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:The answer to this question is currently unknown, but experimental evidence so far suggests that non-motile mutants are at a severe disadvantage for establishing and maintaining interactions with these hosts (Black et al., 1988; Nachamkin et al., 1993; Wassenaar et al., 1993; Hendrixson and DiRita, 2004; Wösten et al., 2004).
Category: