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 and growth conditions Listeria monocytogenes EGDe was used for all experiments except for the D-alanine auxotrophy experiments, which were performed with L.monocytogenes strain DP-L3506 [23].
Category: