Thymol is a cheap and effective way to keep "bugs" from growing in glop. I've discovered a year old container of glop in the back of the fridge that was still clean and usable. Keeping containers, stirrers etc. scrupulously clean, and using sterile (boiled) water lessens the chances of starting off with an inoculated mixture.

Dequalinium (available as an 'oral paint') is also an effective bug killer.