The perfect Contact plate

Over the last 25 years Cherwell Laboratories have developed a range of contact plate products to overcome both general problems of handling and specific problems brought to us by individual customers. We currently manufacture 62 varieties of contact plate products all of which now use the latest grip-lid plastic dish. The grip-lid design ensures that the plates are secure if stacked on the bench or in the incubator and the lid remains in place until it is removed by the user. When the lid is replaced after sampling, inspection or subculture it is again secure, yet it is no more difficult to manipulate with one hand than the traditional design. Contact plates, sometimes known as RODAC (Replicate Organism Detection and Counting) plates, take significant skill to fill perfectly but if you can buy them pre-filled they have unrivalled convenience. The contact plate is a plastic dish filled with agar to give a convex surface with an area of 25cm². The two principal applications are surface sampling, so that the count of colonies after incubation can be directly related to the contamination as cfu/unit area, and air sampling, where the contact plate is used in a SAS active air sampler to produce cfu/unit volume. The choice of media and pack option is dictated by the environment and the microbiological risk at the test site combined with the regulatory requirements or the preference of the microbiologist. Total viable counts in the pharmaceutical industry use mostly TSA, with SDA for yeasts and moulds in air, while others prefer Nutrient Agar or Blood Agar with Malt Extract Agar or Rose Bengal. When specific contaminants are suspected selective and differential media can be helpful. MacConkey for Coliforms, Pseudomonas selective agars, Baird Parker for Staphylococcus are common examples. If the surfaces to be sampled have been cleaned with disinfectants it is necessary to incorporate an appropriate neutraliser for each biocide into the medium. Lecithin and Tween (Polysorbate) are common with Thiosulphate, L-Histidine, Thioglycollate and bisulphate in various combinations. Where the environment might contain other growth inhibitors these also need to be considered. For example we incorporate b -Lactamase enzyme in plates for use in Cephalosporin production areas.

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