Data from: Escherichia coli populations in unpredictably fluctuating environments evolve to face novel stresses through enhanced efflux activity

There is considerable understanding about how laboratory populations respond to predictable (constant or deteriorating environment) selection for single environmental variables such as temperature or pH. However, such insights may not apply when selection environments comprise multiple variables that fluctuate unpredictably, as is common in nature. To address this issue, we grew replicate laboratory populations of Escherichia coli in nutrient broth whose pH and concentrations of salt (NaCl) and hydrogen peroxide (H2O2) were randomly changed daily. After ~170 generations, the fitness of the selected populations had not increased in any of the three selection environments. However, these selected populations had significantly greater fitness in four novel environments which have no known fitness-correlation with tolerance to pH, NaCl or H2O2. Interestingly, contrary to expectations, hypermutators did not evolve. Instead, the selected populations evolved an increased ability for energy-dependent efflux activity that might enable them to throw out toxins, including antibiotics, from the cell at a faster rate. This provides an alternate mechanism for how evolvability can evolve in bacteria and potentially lead to broad-spectrum antibiotic resistance, even in the absence of prior antibiotic exposure. Given that environmental variability is increasing in nature, this might have serious consequences for public health.

目前学界已较为充分地阐明，实验室种群如何响应针对温度、pH等单一环境变量的可预测选择（环境为恒定或退化状态）。然而，当选择环境包含多个不可预测波动的变量时，这类认知或许不再适用——而这种情况在自然环境中极为常见。为解决这一问题，我们在营养肉汤中培养了多组平行的大肠杆菌（Escherichia coli）实验室种群，该肉汤的pH值、氯化钠（NaCl）与过氧化氢（H₂O₂）浓度每日随机变动。经过约170代培育后，所选育种群在三种选择环境中的适合度均未出现提升。但这些选育种群在四种全新环境中的适合度显著更高，且目前已知这些环境与pH、NaCl或H₂O₂耐受性不存在适合度相关性。有趣的是，与预期相悖的是，实验中并未演化出超突变株（hypermutator）。相反，选育种群演化出了增强的能量依赖性外排活性，这或许能帮助它们以更快速率将包括抗生素在内的各类毒素排出细胞外。这为细菌可演化性的演化提供了一种新机制，且即便在未预先接触抗生素的情况下，也可能引发广谱抗生素耐药性。鉴于自然环境的波动性正不断加剧，这一现象或许会对公共健康造成严重影响。