IHAP, Université de Toulouse, INRA, ENVT, Toulouse, France.
UMR Mycoplasmoses of ruminants, ANSES, VetAgro Sup, University of Lyon, Lyon, France.
PLoS Genet. 2019 Jan 22;15(1):e1007910. doi: 10.1371/journal.pgen.1007910. eCollection 2019 Jan.
Horizontal Gene Transfer was long thought to be marginal in Mycoplasma a large group of wall-less bacteria often portrayed as minimal cells because of their reduced genomes (ca. 0.5 to 2.0 Mb) and their limited metabolic pathways. This view was recently challenged by the discovery of conjugative exchanges of large chromosomal fragments that equally affected all parts of the chromosome via an unconventional mechanism, so that the whole mycoplasma genome is potentially mobile. By combining next generation sequencing to classical mating and evolutionary experiments, the current study further explored the contribution and impact of this phenomenon on mycoplasma evolution and adaptation using the fluoroquinolone enrofloxacin (Enro), for selective pressure and the ruminant pathogen Mycoplasma agalactiae, as a model organism. For this purpose, we generated isogenic lineages that displayed different combination of spontaneous mutations in Enro target genes (gyrA, gyrB, parC and parE) in association to gradual level of resistance to Enro. We then tested whether these mutations can be acquired by a susceptible population via conjugative chromosomal transfer knowing that, in our model organism, the 4 target genes are scattered in three distinct and distant loci. Our data show that under antibiotic selective pressure, the time scale of the mutational pathway leading to high-level of Enro resistance can be readily compressed into a single conjugative step, in which several EnroR alleles were transferred from resistant to susceptible mycoplasma cells. In addition to acting as an accelerator for antimicrobial dissemination, mycoplasma chromosomal transfer reshuffled genomes beyond expectations and created a mosaic of resistant sub-populations with unpredicted and unrelated features. Our findings provide insights into the process that may drive evolution and adaptability of several pathogenic Mycoplasma spp. via an unconventional conjugative mechanism.
水平基因转移长期以来被认为在支原体中微不足道，支原体是一组无壁细菌，由于其基因组较小（约 0.5 至 2.0 Mb）和有限的代谢途径，常被描绘为最小的细胞。这种观点最近受到了挑战，因为发现了大染色体片段的可接合交换，这些片段通过一种非传统的机制同样影响染色体的所有部分，从而使整个支原体基因组具有潜在的可移动性。通过将下一代测序与经典交配和进化实验相结合，本研究利用氟喹诺酮恩诺沙星（Enro）作为选择压力和反刍动物病原体支原体（Mycoplasma agalactiae）作为模型生物，进一步探讨了这种现象对支原体进化和适应的贡献和影响。为此，我们生成了具有不同自发突变组合的同基因系，这些突变与 Enro 靶基因（gyrA、gyrB、parC 和 parE）相关联，并与 Enro 的逐渐抗性水平相关联。然后，我们测试了这些突变是否可以通过可接触的群体通过可接合的染色体转移获得，因为在我们的模型生物中，4 个靶基因散布在三个不同的、遥远的基因座中。我们的数据表明，在抗生素选择压力下，导致高水平 Enro 抗性的突变途径的时间尺度可以很容易地压缩成一个单一的可接合步骤，在这个步骤中，几个 EnroR 等位基因从抗性支原体细胞转移到易感支原体细胞。除了作为抗菌药物传播的加速器外，支原体染色体转移还出人意料地重新排列了基因组，并创建了具有不可预测和不相关特征的抗性亚群马赛克。我们的研究结果为了解可能通过非传统的接合机制驱动几种致病性支原体进化和适应性的过程提供了线索。
