Data from: Antibiotic resistance correlates with transmission in plasmid evolution

Conjugative (horizontally transmissible) plasmids are autonomous replicators, whose ‘self-interests’ do not necessarily overlap with those of their hosts. This situation causes plasmids and bacteria to sometimes experience differing selection pressures. Escherichia coli plasmid pB15 contains genes for resistance to several antibiotics, including tetracycline. When plasmid-bearing cells were experimentally evolved in the laboratory, changes in resistance level in the unselected tetracycline marker coincided with changes in plasmid rates of vertical versus horizontal transmission. Here we used minimum-inhibitory assays that measure resistance levels as quantitative traits to determine phenotypic correlations among plasmid characters and to estimate divergence among plasmid lineages. Results suggested that plasmid-level evolution led to formation of two phenotypically-dissimilar groups: virulent (highly infectious) and avirulent (weakly infectious) plasmids. In contrast, measures of carbon-source utilization, and fitness assays relative to a common competitor revealed that bacterial hosts generally converged in phenotypic performance, despite divergence among their associated plasmids. Preliminary sequence analyses suggested that divergence in plasmid conjugation was due to altered configurations of a shufflon region (a site-specific recombination system), where genetic rearrangements affect conjugative ability. Furthermore, we proposed that correlated resistance and transmission in pB15 derivatives were caused by a tetracycline-resistance transposon inserted into a transfer operon, allowing transcription from its promoter to simultaneously affect both plasmid resistance and transmission.接合型（可水平传递）质粒（Conjugative plasmid）是一类自主复制子，其自身的演化利益未必与其宿主的演化利益重合。这种情况会导致质粒与细菌有时面临不同的选择压力。大肠杆菌（Escherichia coli）质粒pB15携带有针对包括四环素在内的多种抗生素的抗性基因。当携带质粒的细胞在实验室中进行实验性演化时，未受选择的四环素抗性标记的抗性水平变化，与质粒垂直传递与水平传递的速率变化相一致。本研究采用以抗性水平作为数量性状进行量化的最低抑菌测定法，以此探究质粒各性状间的表型相关性，并估算质粒谱系间的分化程度。研究结果显示，质粒层面的演化催生了两类表型迥异的群组：强侵染性（高传染性）与弱侵染性（低传染性）质粒。与之相反，通过碳源利用能力测定以及以共通竞争菌株为参照的适合度实验，研究发现尽管其所携带的质粒存在分化，细菌宿主的表型表现总体上趋于一致。初步序列分析结果表明，质粒接合能力的分化源于洗牌子（shufflon，一种位点特异性重组系统）区域的构型改变，该区域内的遗传重排会影响质粒的接合能力。此外，本研究提出，pB15衍生株中抗性与传递性状的相关性，源于四环素抗性转座子插入至转移操纵子中：该转座子的启动子转录过程可同时影响质粒的抗性与传递能力。