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:Several studies previously reported that the abundant proteins or LPS on the surface of outer membrane were suitable as targets for vaccine and diagnosis of leptospirosis such as outer membrane proteins [38,39], LIC11207 [40], OmpL1 [41,42], MPL17 and MPL21 [43], HbpA [44], LigA [45], LP29 and LP49 [46], LipL32 [47-50], LipL21 [50,51], LipL41 [42], flagellin protein [52].
Category: