It can be challenging to understand colony forming units calculation, considering the work that goes into collecting, diluting, and then counting the CFUs. For this reason, it helps to follow a simple path from collection to identification and calculation. That’s the purpose of this article.
You can also get access For Free to our comprehensive How to Count Colonies on Agar Plates PDF Guide (19 pages) put together by our specialists, Dr. Katja Schulze, who specialized in bioinformatics and image recognition of microscopic images, and Dr. Ulrich M. Tillich, who specialized in laboratory automation!
How Bacteria and Yeast Are Collected
First, it helps to understand how bacteria and yeast are collected.
To collect bacteria and yeast, scientists select from various sources – from slices of bread, fruits and vegetables, to dead animals, water, and the human body.
The samples should be selected with particular emphasis on isolating the sample only in the most hygienic conditions possible. Be sure to wear sanitary gloves and choose a sanitary vessel to bring the sample back to your lab. Then, refrigerate the sample until you’re ready to swab the sample onto an agar plate.
Why and How Bacteria and Yeast Are Diluted
To example the sample, you’ll have to dilute it, a step that is commonly missed. Dilution is necessary because without it, the bacteria will likely overrun the Petri dish so much that you won’t be able to differentiate between CFUs.
The sample should be diluted at least twice, and more if necessary. You’ll know you need to dilute further if there are so many CFUs on the agar that they are touching.
To dilute your sample, you’ll take one part of the sample to 10 parts of saline or filtered water. Shake the diluted sample gently and dilute the sample again in the same way.
Be sure to track your dilutions so you know how to factor back up when you’re performing your count.
Once you’ve diluted the sample at least twice, you’ll take a cotton swab and collect some of the final dilution. Swipe that sample in a tight zigzag pattern so that it reaches from edge to edge of the plate.
Then, turn the plate 90 degrees, collect another sample, and swipe again in the same manner. This process will ensure the entire agar plate is covered over with bacterial sample.
Now, you can place the lid on your Petri dish and turn the plate upside down.
Incubate your bacteria as required by your specific strain, usually between 18 and 36 hours.
Once the incubation period has passed, you can calculate your CFUs.
Colony Forming Units Calculation
To calculate colony forming units, you’ll look at the underside of the Petri dish. You can count the number of CFUs you can identify with your naked eye. Then, you’ll need to scale that number up according to how much you diluted it.
If you diluted your sample by a factor of 10, for example, you’ll need to multiply by a factor of 10. If you diluted twice, you’ll need to multiply by a factor of 100.
So, for example. If you count 22 CFUs on your agar, and you’ve diluted the sample twice as described above, you’ll multiply 22 by 100, to get 2,200 CFUs in your original sample.
The calculation would go as follows:
(number of CFUs found on agar) x (factor of dilution) = expected CFUs in total bacterial sample
Oculyze Can Help
In the end, you can cut time, energy, and money by allowing an app to count your CFUs for you and do the calculations to factor back up. This approach also allows you to trust human error has been eliminated as long as you use a reputable provider.
Oculyze has come onto the market to offer a more intelligently designed and extremely accurate colony counting software. We’ve partnered up with agar plates producers to provide the simplest process and solution for lab people.
Once you’ve swabbed your culture and you’re ready to begin counting, you simply have to:
- Scan the QR code that came with the pre-poured plate or the dehydrated agar.
- Immediately, the Oculyze Web App (no installation required) gets loaded with the image recognition tailored to the specific type of plate being analyzed.
- Take an image with your smartphone, tablet, or desktop PC with an attached camera. The results are shown in less than a second.
You can read more about this on our Colony Counter page. And next time you talk to your agar plates distributor, ask them about the possibility of integrating this service.
You can also Test our Colony Counter completely free of charge and with no commitment to purchase.
Just please note that the recognition provided here is solely for demonstration purposes and may not accurately represent the performance of our product. Our customers receive customized recognitions tailored to their specific needs, which ensures high levels of accuracy for their plates.