SP1 and infection: In contrast, heterogeneous expression of the SP-1 T3SS by wild-type bacteria maintains a reservoir of genetically wild-type but phenotypically avirulent individuals that benefit from the shared public good provided by SP-1-expressing cells, grow rapidly enough to prevent the population from being overtaken by SP-1-deficient mutants, and retain the ability to express SP-1 T3SS proteins necessary for successful infection and transmission.