about bacteria—tiny microorganisms present everywhere. Ask students to mention what they know or have heard about bacteria. List their ideas on the board.

2. Point out that bacteria are a major source of food contamination, and that students will be investigating where bacteria might be present. Ask, Can we see where bacteria are? How might we be able to find out where the most bacteria are in the room (school, etc.)?

3. Tell students that one way to study bacteria is to let them grow until they form a clump that is large enough to see. Mention that they will be finding and growing bacteria.

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

Houston Endowment Inc.

Foundations for the Future: Capitalizing on Technology to Promote Equity, Access and Quality in Elementary Science Education