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:Group representative strains were screened for nifH, the gene encoding the nitrogenase iron protein dinitrogenase reductase , by dot blot hybridization using a biotinylated nifH specific DNA probe that detects a very wide range of diazotroph taxa Bagwell et al. 1998; 2001; Bergholz et al. 2001 .
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