How to Use a Microscope to See Bacteria

June 1, 2023

Bacteria are incredibly tiny, and they can be quite hard to see, even with a microscope. However, with the right magnification and a bit of patience you can easily see bacteria.

Most home microscopes are bright field microscopes, and while these can be used to find sections of specimens that have been fixed and stained (such as a tissue section) they are not very good at finding living, unpigmented microbes such as bacteria or protists. These types of organisms do not produce sufficient contrast to allow them to be located in a bright field image. Instead, these microbes rely on their natural pigmentation or stains to absorb light differentially and become visible to the microscope viewer through a lens beneath the microscope stage called the condenser, then the objective lens and finally the ocular eyepiece.

To see these organisms you will need a high magnification objective and a technique that uses phase contrast to increase the contrast of your image. The first step in preparing to view bacteria is to ensure that you have a sample to work with. Prepare a slide with a bacterial culture or a fecal smear, and choose a suitable magnification for your specimen.

Begin with the lowest magnification, and focus on a moderately dense area of the smear. When you have a location that has a reasonable number of bacteria, apply a drop of immersion oil to the microscope objectives. Be sure to wipe off any excess oil and clean the objectives after each use, as a buildup of oil can gum up other parts of your microscope.


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