What is a Wet Mount?

Have you ever walked by a greenish-brown pond or lake and wondered what lives there? Creating a type of slide called a wet mount, and viewing it under the microscope, is one way to find out! Making a wet mount is rather simple: clean off your microscope slide, add a few drops of liquid mixed with bacteria (such as a prepared bacterial broth, a sample of pond water, or bacteria from a solid plate mixed with water), and drop a cover slip (a smaller piece of glass or plastic) over your sample on your slide. With a wet mount, you do not heat fix your bacteria or add any stain to your bacteria. Why do you think that is useful? Well, avoiding heat fixing and staining allows you to keep the bacteria alive, so you can watch them move around!

When you've made your slide, you'll place it on a microscope, and typically use phase contrast. That is a type of microscopy that increases the contrast between the unstained bacteria and the background. It might be difficult to find the bacteria because they're translucent, but once you get them into focus you should see them moving around. They might look like they're randomly dancing in place, or flowing in one direction as if they're being swept down a river. They might also appear to move independently, on their own paths; for example, a bacterium could move left across the slide, stop, and move to the right. If your bacteria move independently, that means that they are motile, or able to move of their own accord!

In a medical diagnostics lab, you can create a wet mount out of certain bodily fluids to look for organisms that may be causing disease in your patients.