Abstract: A method of producing a secondary metabolite includes providing an elongate porous substrate with a biofilm of micro-organism and which is arranged with one end of the substrate being at a higher elevation than the other end of the substrate so that the substrate is at an angle to the horizontal. A nutrient solution flows through the substrate, at a rate which is sufficiently low for a nutrient gradient to be established across the biofilm such that the nutrient concentration at a high level along the gradient is sufficiently high to support primary growth of the micro-organism, and the nutrient concentration at a low level along the gradient is sufficiently low to induce secondary growth of the micro-organism. The substrate's angle with the horizontal ensures that any droplets of nutrient permeate forming on the biofilm run towards the lower end of the substrate.