Data from: Experimental evolution of Legionella pneumophila in mouse macrophages leads to strains with altered determinants of environmental survival

The Gram-negative bacterium, Legionella pneumophila, is a protozoan parasite and accidental intracellular pathogen of humans. We propose a model in which host cycling through multiple protozoan hosts in the environment holds L. pneumophila in a state of evolutionary stasis as a broad host-range pathogen. Using an experimental evolution approach, we tested this hypothesis by restricting L. pneumophila to growth within mouse macrophages for hundreds of generations. Whole-genome resequencing and high-throughput genotyping identified several parallel adaptive mutations and population dynamics that led to improved replication within macrophages. Based on these results, we provide a detailed view of the population dynamics of an experimentally evolving bacterial population, punctuated by frequent instances of transient clonal interference and selective sweeps. Non-synonymous point mutations in the flagellar regulator, fleN, resulted in increased uptake and broadly increased replication in both macrophages and amoebae. Mutations in multiple steps of the lysine biosynthesis pathway were also independently isolated, resulting in lysine auxotrophy and reduced replication in amoebae. These results demonstrate that under laboratory conditions, host restriction is sufficient to rapidly modify L. pneumophila fitness and host range. We hypothesize that, in the environment, host cycling prevents L. pneumophila host-specialization by maintaining pathways that are deleterious for growth in macrophages and other hosts.革兰氏阴性菌嗜肺军团菌（Legionella pneumophila）是一种原生动物寄生虫，同时也是可感染人类的偶然性胞内致病菌。我们提出如下模型：在自然环境中，嗜肺军团菌通过多种原生动物宿主进行宿主循环，使其作为广谱宿主范围致病菌处于进化停滞状态。本研究采用实验进化方法，通过将嗜肺军团菌限制在小鼠巨噬细胞内传代数百代，对该假说进行验证。全基因组重测序与高通量基因分型技术鉴定出多组平行适应性突变及种群动态变化，这些突变与变化可提升嗜肺军团菌在巨噬细胞内的增殖能力。基于上述结果，我们详细解析了实验进化细菌种群的种群动态特征，其演化过程常伴随频繁的短暂克隆干扰与选择性清除事件。鞭毛调控因子fleN的非同义点突变，可提升嗜肺军团菌的吞噬摄取效率，并在巨噬细胞与变形虫中均能广泛增强其增殖能力。赖氨酸生物合成通路多个环节的突变也被独立筛选得到，这些突变可导致赖氨酸营养缺陷型表型，并降低嗜肺军团菌在变形虫内的增殖能力。上述结果表明，在实验室条件下，宿主限制足以快速改变嗜肺军团菌的适合度与宿主范围。我们推测，在自然环境中，宿主循环过程可通过维持对巨噬细胞及其他宿主的增殖存在不利影响的通路，阻止嗜肺军团菌发生宿主特化。