Agar Desiccation – The Causes and How to Address Them

Abstract

Solid culture media, or agar plates, are prone to desiccation. The combination of warm, dry air and limited moisture content to achieve a solid finish leads to inevitable drying out and desiccation. Cracks in the media, a loss of volume or shrinkage away from the sides of the Petri dish are evidence of desiccation. Overall, this reduces the capability of the media to support the growth of organisms. The reduction in growth support capability is particularly so if the microorganisms deposited on the surface of the media are stressed. Why is Desiccation an Issue It is a well-known fact that microbes have three basic requirements for growth; a food source, warmth and moisture. Reducing the moisture content risks inactivating and or inhibiting growth. The purpose of monitoring for microbes through the use of solid media is to capture microorganisms from the manufacturing environment and create favourable conditions to support their optimum growth, thus allowing them to replicate and form visible colonies for inspection. Viable environmental monitoring is a challenging process to optimise for; you are looking for the chance of capturing one of the very few microorganisms present in your extensive manufacturing facility. To do so, you use solid culture media to take samples, snapshots in time, of your environment. Firstly, you have a vast space to monitor, and your monitoring activity needs to find the very few microorganisms present. If you have set your monitoring programme up suitably and you successfully capture organism present in the environment but then add desiccation you risk losing critical data. Desiccation leads to a reduction in these favourable conditions and potentially a loss of critical data and false-negative results. Desiccation of media is considered a significant quality issue as it can lead to the inhibition of growth and or cell death. The action of drying out of the media can lead to the formation of a skin on the surface. The skin formation inhibits the recovery of organisms, through «bounce» or «air bounce». As the purpose of environmental monitoring is to capture and support the growth of contaminants, this issue will result in inconclusive data gathering. Desiccation is an inevitability. Cleanroom environments are by their very nature, high airflow facilities which, as you will see below, is a primary reason for the loss of moisture from agar plates. It is, therefore, important to ensure media qualification studies include a review that post use, your media, can meet the 70% growth rate recovery as specified in USP <1227>.