Petrifilm® Plate Technical Tips: A Review of Best Practices and Procedures
March 15, 2024

Recently, our Food Safety experts Annie Simmons and Grant Hedblom held a technical training on current procedures and best practices for Neogen Petrifilm Plate technology. Throughout the webinar, Annie and Grant answered top-of-mind topics, like important considerations for sample preparation, interpreting plates and calculating results in various scenarios, and the difference between Rapid and Standard Petrifilm Plates.
When the training concluded, they were able to address a few live questions to help participants further understand the process of validation and verification with Petrifilm Plates, using dilutions for environmental monitoring samples, and how to accurately report colonies outside of a Petrifilm’s recommended countable range.
What method can be used to validate or verify Petrifilm® Rapid Aerobic Count Plates in the lab?
We ensure our methods are accredited by third-party organizations such as ANOR, MicroVal, and AOAC® by comparing them to standards such as ISO and FDA BAM. A good place to start when adopting a new method would be to see if any of your products are covered under the existing validation scope. If that's the case, then you should perform a verification in-house to check if you are able to get comparable results and that the method works in your laboratory testing environment. After, you can compare it to the reference method or check to ensure that you are getting the expected recovery when using certified reference materials. If the products you are testing are not covered under that validation scope, you may want to perform a more extensive internal validation study. This study could involve comparing products to reference methods across several different conditions and representative versions of those products, like different lots and manufacturing runs, generating data to demonstrate equivalence. You start by validating that the method works with your product, but you must also verify that you can perform this method in your lab environment with your technicians and lab equipment. Essentially, it is a two-step process starting with validation to see that it works with the product then verification that you can perform the method in your own lab. Please see our webinar about method verification and validation for additional information.
Can you perform dilutions with environmental samples?
Yes, you can perform dilutions with environmental samples. We've seen users do this when they are plating a high volume of plates from one swab. Our quick swabs come with 1 mL of media, and it is not possible to plate several plate types with that 1 mL. In that situation, they'll add dilution media to that swab, then add any compatible and acceptable buffer listed on the "instructions for use" document for the plates. If you're collecting an environmental sample with Letheen broth, it does not necessaily mean you have to add a Letheen broth to the swab; for example, you can add Butterfield Phosphate buffer to the swab to give you a higher volume. It is important to calculate the volume added to the sponge or swab to report counts accurately. For example, if the swab contains 1 mL and I add 9 mL, that would be considered a 1 to 10 dilution and no longer a 1 to 1 dilution, so any counts that I see I would multiply by 10 because I now have a total volume of 10 mL. Otherwise, I would report less than ten if I saw no count on that Petrifilm.
If we're able to count colonies, but they're outside the recommended countable range, do we report as too numerous to count or the count itself?
Countable ranges exist to help you optimize the counting of the plates and are based on limits established in reference methods like the FDA BAM and ISO standards. These standards will recommend specific countable ranges to get accurate results and help with statistical analyses. Above the countable range for the plate, organisms can begin to compete for nutrients and resources, which can make interpretation challenging. For example, if you're working in the Petrifilm® Coliform Count Plate where you're looking for colonies with associated gas bubbles, if you have counts above that countable range — it becomes challenging to easily distinguish which colonies are producing gas or which colonies may not be producing gas. At high concentrations, organisms that would normally behave as a typical colony may be stressed when competing with the organisms present on the plate. When you are above that countable range, it is important to express that plate as too numerous to count or to perform an estimation calculation following the protocols outlined in the "instructions for use" document for each plate type.
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About the Speakers
Annie Simmons – Field Application Scientist, Neogen
Annie is a Field Application Scientist for Neogen® Petrifilm® Plates. With over 10 years of industry experience as a technical services representative, Annie uses her expertise in microbiology, quality assurance, and food safety to assist food processors with test method implementation and advanced food safety applications. She also works with industry regulators to help ensure best practices in method implementation. Annie holds a Bachelor of Science degree in Microbiology from Minnesota State University.
Grant Hedblom, PhD, Microbiology Product Specialist, Neogen
Grant is a Microbiology Product Specialist for Neogen with a focus on food safety products, Neogen® Petrifilm® Plates, and the Soleris® Next Generation System. Grant uses his experience in microbiology, food science education, and curriculum development to assist food processors with applied food safety and test method development and implementation. Grant combines his experience with customers and academic background to bridge the gaps between theory and practical applications of food safety concepts. He received a Bachelor of Science degree in Biology and a Ph.D. in Food Science from the University of Minnesota, Twin Cities.
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Category: Food Safety, Consumer Goods, Dietary Supplements, Food & Beverage, Pet Food, Environmental Monitoring