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（水平可传递型）质粒（conjugative plasmid）是一类自主复制因子，其‘自身利益’未必与其宿主的利益相契合。此种情形会导致质粒与其宿主细菌有时面临差异化的选择压力。大肠杆菌（Escherichia coli）质粒pB15携带有多种抗生素抗性基因，其中包含四环素抗性基因。当携带质粒的细胞在实验室中开展实验性进化时，未施加选择压力的四环素标记抗性水平的变化，与质粒垂直传递与水平传递速率的变化呈现一致性。本研究采用将抗性水平作为数量性状进行量化的最低抑菌测定（minimum-inhibitory assays），以解析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，质粒层面的进化形成了两类表型迥异的群组：强毒力（高侵染性）质粒与弱毒力（低侵染性）质粒。与之相反，通过碳源利用测定以及以共同参照菌株为对照的适应性试验发现，尽管其所携带的质粒谱系间存在分化，但宿主细菌的表型性能总体上趋于一致。初步序列分析表明，质粒接合能力的分化源于重组跳跃区（shufflon region，一种位点特异性重组系统）的构型改变，该区域的遗传重排会影响质粒的接合能力。此外，本研究提出，pB15衍生质粒的抗性与传递性状间的相关性，源于一段四环素抗性转座子（tetracycline-resistance transposon）插入至接合转移操纵子中，使其启动子的转录可同时影响质粒的抗性与传递能力。