UTEX tests for contamination by plating samples of the strain on 3 -4 different types of agar media plates:
These media contain organic components that bacterial and fungal contaminants can utilize. The plates are incubated in the dark at 37 °C for 3-7 days. Keeping the plates in the dark helps prevent algae from growing, remember we are trying to grow contaminants so the culturing conditions are meant for bacteria and fungus. After the incubation period, the plates are examined under a stereomicroscope for growth.
Any plates that do not show bacterial growth on all of the 3-4 agar media plates are considered to be possibly free of contaminants. However, there isn't a single media recipe that can grow every single type of contaminant, so final verification is performed through microscopic examination.