Neogen® Petrifilm® 101 with Neogen® Technical Services

April 10, 2023

Learn tricks for Petrifilm from our technical experts and find answers to common questions in this Q&A style blog.

Jump to your Petrifilm question:

 

Q: What happens if I cannot read the plates right at the end of the incubation window? What options do I have?

A: If users cannot read the Petrifilm plates within one hour from the removal of the incubator, then they have the option to place them in a plastic container and freeze them for up to seven days for later interpretation. This allows for more flexibility for the time needed to interpret the results.

Q: Do I need to pH adjust my samples before plating onto Petrifilm? How do I complete this?

A: Yes, it’s important to verify that the pH of your sample is within the pH range listed for the plate. The range for each plate type can be found in the product instructions. There are a few reasons why the pH is important for testing.

The first is that most of the organisms the plates are trying to recover prefer a neutral pH range to grow. If the pH is too low or too high, they may not recover within the validated incubation conditions because it’s too stressful for them.

Additionally, there are indicator dyes in the Petrifilm plate that can be tripped by the pH of the sample, which could impact ease of interpretation. You should be testing the pH of the diluted sample and not just the sample itself. For example, if you are testing a juice that has a pH of 5 but after performing a 1:10 dilution in Butterfield’s buffer, the pH is now 7, no further pH adjustment is needed. However, if it is outside of the pH range for the plate, then the diluted sample should be adjusted using a HCl (if more acidic) or NaOH (if more basic) to be within the pH window. Typically, 1N solutions are used but if more than 1 mL of HCl or NaOH is being added to adjust the pH of the sample, then users can try using 5N or 10N instead.

The newer Rapid Petrifilm Plates do have some advanced buffer capabilities compared to the standard plates so this could help eliminate some pH adjusting steps.

Q:What spreader should be used with the plates?

A: There are five different spreaders that we offer depending on the plate type being used. It’s important to use the correct spreader to make sure the sample distributes evenly throughout the inoculation area. The type of spreader being used will depend on if the plate has a foam barrier and the size of the inoculation area. 

Q: What are the proper storage conditions for opened and unopened pouches?

A: The storage requirements for the Petrifilm plates will depend on if the foil pouch has been opened or not. Unopened Petrifilm plates should be stored at refrigerated or frozen temperatures for long term performance and shelf-life. Once users have opened the foil pouch, they no longer want to store them in fridge due to condensation concerns. For opened pouches, users can place them back in the foil pouch and store them at room temperature for up to four weeks. Additionally, some Petrifilm plates do allow for the opened pouches to be stored in a plastic container and placed in the freezer, and they are good for the remainder of their shelf-life.

Q: What dilutions should I plate for my samples?

A: There are a few factors that will determine what dilutions should be plated on the Petrifilm for different samples:

  • The flora in the sample: Depending on the amount of organisms present, the sample needs to be diluted so that it is within the countable range of the Petrifilm plates

  • The method being used: Some methods require specific dilutions for the sample types

  • Regulatory requirements: There are some industries that have specific dilutions that are suggested based on the sample type. For example, dairy users typically follow dilutions called out in the Standard Method for Examination of Dairy Products

  • Properties of the sample: Some samples have some inhibitory components that need to be diluted out to get better recovery of the organisms present. For example, spice samples typically need to be diluted further out to help reduce the inhibitory components present

The most common dilution to start with for food testing is a 1:10 dilution, and then perform serial dilutions after that if further dilutions are needed for reasons outlined above.

Q: What is the dilution factor when testing a sponge or swab?

A: The dilution factor for a sponge/swab sample will be the total amount of media used to hydrate the sponge. For example, if you are testing a sponge that contains 10 mL of media inside, the dilution factor is a 1:10. So, any counts on the plate would be multiplied by ten to report calculated counts. So, if there are 50 cells on a swab or sponge and 10 mL is added, the 50 cells are diluted 10 X to create a concentration of five cells per mL.

Q: What happens if I deviate from the validated incubation conditions? (What happens if my incubator fluctuates on temperature during incubation? For example, my power goes out for a few hours. Are the plates inside the incubator still okay?)

A: If the temperature the plates are being incubated at strays from our validated temperatures, you run the risk of having inaccurate counts. If the temperature or time fluctuates beyond the validated range, you run the risk of having your target organisms grow at a reduced rate and you also run the risk of having non-target organisms break through. This can cause an over- or underestimation of the overall microbial count and can interfere with the accuracy of your results. The safest thing to do would be to retest in these types of situations.

