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，同时也是人类的偶发性胞内致病菌。本研究提出一项模型：在自然环境中，嗜肺军团菌的宿主在多种原生动物宿主间循环的过程，可使该菌维持作为广谱宿主范围病原体的进化停滞状态。我们采用实验进化方法，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群体动态变化，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群体的群体动态特征，其过程穿插频繁的瞬时克隆干扰与选择性清除事件。鞭毛调节因子fleN的非同义点突变，可提升嗜肺军团菌的吞噬摄取效率，并显著增强其在巨噬细胞与变形虫中的增殖能力。赖氨酸生物合成通路多环节的突变也被独立筛选得到，这些突变会导致嗜肺军团菌出现赖氨酸营养缺陷型，并降低其在变形虫中的增殖能力。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。我们据此提出假说：在自然环境中，宿主循环过程通过维持对巨噬细胞及其他宿主内增殖有害的通路，阻止嗜肺军团菌发生宿主特化。