A2. The plates have been exposed to CO2 during storage. They must be stored in a CO2-free environment. This can be achieved by using a sealed box or dessicator containing self-indicating soda lime (and a separate container of water to minimise drying of the plates) and replacing soda lime frequently. Using self-indicating soda lime gives the best indication of when the soda lime requires replacing.
Do not use plates if they are not of uniform colour or if the agar has started to dry out.