Cooperative virulence can emerge via horizontal gene transfer but is stabilized by transmission

Intestinal inflammation fuels Salmonella Typhimurium (S.Tm) transmission despite a fitness cost associated with the expression of virulence. Cheater mutants can emerge that profit from inflammation without enduring this cost. Intestinal virulence in S.Tm is therefore a cooperative trait, and its evolution a conundrum. Horizontal gene transfer (HGT) of cooperative alleles may facilitate the emergence of cooperative virulence, despite its instability. To test this hypothesis, we cloned hilD, coding for a master regulator of virulence, into a conjugative plasmid that is highly transferrable during intestinal colonization. We demonstrate that virulence can emerge by hilD transfer between avirulent strains in vivo. However, this was indeed unstable and hilD mutant cheaters arose within a few days. The timing of cheater emergence depended on the cost. We further show that stabilization of cooperative virulence in S.Tm is dependent on transmission dynamics, strengthened by population bottlenecks, leading cheaters to extinction and allowing cooperators to thrive.