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, as previously discussed, choice of host species has been shown to greatly influence the outcome of a Salmonella infection, and several studies utilizing a variety of advanced model systems have shown that T3SSs are necessary for Salmonella invasion and infection of these models [17,18,19,21,22,23,26,39].
Category: