Are all Contact plates the same when it comes to EM?

By Andy Whittard

Contact plates (or RODAC®) have been around since the 1950’s and are now a very commonly used device within pharmaceutical manufacturing for environmental monitoring of surfaces. The media within the plate typically contains a mix of neutralisers to neutralise the effect of any residual disinfectant that might be transferred to the agar surface. Contact plates are also widely used for air sampling applications within devices such as the SAS Super range of air samplers supplied by Cherwell Laboratories.

Within media formulations there are many variants, and it is important that users understand the needs of their application as well as the factors that can influence formulation choice. An important consideration is choice of neutraliser, a common combination being Lecithin and Tween. However, this mix only neutralises a limited range of disinfectants, notably Quaternary Ammonium Compounds (QAC’s), parahydroxybenzoates (parabens), bis-biguanides and iodine(*). An increasingly popular neutraliser combination is Lecithin, Tween, Sodium thiosulphate and L-Histidine. The various media manufacturers refer to this mix differently, in Cherwell’s case we name it Neutraliser 4. This combination has a broader range of efficacy against also mercurials, halogens and aldehydes (*).

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The key point here is users must understand what disinfectants they are using, which category they fall into, and therefore which neutralisers are needed. Without this fundamental knowledge EM data can be affected and lead to false negative results.

Whilst neutralisers are less of an issue with air sampling and monitoring, the actual size of the plate can be a significant factor. A key performance factor in any sieve sampler is its collection efficiency. This is determined by the design parameters and the air flow through the sampler. Air flow will affect impaction velocity of particles onto the agar surface and ultimately determine the collection efficiency. This is most easily seen by the impaction imprint, or ‘dimples’, on the agar surface of the plate (as shown by the image).

The gap between the sieve and the agar surface of the Contact plate can alter this performance. If agar has shrunk and/or partially desiccated, it is possible to have no dimples, suggesting impaction velocity has decreased and particles have not been captured. However, if the Contact plate is too tall it is possible to restrict air flow and again affect velocity and reduce dimpling on the agar. In extreme cases it is possible to even block the sieve. The best indicator is that from a 1000 litre sample there should be an even, well pronounced dimple pattern across the entire agar surface. This gives confidence of impaction and therefore particle collection. If the pattern is slight or uneven, or worst still not present, urgent further investigation is necessary because the quality of the EM results will be compromised.

Cherwell’s service team can offer advice on any mechanical issues with the sampler and our Product Specialists can help with selection of the correct media for your application.

(*) EP 2.6.12 Microbial examination of non-sterile products: Microbial enumeration tests - Table 2.6.12.-2. – Common neutralising agents for interfering substances.


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