Pathoadaptive Mutations in <i>Salmonella enterica</i> Isolated after Serial Passage in Mice

How pathogenic bacteria adapt and evolve in the complex and variable environment of the host remains a largely unresolved question. Here we have used whole genome sequencing of Salmonella enterica serovar Typhimurium LT2 populations serially passaged in mice to identify mutations that adapt bacteria to systemic growth in mice. We found unique pathoadaptive mutations in two global regulators, phoQ and stpA, which increase the competitive indexes of the bacteria 3- to 5-fold. Also, all mouse-adapted lineages had changed the orientation of the hin invertable element, resulting in production of a FliC type of flagellum. Competition experiments in mice with locked flagellum mutants showed that strains expressing the FliC type of flagellum had a 5-fold increase in competitive index as compared to those expressing FljB type flagellum. Combination of the flagellum cassette inversion with the stpA mutation increased competitive indexes up to 20-fold. These experiments show that Salmonella can rapidly adapt to a mouse environment by acquiring a few mutations of moderate individual effect that when combined confer substantial increases in growth.病原菌如何在宿主复杂多变的环境中适应与进化，仍是一个尚未被充分阐明的核心科学问题。本研究通过对在小鼠体内连续传代培养的肠炎沙门氏菌鼠伤寒血清型LT2（Salmonella enterica serovar Typhimurium LT2）种群开展全基因组测序（whole genome sequencing），筛选出可帮助细菌适应小鼠体内系统性增殖的突变位点。我们在两个全局调控因子（global regulator）phoQ与stpA中发现了独特的致病适应性突变（pathoadaptive mutations），这类突变可使细菌的竞争指数（competitive index）提升3至5倍。此外，所有经小鼠适应的菌株谱系均发生了hin可逆元件（hin invertible element）的方向反转，进而表达FliC型鞭毛（FliC flagellum）。针对固定鞭毛类型的突变株开展的小鼠体内竞争实验显示，表达FliC型鞭毛的菌株，其竞争指数相较于表达FljB型鞭毛（FljB flagellum）的菌株提升了5倍。将鞭毛基因盒反转与stpA突变相结合，可使竞争指数提升至原本的20倍。本研究结果表明，沙门氏菌可通过获取若干单个效应中等的突变，快速适应小鼠体内环境；这些突变组合后可显著提升细菌的增殖能力。