Have questions about interpreting results on 3M™ Petrifilm™ Plates? We have answers. (Part 2 of 5)

Have you ever seen results on your plates that you aren’t sure how to interpret? Here are some questions asked by food safety labs that have been answered by our 3M U.S. and global 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: How does the film reader interpret too numerous to count (TNTC) count?

Answer: Counts above the countable range will be estimated by the 3M™ Petrifilm™ Plate Reader to the extent that it is able. Once the number of colonies exceeds the threshold of counting individual colonies (usually around 900 colonies) the unit will report a 999 (TNTC) value.

Question: How can you tell the difference between enzymatic hues from bacteria colonies on a 3M™ Petrifilm ™ Rapid Aerobic Count Plate?

Answer: On the 3M Petrifilm Rapid Aerobic Count Plate, usually an enzymatic reaction creates a uniform, even blue background. This appears different than TNTC in which the background is not uniform. If the food product, such as spices or granulated products, is causing an enzymatic reaction this will appear as intense, pinpoint blue specks.

Question: Are there any recommended methodologies for verifying the validity of results (i.e. control plates, indications of contamination)? This is specifically for use of 3M™ Petrifilm™ Aerobic Count Plate.

Answer: Each 3M™ Petrifilm™ Plate undergoes release testing after manufacture. To see the specific testing that each batch of 3M Petrifilm Plates undergoes, you can refer to the Certificate of Analysis and our Product Information Sheets which include the sampling plan, contamination testing criteria and performance specifications. It is up to each individual lab to determine if they would like to conduct their own additional quality control testing. The best resource that we are aware of on methodology is ISO 11133.

Question: How can we differentiate between total plate count with probiotic on aerobic film?

Answer: There is not an aerobic plate count method that can differentiate between probiotic counts and other organisms present in the sample. Some processors may try to utilize raw material or in-process testing to ensure that the product is of appropriate quality prior to adding the probiotics. Naturally, this is product and process specific.

Question: I am using the 3M™ Petrifilm™ Staph Express Plate and it has blue colonies. When I use the 3M™ Petrifilm™ Staph Express Confirmation Disk, there are pink zones without colonies. Are those still counted positive despite the blue previous colonies not having a pink zone?

Answer: If there is a pink zone without a colony, do not count the pink zones. Blue and green colonies are not S. aureus and also should not react with the 3M Petrifilm Staph Express Confirmation Disk.

Question: Why do the 3M Petrifilm Rapid Aerobic Count Plates not have a grid for easy counting?

Answer: 3M Petrifilm Rapid Aerobic Count Plates do have gridlines to facilitate easier counting. To see the gridlines, use a backlight from either a lightbox or a magnified light colony counter.

Question: The black colonies on the 3M Petrifilm Staph Express Count Plates can be considered S. aureus? If so, further confirmation is needed, correct?

Answer: Correct. Black colonies may be stressed Staphylococcus aureus or another coagulase positive staphylococci such as S. hyicus or S. intermedius. To check, the 3M Petrifilm Staph Express Disk should be used.

Question: Is the reader machine able to read all bacteria or only specific bacteria?

Answer: The 3M Petrifilm Plate Reader can read the 3M Petrifilm Aerobic Count Plate, 3M Petrifilm Rapid Aerobic Count Plate, 3M™ Petrifilm™ Coliform Count Plate, 3M™ Petrifilm™ E. coli/Coliform Count Plate, 3M™ Petrifilm™ Enterobacteriaceae Count Plate and 3M™ Petrifilm™ Select E. coli Count Plate (available only in Europe). The 3M Petrifilm Plate Reader will enumerate the colonies on the plate according to our package insert and interpretation guide.

Question: How do you calculate the final results if you have two dilutions that are in the countable range?

Answer: To calculate this result, refer to FDA Bacteriological Analytical Manual (BAM), Chapter 3, Aerobic Plate Count Section D. Computing and recording counts or ISO 7218. Here is an example: 150 colonies (C1) on 1:10 (D1) and 50 colonies (C2) on the 1:100 (D2). Use the following equation (C1+C2)/(D1+D2) where C=plate counts and D=dilution. The result would be (150+60)/(0.1+0.01)= 1909 CFU/g. If you have multiple plates for each dilution, take the average of the counts for each dilution first before using the equation.

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.