Q: Is it okay if some of the sample wicks onto the foam barrier?

A: Yes! This is a totally normal part of using Petrifilm plates. This spreading beyond the foam barrier is a feature that is intrinsic to the design and formulation of Petrifilm plates and is even demonstrated in our interpretation guides for our plates. Each lot is tested by our manufacturing and quality assurance teams to ensure that samples do not spread beyond specifications so you can feel confident that if a little bit of sample wicks onto the foam barrier, it will not impact the accuracy of your results. It is important to apply enough pressure on the spreader to evenly distribute the sample throughout the entire inoculation window, but also not press too hard where the sample is spilling out—but there will always be a normal amount of wicking present.

Q: How high can I stack the plates inside the incubator?

A: This depends on the plate type, but there are specified stack height limits in each of the instructions for use documents for each plate under the incubation section. For example, the Aerobic Count Petrifilm plates have a maximum plate stack height of 20 plates. This can vary by plate type so make sure you verify you are not exceeding the stack height for the plates you are utilizing. View specific Petrifilm plate instructions.

Q: How to prevent air bubbles getting trapped in plates with foam barrier? What are the plating techniques if I am plating onto Petrifilm with and without a barrier?

A: To prevent gas bubbles getting trapped in Petrifilm plates it is important that you are plating your samples correctly. When using Petrifilm plates with a foam barrier like our E. coli Coliform Count Petrifilm Plates, you will want to lift the top film of the Petrifilm plate, pipette 1 mL of your sample directly to the middle of the plating area, then slowly roll down the top film and use the appropriate spreader to encourage the sample to fill the area. This chases out gas bubbles that may have been left by pipetting technique. When using Petrifilm plates without a foam barrier, such as our Aerobic Count Petrifilm, you will follow the same procedure listed above, except you will drop the top film instead of rolling it back down to help contain the sample within an area to spread it more easily.

Q: How do I dispose of the plates?

A: Petrifilm plates are designed to encourage the growth of microorganisms, some of these organisms could potentially cause a biological safety risk if not handled appropriately. When you are done with a Petrifilm test, it is recommended that you dispose of the waste as a potential biohazard, using one of the following two methods. First would be utilizing an autoclave using a general protocol of 121°C/205°F and 15 pounds per square inch (psi) for 30 minutes. The other option is to contract a professional disposal service to collect and dispose of your biological waste offsite. Additionally, its important to make sure you are compliant with your local waste management guidelines on disposal for biohazardous waste.

Q: Can I perform air testing using Petrifilm plates?

A: Yes, it is very easy to use the Petrifilm plates for air testing! You simply hydrate the plate with an appropriate sterile diluent and then let the plates solidify for at least 1 hour before use. Then, peel open the plate and expose it to the needed air for typically 15 minutes. Then, seal up the plate and incubate per the validated temperature/time for the plate type you are using. See our post on Optimizing Environmental Monitoring Programs.

 

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About the Authors

 

Taylor Lecy — Food Safety Technical Service Representative, Neogen®

Taylor Lecy is a field technical sales representative for Neogen with a focus on food safety products. Taylor has over seven years of experience in the food industry. She uses her expertise in microbiology, laboratory techniques, and food safety to assist customers with test method implementation. She has a deep understanding of creating integrated food testing programs and increasing lab efficiencies. She formerly spent two years working in research and development on the Neogen® Molecular Detection System and now helps train customers on the platform, as well as other products. She received a Bachelor of Science in Biology from the University of Wisconsin, Madison.

 

Email: tlecy@neogen.com

Phone: 1.800.328.6553 — ask for food safety

LinkedIn: Taylor Lecy

 

Grant Hedblom, Ph.D. — U.S. Field Technical Sales Representative, Neogen®

Grant Hedblom is a field technical sales representative for Neogen with a focus on food safety products and Petrifilm Plates. 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 B.S. in Biology and a Ph.D. in Food Science from the University of Minnesota, Twin Cities.

 

Email: ghedblom@neogen.com

Phone: 612.642.2656

LinkedIn: Grant Hedblom

 


Category: Food Safety, Food & Beverage, Microbiology, Pathogens