Escherichia coli K-12 is a model organism for bacteriology and has served as a workhorse for molecular biology and biochemistry for over a century since its first isolation in 1922. However, Escherichia coli K-12 strains are phenotypically devoid of an O antigen (OAg) since early reports in the scientific literature. Recent studies have reported the presence of independent mutations that abolish OAg repeating-unit (RU) biogenesis in E. coli K-12 strains from the same original source, suggesting unknown evolutionary forces have selected for inactivation of OAg biogenesis during the early propagation of K-12. Here, we show for the first time that restoration of OAg in E. coli K-12 strain MG1655 synergistically sensitises bacteria to vancomycin with bile salts (VBS). Suppressor mutants surviving lethal doses of VBS primarily contained disruptions in OAg biogenesis. We present data supporting a model where the transient presence and accumulation of lipid-linked OAg intermediates in the periplasmic leaflet of the inner membrane interfere with peptidoglycan sacculus biosynthesis, causing growth defects that are synergistically enhanced by bile salts. Lastly, we demonstrate that continuous bile salt exposure of OAg-producing MG1655 in the laboratory, can recreate a scenario where OAg disruption is selected for as an evolutionary fitness benefit. Our work thus provides a plausible explanation for the long-held mystery of the selective pressure that may have led to the loss of OAg biogenesis in E. coli K-12; this opens new avenues for exploring long-standing questions on the intricate network coordinating the synthesis of different cell envelope components in Gram-negative bacteria.

中文翻译：

---

O 抗原的生物发生使大肠杆菌 K-12 对胆汁盐敏感，为其进化损失提供了合理的解释。

大肠杆菌 K-12 是细菌学的模式生物，自 1922 年首次分离以来一个多世纪以来一直是分子生物学和生物化学的主力。然而，大肠杆菌 K-12 菌株在表型上缺乏 O 抗原 (OAg) ）自科学文献的早期报道以来。最近的研究报道了来自同一原始来源的大肠杆菌 K-12 菌株中存在独立突变，这些突变废除了 OAg 重复单元 (RU) 的生物发生，这表明未知的进化力量在 K 的早期繁殖过程中选择了 OAg 生物发生的失活。 -12。在这里，我们首次表明，大肠杆菌 K-12 菌株 MG1655 中 OAg 的恢复可协同使细菌对带有胆盐 (VBS) 的万古霉素敏感。在致死剂量的 VBS 中存活下来的抑制突变体主要包含 OAg 生物合成的破坏。我们提供的数据支持一个模型，其中内膜周质小叶中脂质连接的 OAg 中间体的短暂存在和积累会干扰肽聚糖球囊生物合成，导致生长缺陷，而胆汁盐会协同增强生长缺陷。最后，我们证明在实验室中持续暴露于产生 OAg 的 MG1655 的胆汁盐，可以重现选择 OAg 破坏作为进化适应性益处的场景。因此，我们的工作为长期存在的选择压力之谜提供了一个合理的解释，选择压力可能导致大肠杆菌 K-12 中 OAg 生物发生的丧失。这为探索协调革兰氏阴性细菌中不同细胞包膜成分合成的复杂网络这一长期存在的问题开辟了新途径。