How to Read Bacteria Types on Fruit with a Swab on Nutrient Agar

Always wear gloves when working with bacteria.

Bacteria are found everywhere and are easy to collect and grow. The most common method of growing bacteria is in petri dishes containing agar. Agar contains nutrients that allow microorganisms, such as bacteria, to grow in large numbers. This helps the scientist or student view the number and type of cultures on the agar plate.

Things You'll Need

• Protective gloves
• Fruit
• Petri dishes partially filled with agar
• Bunsen burner
• Inoculating needle
• Incubator
• Bacteria identification chart or book
• Glass slides
• Distilled water
• Slide rack
• Plastic tub container
• Crystal violet stain
• 95% ethyl alcohol
• Compound microscope

Instructions

1. Put on your protective laboratory gloves. Get the inoculating loop and the piece of fruit.

2. Turn on the Bunsen burner and put the inoculating loop over the flame. Make sure the loop turns red to kill any bacteria already existing on the loop.

3. Take the inoculating loop and rub it against the fruit. Immediately open a fresh petri dish containing agar and streak the agar with the loop. After streaking, place the loop over the flame again to remove any bacteria. Replace the lid on the petri dish.

4. Repeat this steps for two additional agar plates. You want to have more than one plate to compare your results.

5. Place the agar plates in a 37 degree (Celsius) incubator for 48 hours.

6. Remove the plates from the incubator and note the number, color and shape of colonies that formed on the agar plates. Refer to a bacteria identification chart or book to help establish type of colony.

7. Take the inoculating loop, put it over the Bunsen burner flame, and swipe it across one of the colonies on your agar plates. Spread the loop across a glass slide. Add a drop of distilled water to the slide and let the slide dry. Put the slide briefly over the Bunsen burner flame to fix the sample.

8. Put the slide on the slide rack inside the plastic tub container and stain the slides with the gram stain for one minute. Gram staining requires placing drops of the stain over the slide. The tub container will catch any stain that falls off of the slide. Staining will make it easier to view the bacteria and you will be able to determine if they are gram positive or gram negative bacteria.

9. Wash slide with distilled water. Wash the slide with 95% ethyl alcohol to remove the color from the bacteria with thin cell walls. Rewash with distilled water.

10. View the slide under the compound microscope. Gram positive bacteria will be dark purple with thick cell walls. Gram negative bacteria will be light purple with thin cell walls. Make sure you note the shapes and numbers of bacteria as well.

References

• Science Company: Bacteria Growing Experiments in Petri Plates
• Science Projects: Determining the Dirtiness of Coins
• Clinton Community College:Prokaryotes