Culture Media Training Academy: Importance of Appropriate Drying When Preparing Media

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It is sometimes necessary to dry agar plates before use, especially if they've been stored at two to eight degrees Celsius as condensation may have occurred.

If a wet plate is inoculated, the resulting growth may be smeared and difficult to identify and count.

A normal method of drying plates is by using a laminar flow cabinet where clean air is passed over to the agar surface to evaporate any moisture present, however, excessive drying can lead to performance problems, especially with an agar that is selective.

Here is a Listeria chromogenic agar as described in ISO 11290. Pathogenic Listeria monocytogenes grows on the agar as a green colony surrounded by an opaque halo.

The agar contains a cocktail of antibiotics that exert selective pressure on non-target organisms and inhibits them. This is what Listeria monocytogenes looks like on agar when dried for around 10 minutes using a laminar flow.

Here are healthy green colonies with large opaque halos.

You can see as we increase the drying time, we start to see a reduction in size, color intensity and halo production.

We also start to see a drop in number.

As the drying time increases, we start to fail to identify the dangerous pathogen Listeria monocytogenes. This is because the dehydration adds an extra selective hurdle that reduces or suppresses the target organism.

To prevent this from happening, users should monitor how long agar plates are dried for taking into account the intensity of their drying procedure.