The biggest issue facing operators when exposing settle plates in high airflow environments, such as laminar airflow cabinest; class II microbiology safety cabinets and isolators, is the desiccation of the medium during the four-hour exposure time, which is the maximum recommended exposure time. It is important during exposure that the agar does not shrink from the edge of the petri dish or crack down the middle. This is to ensure the maximum surface area is constantly exposed to the environment. Possibly more importantly, is that the agar is still able to show good recovery of your regular test organisms; both post exposure and post incubation1.
Reducing the effect of desiccation of agar
Selecting media fill volume
You cannot avoid the desiccation of agar. An agar plate exposed in a LAF running at 0.45m/s ±20% for four hours is going to desiccate. The Redipor range of irradiated agar comes in a number of fill volumes to meet the needs of your working environment:
18ml is the standard fill volume for 90mm petri dishes
27ml is our deep filled equivalent. This is the most popular for high air flow environments
32ml is our extra deep fill and also the maximum we can fill to. These are for environments where dehydration is a real problem.
Assessing operating environment
In regards to how the agar shrinks, ideally it will shrink downwards before it shrinks inwards. If there is excessive shrinkage even in deep filled plates, there may be issues with your operating environment, i.e. the airflow is too high and/or the air is too warm - I call this the hair dryer effect! If you cannot alter this, then you may need to replace your settle plates every couple of hours rather than waiting the full hour hours.
Selecting media formulation
We are aware of issues where the agar cracks down the middle as it shrinks. This may happen with irradiated media containing disinfectant neutralisers, more so in high airflow environments. Should this happen, perhaps you should reconsider the use of neutralisers in your settle plates? Neutralisers are necessary inpetri dishes when they are used as glove dabs or streaking swabs. Are neutralisers necessary in media for passive air sampling?
Hopefully the above is good advice for helping you choose the appropriate fill volume and formulation. If you have any further queries or comments, we are happy to assist you.
Microbiology Product Specialist,
1. Sandle T. Settle plate exposure under unidirectional airflow and the effect of weight loss upon microbial growth. European Journal of Parenteral & Pharmaceutical Sciences 2015;20(2):45-50
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