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:Cattle and sheep are thought to be the major reservoir of STEC, since the species often carry STEC in their intestinal flora and serve as a source of food contamination (Martins da Costa et al., 2011). There are some studies about the presence and prevalence of STEC and EPEC in healthy poultry feces and product. The results of the present study revealed the presence of two stx2 positive isolates, which were considered as STEC pathotype. On the other hand, 11% of the examined external and visceral cavity isolates were only positive for the eae gene, which were considered as EPEC strains. Moreover, one of the isolates possessed both stx2 and eae genes which is considered as highly pathogenic. This isolate belonged to phylo-groupD and phylo-subgroupD2. Salehi (2012) in Iranhas evaluated the presence of virulence factors in 290 fecal E.
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