Derby adaptation to swine and simultaneous attenuation for humans go through decay of Pathogenicity Island I.

This study integrated population data with assessment of virulence phenotypes to unveil that a considerable part of the global population of Derby is evolving to enhance its host adaptation to the swine host and that this evolution is simultaneously increasing its attenuation for humans. The study shows that the fixation of deleterious mutations in SPI-1 has a role in this process. This evidence indicates that SPI-1 has a key role for . Derby virulence in humans but not for its circulation in swine. The results show that genes generally considered essential for pathogenesis do not play the same key role for all serovars or lineages and/or all hosts. The study helps in understanding the molecular mechanisms underlying the ecology and host adaptation of showing that the adaptation process can vary for different types of and hosts.