Bouma J E, Lenski R E
Department of Ecology and Evolutionary Biology, University of California, Irvine 92717.
Nature. 1988 Sep 22;335(6188):351-2. doi: 10.1038/335351a0.
Associations between bacteria and their accessory elements (viruses, plasmids and transposons) range from antagonistic to mutualistic. A number of previous studies have demonstrated that plasmid carriage reduces bacterial fitness in the absence of selection for specific functions such as antibiotic resistance. Many studies have demonstrated increased fitness of evolving microbial populations in laboratory environments, but we are aware of only one study in which fitness gains were partitioned between a plasmid and its host. Here, we examine the evolution of an association between a plasmid and its bacterial host. Carriage of the non-conjugative plasmid pACYC184 initially reduced the fitness of Escherichia coli B in the absence of antibiotic. We then cultured plasmid-bearing bacteria for 500 generations in the presence of antibiotic. The fitness of each combination of host and plasmid, with and without the culture history, was determined by competing it against a baseline strain. The results indicate adaptation by the host genome, but no plasmid adaptation. We also competed the evolved host, transformed with the baseline plasmid, against its isogenic plasmid-free counterpart. The plasmid now increased the fitness of its host.
细菌与其附属元件（病毒、质粒和转座子）之间的关系从拮抗到互利共生不等。此前的一些研究表明，在没有针对特定功能（如抗生素抗性）进行选择的情况下，携带质粒会降低细菌的适应性。许多研究已经证明，在实验室环境中进化的微生物群体适应性增强，但我们只知道有一项研究将适应性增益在质粒及其宿主之间进行了划分。在这里，我们研究了质粒与其细菌宿主之间关系的进化。在没有抗生素的情况下，携带非接合性质粒pACYC184最初会降低大肠杆菌B的适应性。然后，我们在有抗生素的情况下将携带质粒的细菌培养500代。通过将宿主和质粒的每种组合与一个基线菌株竞争，来确定有无培养历史的宿主和质粒组合的适应性。结果表明宿主基因组发生了适应性变化，但质粒没有适应性变化。我们还将用基线质粒转化的进化宿主与其同基因无质粒对应物进行了竞争。现在，该质粒提高了其宿主的适应性。
