the invention discloses a method of introducing microorganisms into a heap of material for bio - assisted heap leaching by prepraring microorganisms substantially without exopolymers on their external cell walls ; adding such microorganisms to the heap ; and re - activating the production of exopolymers on the external cells walls of the microorganisms in the heap . the invention also extends to a method of enriching the environment of microorganisms &# 39 ; embedded in a heap for bio - assisted heap leaching .