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:however, been shown to be essential for the entry and replication of Salmonella Typhi in epithelial cells [43] and for the survival and virulence of Mycobacterium bovis in mice [44,45].
Category: