The upstream part of a bioprocess refers to the first step in which microbes/cells are grown, e.g. bacterial or mammalian cell lines (see cell culture), in bioreactors. Upstream processing involves all the steps related to inoculum development, media development, improvement of inoculum by genetic engineering process, optimization of growth kinetics so that product development can improve tremendously. Fermentation has two parts: upstream and downstream. After product development, the next step is the purification of product for desired quality. When they reach the desired density (for batch and fed-batch cultures) they are harvested and moved to the downstream section of the bioprocess.