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:They comprised 136 strains isolated from clinical environments; 42 strains isolated from intertidal sand contaminated by Prestige’s crude oil [23]; 50 strains from our laboratory collection, which have been previously studied by multilocus sequence analysis, MLSA [6, 20, 22]; and strain PE, which was isolated from a bioreactor seeded with the commercial compound Putidoil®.
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