Brewers yeast is inactive yeast, so it doesn't actually inhibit mold, but it provides nutrients and protein. I've heard of alot of people sprinkling a bit of live active yeast, such as you would use to bake bread at home, on top of the culture and that is supposed to prevent mold.
Do you find that your recipe starts to smell after a while? Does it produce lots of flies?
When I can no more stir my soul to move, and life is but the ashes of a fire
When I can but remember that my heart once used to live and love, long and aspire
Oh, be thou then the first, the one thou art, be thou the calling, before all answering love
And in me wake hope, fear, boundless desire.