Growing mushrooms or any species of fungi needs to be done in a completely sterile environment. If even a few cells from an unwanted microbe make it into the growth substrate, the entire batch is ruined and a lot of time is lost. To insure that each kit only contains the desired species of fungus, we pressure cook the growth substrate in special bags. We then take the sealed and now sterile bags into our lab where we have a laminar flow hood. The laminar flow hood consists of an industrial blower and a high efficiency filter with 0.2 micron pores. The filter acts as strainer for microorganisms. So when the blower is turned on, air from the room is pushed through the filter and any air that passes through the pores is completely sterile. (When we open the pressure-cooked bags to add our fungus, we do so in front of the laminar flow hood. When you open your kit at home, the fungus is already well established and can outcompete any microbes in your house.