Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:Therefore, unlike inorganic nitrogen sources, most of the organic nitrogen sources, in addition to acting as a nitrogen source, supply vitamins, minerals and accessory growth factors in the cultivation medium that enhance bacterial growth and enzyme production (Gupta et al., 2004).
Category: