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:The vast majority of the Dot/Icm T4S effectors that gain access to the host cell cytoplasm are not absolutely required for bacterial intracellular replication, as the individual effector mutants do not replicate to lower numbers than WT bacteria (91).
Category: