When it comes to colony identifying bacteria on agar plates, and anything else, for that matter, it seems pictures are worth a thousand words. Still, it’s nice to have some words to describe what you’re seeing. After all, there is a wide range of language to use when identifying and describing bacteria colonies and their morphology.
Also, check out our Complete Guide (19 pages) on How to Count Colonies on Agar Plates!
Collecting Bacteria on Agar Plates
The first and perhaps most important step when it comes to identifying and describing colony forming units is to ensure you collect your samples and inoculate the Petri dish correctly.
From a sample in a vial, transfer one part of the sample to a separate vial and add 10 parts of a dilutant, usually saline or filtered water. This dilution will allow you to observe the colony forming units, or CFUs, properly. It is typically advised to dilute the diluted sample at least one more time at a 1:10 ratio. Be sure to place the lid on the vial and shake well so you thoroughly mix the sample with the water.
Once you have a nicely diluted sample, be sure to take note of how much you diluted it so you can scale up in future calculations.
Now, take a cotton swab and collect some of the diluted sample. Then, swipe the sample onto your agar plate in a tight, zigzagging motion to be sure to cover the entire surface. Turn the plate 90 degrees and repeat this process, ensuring you swipe fully, from edge to edge of the Petri dish.
Next, you’ll place the lid on the Petri dish and incubate the plate, upside down, according to the directions for your particular bacteria.
After a period of, usually, 24 to 36 hours, you can remove the plate and begin to count and identify the CFUs.
Identifying Bacteria on Agar Plates
When it comes to counting the CFUs, you should only be using the naked eye. This ensures you don’t miscount or try to identify microscopic details that are not actually CFUs.
Alternatively, you could invest in an automated cell counter to avoid the likely potential for human error.
Track the number of CFUs you can identify and then move on to describing what you see.
Categories for Descriptions
When identifying colonies of bacteria on agar plates, you can use the naked eye or a microscope. There is no reason not to get a closer look if you can. Here are the factors you’ll be looking for.
Size can often be determined using a small ruler that has millimeters on it. You can also use the above-mentioned automated cell counter, which will take accurate measurements for you from an uploaded image of the agar plate. You’ll want to note the size of each CFU in millimeters, from edge to edge, across the middle.
Color descriptions are relatively straightforward, and you can usually use obvious color descriptions like red, black, white, etc.
Form descriptions will reveal the shape of the CFU. You’ll typically use language like circular, boxy, and irregular.
You can turn the Petri dish to the side to get a side angle of the CFU and determine its shape and size as it rises. You’ll use words like flat, convex, or raised, for example.
The surface of a CFU will usually be shiny, moist, wrinkled, or matte.
When it’s safe, and using all precautions, you can use a sterile tool to manipulate the surface of the CFU and see how it reacts. You should be able to tell if it is sticky, gooey, dry, brittle, or mucoid.
Describing the edges involves exactly what it sounds like. You’ll view the edges for charateristics like undulating, ridged, curled, or entire.
Opacity will tell you if the CFU is translucent or clear, iridescent or solid, etc.
Only when it’s safe, you can take a whiff of the CFUs from a distance and see if you pick up any interesting notes like putrid, grape, sweet, etc.
Ultimately, the purpose of taking notes is to get them as accurate as you can so you can watch the progress of a particular bacterial colony forming unit.
Of course, this work can be done on more than just potentially harmful bacteria.
In the scientific community, revolutionary work is being done to understand good bacteria as well, which has been shown in recent years to help combat harmful bacteria in the body.
Furthermore, breweries, wineries, and others in the fermentation industries are doing groundbreaking work to understand yeast and its role in creating the delicious, fermented foods and beverages we all enjoy.
The key to all identification and description of bacterial and yeast colonies is to better understand these ancient life forms that predate human existence by millions of years.
Because the better we understand these colonies, the better we understand ourselves, and the better work we can do to coexist and grow.
As for those promised colony identifying bacteria on agar plates pictures, here are a couple of CFU counts done by the Oculyze automated colony counting system:
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