Of all known Salmonella enterica serovars, S. Infantis is one of the most commonly isolated and has been recently emerging worldwide. To understand the recent emergence of S. Infantis in Israel we performed extensive comparative analyses between pre-emergent and the clonal emergent S. Infantis populations. We demonstrate a horizontal acquisition of an emergent-specific plasmid, designated pESI. This self-transferred episome is a mosaic megaplasmid (~280 kb), which increases bacterial tolerance to environmental mercury (mer operon), oxidative stress and provides further resistance to tetracycline (tetRA), sulfamethoxazole (sulI) and trimethoprim (dfrA). Moreover, pESI carries the yersiniabactin siderophore system and two novel chaperone usher fimbriae. In -vitro studies established that the emergent clone presents superior biofilm formation, adhesion and invasion to avian and mammalian host cells in a pESI-dependent manner. In-vivo mouse infections demonstrated higher pathogenicity and increased intestinal inflammation caused by the emergent clone compared to a plasmidless pre-emergent strain. Our results indicate that pESI acquisition facilitated the emergence of a fitter, resistant and more virulent strain that replaced the local S. Infantis community in an exceptionally short time of only 2-3 years.
