Composting In Schools

Preparing Slides of Microorganisms


Use sterile techniques to prepare your slide! Use an inoculating needle to add a drop of saline to a clean slide. Take a sample of a single bacterial colony and mix it into the saline. Let air dry until a white film appears. Heat fix by passing the slide through a flame a few times.


Follow the above procedure but try to get a portion of the colony on the slide intact. You can try lifting it with a sterile scalpel. It will be too crowded to observe on most of the slide, but at the edges of the colony you will be able to see the pattern that the filaments form.

Lift a portion of the colony intact onto a clean slide (it will still be attached to the agar), add a cover slip and observe without staining. Look at the edges of the colony where the sample will be thinner and there will be enough light to observe.

Staining Slides

Gram staining can be used to make slides of bacteria and actinomycetes:

Preparation of Gram Stains

Crystal Violet:

Dissolve 2 g of crystal violet in 20 ml of 95% ethanol. Add this solution to 80 ml of a 1% Ammonium Oxalate solution. Let stand for 24 hours and filter.

Gram Iodine:

Add 1 g Iodine and 3 g Potassium Iodide to 300 ml distilled water. Store in an amber bottle.

Decolorizer: 95% Ethyl Alcohol


Add 2.5 g safranin to 10 ml 95% ethanol. Add this solution to 100 ml distilled water

Procedure for Gram Staining

1. Flood slide with crystal violet - 20 sec.

2. Wash with distilled water - 2 sec.

3. Flood slide with Gram iodine - 1 min.

4. Decolorize by tilting slide and drop by drop rinsing with 95% ethanol until

ethanol runs clear - about 10 to 20 sec.

5. Wash with distilled water - 2 sec.

6. Flood with safranin - 20 sec.

7. Wash with distilled water - 2 sec.

8. Blot dry.

These protocols were written by Elaina Olynciw, biology teacher at A. Philip Randolph High School, New York City, while working in the laboratory of Dr. Eric Nelson at Cornell University as part of the Teacher Institute of Environmental Sciences.


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