Have questions about interpreting results on 3M™ Petrifilm™ Plates? We have answers. (Part 3 of 5)

Are you seeing something you didn’t expect to see on your plates? Here are some questions asked by food safety labs that have been answered by 3M global and U.S. technical specialists. Additional information is available for you as well on both the 3M™ Petrifilm™ Rapid Plates Portfolio as well as the 3M™ Petrifilm™ Standard Plate Portfolio.

Question: On the lactic acid plate, sometimes the colonies are hard to count because they fuse with each other or does not form in a perfect round shape. How would you interpret that?

Answer: We suggest that if the colonies do not form a perfect round shape but seem to connect and be part of the same colony, count as 1 CFU. If there is clear, distinct separation between two or more red spots, count each as individual CFU. We can provide examples of what you are experiencing and help provide some training.

Question: What is the reason for the different color species on the 3M™ Petrifilm™ Rapid Aerobic Count Plate?

Answer: The 3M Petrifilm Rapid Aerobic Count Plates have a dual indicator system. The technology in these multiple indicators allow us to visualize colonies faster than traditional methods. The different colors do not provide any information to differentiate between different microorganism types.

Question: What is the reason for the 3M™ Petrifilm™ Lactic Acid Bacteria Count Plates being hard to count? It usually turns brown when left for over three days and colonies are hard to see or it has so much gas the colonies are not clear?

Answer: If there is an excess of gas, an additional dilution might help. If the plate is turning brown, it could be because the incubation temperature is very high and the plates have been incubated too long as the incubation time is two days. We’d be happy to help if you can send us pictures or set up some virtual or on-site training.

Question: Do all red violet colonies on the 3M™ Petrifilm™ Staph Express Count Plate cause coagulase reactions? If not, why?

Answer: 3M Petrifilm Staph Express Count Plates have been validated comparing to the reference method including confirmation (coagulase test). Our validations demonstrate that red-violet colony counts are equivalent to the results of the reference method + coagulase test. As with any microbiological methods there is the possibility of exceptions. In the case of the 3M Petrifilm Staph Express Count Plate, a red-violet colony that doesn’t confirm via the coagulase method could be a coagulase negative S. aureus or it could be a phenotypically similar/related organism that has picked up the same enzyme used to react with the red-violet indicator in the 3M Petrifilm Staph Express Count Plate.

The 3M™ Petrifilm™ Staph Express Confirmation Disk looks for the presence of DNase. There is a high correlation of coagulase positive staphylococci that also carry DNase. If there is ever any doubt about a red-violet colony, you could use the 3M Petrifilm Staph Express Disk for DNase detection.

Question: Can you explain what to do if a product turns a 3M™ Petrifilm™ Enterobacteriaceae Count Plate yellow?

Answer: If a product turns a 3M Petrifilm Enterobacteriaceae Count Plate yellow, especially at the time the product is plated, it is most likely related to the pH of the sample. Check the pH of the prepared sample and adjust to ensure it falls within the recommended range of 6.5-7.5. If the plate is yellow after incubation it could be related to pH or it could be an indication of too numerous to count. To find out which, dilute the sample further. If there are no colonies or the plate is yellow after the sample is diluted the issue is related to pH.

Question: What should I do when a plate is out of the countable range, but the sample is no longer available to do further testing?

Answer: If you are able, you can estimate by counting the number of colonies in one or more representative squares, take the average of the counts, and multiple by the area of the 3M Petrifilm Plate you are using. If you are unable to estimate the count because there are too many colonies, please refer to FDA Bacteriological Analytical Manual (BAM), Chapter 3 Aerobic Plate Count, section D. Computing and recording counts. Here you will find guidance on how to report counts that cannot be estimated and are too numerous to count.

Question: Does the 3M™ Petrifilm™ Plate Reader automatically estimate plates that are over the plate limit?

Answer: Yes, the 3M Petrifilm Plate Reader will estimate counts over the countable range until its limit is reached (~999 colonies).

Question: What causes the different colors of mold?

Answer: Mold colony color may be different among mold colonies as molds naturally produce a variety of pigments. Often the mold color you see on 3M Petrifilm Plates is their natural color.

Want more information on how we can help with total 3M Food Safety testing solutions? Simply reach out and we’ll be happy to help.