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:Bacterial strains, plasmids, and media
Escherichia coli JM109 (Yanisch-Perron et al. 1985) and pGEM-T Easy (Promega, Madison, USA) were used in routine gene manipulation.
Category: