A method to prevent contamination of feed line(s) and nutrient feed reservoirs(s) is disclosed. In this method the growth medium which flows from the nutrient feed reservoir through the feed line to the bioreactor contains at least one nutritional component supplied at a concentration which is inhibitory to cell growth. The inhibitory nutritional component in the growth medium prevents back growth of cells from the bioreactor into the feed line(s). The growth medium with inhibitory nutritional component is diluted prior to or at entering the bioreactor.