Skip Navigation

Tools and Equipment of Science

Author(s): Roberta Anding, MS, RD/LD, CDE
Showing Results for: science Return to Presentation

Testing for Microbes

Have each group select four locations to test for the presence of microbes. Remember, no body fluids are to be tested. Possible test sites could include the floor, doorknob, writing utensils, etc. Allow students to be creative in their selections, without prompting. Encourage them to choose as many varied locations as possible.

Have each group create a table with two columns, labeled “Location Sampled” and “Predicted Results.” Students should use a piece of masking tape and marker to label the bottom (nutrient agar side) of each Petri dish with the location to be sampled. Each group also should include a unique group name or identifier on the labels, to distinguish their plates from other groups’.

Have groups list selected locations in the “Location Sampled” column. In the “Predicted Results” column, each group should predict if each chosen location will grow microbes. Students also should predict which sample locations will grow the most and fewest microbes.

Have students rub a cotton swab, moistened in boiled water, several times over one of the locations to be tested. Students should open the Petri dish with that location’s name on the label and rub the swab gently in a zigzag pattern across the total surface of the nutrient agar. (This inoculation technique is known as streaking.) Then, students should rotate the Petri dish one quarter turn and streak again. 

Caution students not to break the surface of the agar. Groups should repeat the inoculation process for the remaining three locations, using the appropriately labeled Petri dish and clean swab for each. 

Collect all used swabs and dispose of them in a 10% chlorine bleach solution.

The fifth Petri dish should be inoculated with a clean moist swab and used as a control. Ask, Why are we inoculating with a clean swab? Students should realize that the control plate will help confirm that the nutrient agar, cotton swab, and water used for moistening are not contaminated, and do not cause any microbe growth that could confound the outcomes of the experiment.

All inoculated Petri dishes should be sealed with masking tape and, for safety, not be opened again. 

Collect all inoculated Petri dishes and store them upside down (agar-side up) in a warm place (37°C).