Are you wondering about the best way to plate your sample or how much time to take between steps? 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: What is the best method for opening a plate and placing a confirmation disk without damaging the sample/gel?
Answer: Generally speaking, slowly opening the plate works the best to keep the gel intact. Additionally, there is no need to fret if the gel does split. The 3M™ Petrifilm™ Staph Express Confirmation Disk has indicators coated on both sides to show a reaction regardless of if the gel is on the top film or the bottom film.
Question: What is the proper way to plate a lactic acid sample on a 3M Petrifilm Plate? We’re having issues with it not spreading to the edges, even when using the spreader.
Answer: The best way to plate samples onto the 3M™ Petrifilm™ Lactic Acid Bacteria Count Plate is to use the top film to help distribute the samples as you roll down the top film and quickly place even, firm pressure on the 3M™ Petrifilm™ Flat Spreader immediately after inoculating the plate. We have found that using your thumb with 3M Petrifilm Flat Spreader seems to work better than other fingers or your hand/palm. You can also refer to our how-to videos at www.3M.com/petrifilm. Watch the “How to Use 3M Petrifilm Plates” video to see the 3M Petrifilm Lactic Acid Bacteria Count Plate plating in action.
Question: What is the maximum time lapse allowed from dilution to plating?
Answer: According to FDA Bacteriological Analytical Manual (BAM), Chapter 3, Aerobic Plate Count guidance is to plate within 15 minutes of preparing the original dilution.
Question: What can be done to enhance the blue disk reaction for the staph plates?
Answer: This can be dependent on a variety of factors. Generally speaking, the pink zones can be enhanced by incubating the full three hours. Additionally, ensure that the proper diluent is used.
Question: Can you use expired media if it is unopened?
Answer: We do not recommend using 3M Petrifilm Plates past its expiration, even if unopened. Our 3M Petrifilm Plates have been tested through shelf-life studies to demonstrate no loss in performance at the beginning and end of the shelf life. After the expiration date, we cannot guarantee that the performance will be as expected.
Question: I have a package of plates that are not expired, but it’s been open for a long time. How does that affect the results?
Answer: If the opened pouch has been stored at room temperature, we can only guarantee that the product will perform as expected for four weeks. If the plates were stored in a freezer, the product is still good for four weeks or until the expiration date, depending upon the product type. When stored at room temperature, like all media, the nutrients and selective agents will begin to degrade and lose effectiveness.
Question: Should a sample be shaken once it has been inoculated into the peptone water?
Answer: Yes, it is important to mix any sample properly in the diluent prior to planting to ensure a homogenous distribution of the bacteria. For more information, you can refer to FDA BAM, Chapter 1, Food Sampling/Preparation of Sample Homogenate.
Question: How would you count a sample of 25 g in a 25 ml buffer and plate 1 ml?
Answer: Adding 25g of sample into 25 ml of buffer would be considered a 1:2 dilution. Following the following calculation, the number of colonies on the plate after plating 1 ml would be multiplied by 2 (the dilution factor).
(CFU x dilution factor)/mL plated = CFU/g reported.
For more information, please refer to the 3M Petrifilm Plate Guide to Dilution Preparation.
Question: When reading plates used for air samples, how are the results recorded?
Answer: Results for air samples are recorded as the number of CFU/2x the 3M Petrifilm Plate area. For example, for 3M™ Petrifilm™ Aerobic Count Plate the results are CFU/40 cm2.
Question: Can you filter 100 ml of water through a filter and put it on a standard l 3M Petrifilm Plate instead of using the 3M™ Petrifilm™ Aqua Plates?
Answer: The standard 3M Petrifilm Plate products have not been validated for use with filtered water with the exception of the 3M™ Petrifilm™ Enterobacteriaceae Count Plate which can be used for bottled water testing.
Question: For rapid Aerobic plate count, how would slow-growing bacteria be captured?
Answer: Generally speaking the dual indicator system technology and formulation is used to visualize slow-growing bacteria faster than the reference method. Naturally, no method is perfect. For example, dry products require 48 hours of incubation on the 3M™ Petrifilm™ Rapid Aerobic Count Plate or the temperature may need to be adjusted to the optimal temperature for those organisms that grow slowly.
Question: I am working with Lactic Acid Bacteria count film. If I don’t get any colony formation but I get a high gas formation, can I just dilute the sample further to get a more accurate count?
Answer: Yes, if there is high gas formation on the 3M Petrifilm Lactic Acid Bacteria Count Plate it is possible that you will not see colonies and should dilute the sample further for a more accurate count.
Question: What would be an appropriate spec for aerobic plate count in environmental air monitoring in a food packaging area?
The appropriate specification for aerobic plate count environmental air monitoring samples is dependent on many factors including proximity to the food/line, control measures in place (guard), consequences of elevated aerobic bacteria in the food being manufactured and whether the area is positioned before or after a kill or process control step.
As general guidance, you may consider taking samples and generating data to create a baseline level and acceptance criteria. For more information, we would suggest downloading a free copy of the Environmental Monitoring Handbook for the Food and Beverage Industries, Chapter 5, Environmental Monitoring for Spoilage Organisms.
Question: What is the difference between a gas-producing colony and a non-gas-producing colony for coliforms?
Answer: Coliforms are defined by their biochemical reaction to lactose. Depending on the method (ISO or FDA BAM) the definition may change. According to FDA BAM, coliforms ferment lactose sugar into acid and gas. Colonies that do not produce gas on the 3M™ Petrifilm™ Coliform Count Plate are most likely Gram negatives or Enterobacteriaceae that do not ferment the lactose to gas.
Question: Are there colonies that grow on the Aerobic Count plate that doesn’t take up the dye?
Answer: Yes. While this happens very rarely there are certain microorganisms that cannot utilize the indicator and appear colorless. However, if you look carefully you can see these colorless colonies which appear three-dimensional and create a clearing of the background bubbles.
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