Cherney Blog: Inside The Petridish

Your blog resource for education, experience, and a good read!

Cherney Blog: Inside The Petridish

Your blog resource for education, experience, and a good read!

Do You Follow The Rules: Plate Counting

Posted by Matthew Fenske on Apr 30, 2020 11:49:49 AM

As I scroll through Facebook I occasionally see posts challenging the reader to find the correct answer to a relatively simple mathematical equation. While we can look at it and say there must be only one correct answer, there are typically a variety of different answers in the comments below. Are those people responding unable to add, subtract, multiply, and divide? Possibly, but more commonly they forget simple rules called the order of operations. Simply put conduct the math functions in order parentheses, exponents, multiply, divide, add, and then subtract.

This sample principle holds true in microbiology when conducting a plate count. Your team must follow one set of rules. Do you round at each dilution or is that your final step? How many significant figures should your result have? What is your countable range? How do you handle two dilutions both within a countable range or if none of your dilutions are?

Don’t worry if you do not know where to start when developing your internal rules as there is many resources such as the Compendium of Methods for the Microbiological Examination of Foods, Standard Methods for the Examination of Dairy Products, product suppliers, or a trusted lab with established rules.


There are many benefits to having your rules well established.

  • You will quickly remove sources of error in your process giving you a more consistent result each time.
  • Your team will all be on the same page making it even easier to help one another and develop your staff.
  • Consistent proficiency results within your team and the other participants to further satisfy accreditation, regulatory and customer requirements.
  • And most importantly have the results to ensure your team is providing the world with safe products.

Finally from our staff to yours, happy counting!

Topics: Microbiology, Plate Counts, Lab Rules