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How to Use Contact Plates and Settle Plates for Environmental Monitoring?

Surface monitoring using the contact plate method is ideal for assessing the effectiveness of sanitary procedures on flat, impervious surfaces. This method is ideal for taking samples from flat, impervious surfaces. We use RODAC (Replicate Organisms Detection and Counting) plates to maintain standards and obtain reliable samples. These plates are filled with general or specific microorganism.

Method for Taking Sample:

  • Acquire a surface sample by lightly pressing the RODAC plate against the surface of your choice.
  • Contact plates to assess the presence of bacteria containing TSA and contact plates to assess the presence of yeast and mold containing SDA.
  • If samples are taken immediately after disinfection, a suitable neutralizing agent can be added to the agar if residual disinfectants are suspected. Incubate the plate for the required time.
  • Bacteria require a two-day incubation at 37°C. Yeasts and fungi require incubation at 30°C for at least three days.
  • Colonies are then counted to determine the number of organisms on the surface.

Benefits of Contact Plates:

Contact plates are easy to use and the easiest sampling method. However, it is not suitable for uneven surfaces. The contact plate method may also show confluent growth of microorganisms when colonies are not discrete enough for simple counting. Contact agar plates, also known as Replicate Organism Detection and Counting (RODAC), require a great deal of skill to fill, but when you buy them pre-filled, they are unrivalled. The contact plate is a plastic dish filled with agar to give a convex surface with an area of ​​25 cm². Two main applications are surface sampling, where the number of colonies after culture can be directly related to contamination as His CFU/unit area, and His for volume generation using the contact plate of an active SAS air sampler. Air sampling to CFU/unit.

The test site environment and microbiological risks, in combination with regulatory requirements or microbiologist preferences, determine the media and packaging options. Total viable counts in the pharmaceutical industry primarily use TSA and SDA for airborne yeast and mold. However, some prefer malt extract agar, nutrient agar, or blood agar containing rose Bengal. Selective and differential media are useful when specific contamination is suspected.

Points to Remember during the Testing:

If the surface to be sampled has been cleaned with a disinfectant, an appropriate neutralizer should be incorporated into the medium for each biocide. Lecithin and Tween (polysorbate) combine with thiosulfate, L-histidine, thioglycolate, and bisulfate. If the environment contains other growth inhibitors, these should also be considered. For example, the β-lactamase enzyme is incorporated into the plate for use in cephalosporin-producing regions. When used in isolators or clean rooms, the agar surface and the container and exterior must be sterile. This is accomplished by gamma irradiating the panel after packaging. Most panels in this application are expected to show no growth, so the panels must be free of visible contamination.

The secret lies in the manufacturing process. At Dynatec Labs, plates are automatically filled and inkjet labelled under laminar flow in an ISO class 7 clean room. It is then stored at room temperature for several days before testing. Substrates with visible defects such as cracks or bubbles are sorted out (which also removes contaminated substrates), and perfect substrates are stacked, packaged, heat sealed and labelled in 10 seconds. All this is done in a clean room. The panel to be illuminated is optionally wrapped in an additional layer of clear heat-sealed bag and placed in a plastic-lined carton. Boxes of 100 plates are sealed, labelled, and sent for irradiation on the same day. Boxes should be stored at room temperature and opened near where they will be used. The ultimate guarantee for the user is that the plates can be seen through transparent packaging and sorted in a sterile environment before opening.

Air Monitoring through Settle Plates:

The need for effective air monitoring is increasing in all areas where airborne microorganisms can contaminate or affect industrial products and processes. Air monitoring is, therefore, particularly important for all companies producing filtered air in cleanroom areas. Active air monitoring using settling plates or contact plates requires physical sampling using a microbiological air sampler and passing a specific, predetermined air volume over the agar. Plates are then removed from the air sampler, cultured to develop visible colonies, and counted. The number of visible colonies estimates the number of colony-forming units in the sample air.

Uses of Settling Plates:

Settling Plates can handle the high flow rates and large sample volumes needed to monitor air quality in critically important cleanrooms where the number of microorganisms present is likely to be very low. Settling plate sampling is a direct method of determining the number of microorganisms that can be deposited on a product or surface within a specified period. This is because airborne microorganisms, which are usually attached to larger particles, deposit without affecting open culture plates.

Microorganisms are commonly found in the air in the skin cell’s habitat, and few exist alone. The average size of microbial particles is deposited on the surface by gravity at a velocity of about one cm/s. In sedimentation plate sampling, a petri dish containing an agar medium is exposed for some time to allow particles carrying microorganisms to settle plates on it. The most commonly used Petri dish is 90 mm in diameter (with an internal area of ​​approximately 64 cm2).

The number of microbial-laden particles deposited on the agar surface of the plate during the exposure period is determined by incubating the plate and counting the number of microbial colonies, more commonly known as colony-forming units (CFU). Microbial deposition rate can be expressed as the number deposited in a given area per unit of time. Solid growth media (sedimentation plates or contact plates) are made from an approved recipe.

However, purchasing the material from a commercial manufacturer may be preferable, as it is manufactured with good consistency and is available in various formats (e.g., irradiated). Commercially available materials are batch manufactured to appropriate specifications and quality controlled to test the growth potential of the media.

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