Microbiological surface testing with RODAC Plates: A step-by-step guide for hygiene monitoring
In this blog:
During our training course “Microbiology in Practice,” we combine theory and hands-on practice. Participants learn how to perform RODAC plate sampling themselves and how to assess incubated RODAC plates, as well as more about the rationale behind hygiene and various hygiene measures.
In some cases, a clear work instruction is sufficient. That’s why in this blog we explain step-by-step how you can use RODAC plates to monitor the microbiological condition of your products or surfaces. This is a general approach that may require adjustments in detail to fit your specific situation.
What you need: Sterile RODAC plates with appropriate culture media
Make sure to use sterile RODAC plates filled with the desired culture media, for example Plate Count Agar (PCA) for total viable counts, Violet Red Bile Glucose Agar (VRBGA) for Enterobacteriaceae, or Baird Parker Agar (BPA) for Staphylococcus aureus. The latter two selective media allow targeted detection of relevant microorganisms.
Store the RODAC plates according to the manufacturer’s instructions, for the types above we generally advise a clean environment at a constant temperature between 12 and 20°C. Do not use plates beyond their expiration date, as this compromises validity.
Preparing your sampling plan and hygiene monitoring strategy
First, determine which surfaces you want to test for hygiene in a hygiene monitoring plan. Based on this, create a clear form that assigns a unique identifier to each sampling point. We often use numbers as identification. It is important that the surfaces to be sampled are flat or, in the case of textiles, placed flat on a firm surface.
Working aseptically to prevent contamination
From this point onward, ensure you work hygienically. This means maintaining a clean workspace, performing hand disinfection, and preferably wearing a clean lab coat.
Remove the RODAC plates from their packaging using clean, disinfected hands. You can also use sterile gloves if preferred. When opening the packaging, note that the lid should be loosely placed on the dish. Never touch the culture media or the inside of the lid with your hands, even when wearing sterile gloves. Once unpacked, label each RODAC plate with a unique identifier.
Correct Application of RODAC plates for surface sampling
Before taking a sample, carefully remove the lid of the RODAC plate with one hand. Hold the lid with the open side facing down during sampling to prevent dust particles from settling inside. From this moment, avoid coughing, talking, or deep breathing to prevent contaminating the surface or culture media.
When sampling the RODAC plate, press the culture media onto the surface with approximately 500 grams of pressure for 10 seconds. While tools to assist with this are available, they are not required. After 10 seconds, replace the lid carefully, taking care not to touch the surface of the media. If your RODAC plates have a twist-lock lid, secure it by twisting so the lid cannot fall off.
If you plan to test the same surface with several types of culture media, make sure subsequent samples are done close to – but never overlapping – the previous ones.
After sampling, clean the surface because a small amount of agar medium may have been left behind.
Incubation of RODAC plates: A prerequisite for accurate assessment
Immediately after sampling, place the RODAC plates in an incubator. Plates may be stacked, but only group plates that require incubation at the same temperature.
A digital incubator allows easy setting of the required temperature.
The optimal incubation time and temperature for your RODAC plates can usually be obtained from your supplier. For commonly used media such as TSA, PCA, VRBGA, and BPA (with which we frequently work), the standard incubation is 48 (±2) hours at 37 (±1) °C.
Interpreting and recording Colony Forming Units (CFU)
After incubation, assess the plates for colony growth by counting the number of colonies (CFU = Colony Forming Units) per plate. Record your counts on the form prepared earlier. If you are looking for specific bacteria, it is important to recognize their appearance, as selective media may also support growth of other microorganisms that are not relevant in your case.
Would you like to learn more about performing hygiene monitoring and controlling hygiene within your organization? Register for our practical training ‘Microbiology in Practice’ and learn how to effectively safeguard hygiene and food safety within your organization using RODAC plates.
If, despite these instructions, you would prefer us to carry out the hygiene checks on your site in The Netherlands or Belgium, please feel free to contact us to discuss the options.
