Department of Physics, University of California, San Diego, La Jolla, California.
Department of Physics, University of California, San Diego, La Jolla, California.
Biophys J. 2020 Jun 2;118(11):2816-2828. doi: 10.1016/j.bpj.2020.02.033. Epub 2020 Apr 19.
Bacterial chemotaxis, the directed migration of bacteria in a gradient of chemoattractant, is one of the most well-studied and well-understood processes in cell biology. On the other hand, bacterial thermotaxis, the directed migration of bacteria in a gradient of temperature, is understood relatively poorly, with somewhat conflicting reports by different groups. One of the reasons for that is the relative technical difficulty of the generation of well-defined gradients of temperature that are sufficiently steep to elicit readily detectable thermotaxis. Here, we used a specially designed microfluidic device to study thermotaxis of Escherichia coli in a broad range of thermal gradients with a high rate of data collection. We found that in shallow temperature gradients with narrow temperature ranges, E. coli tended to aggregate near a sidewall of the gradient channel at either the lowest or the highest temperature. On the other hand, in sufficiently steep gradients with wide temperature ranges, E. coli aggregated at intermediate temperatures, with maximal cell concentrations found away from the sidewalls. We observed this intermediate temperature aggregation in a motility buffer that did not contain any major chemoattractants of E. coli, in contradiction to some previous reports, which suggested that this type of aggregation required the presence of at least one major chemoattractant in the medium. Even more surprisingly, the aggregation temperature strongly depended on the gradient steepness, decreasing by ∼10° as the steepness was increased from 27 to 53°C/mm. Our experiments also highlight the fact that assessments of thermal gradients by changes in fluorescence of temperature-sensitive fluorescent dyes need to account for thermophoresis of the dyes.
细菌的化学趋性，即细菌在化学引诱物梯度中的定向迁移，是细胞生物学中研究得最透彻、理解得最深入的过程之一。另一方面，细菌的热趋性，即细菌在温度梯度中的定向迁移，相对来说理解得较少，不同的研究小组报告的结果有些相互矛盾。造成这种情况的原因之一是，产生明确的、足够陡峭的温度梯度来诱发易于检测的趋热性相对具有技术难度。在这里，我们使用专门设计的微流控装置，在较宽的温度梯度范围内以较高的数据采集速率研究大肠杆菌的热趋性。我们发现，在温度梯度较浅且温度范围较窄的情况下，大肠杆菌往往在梯度通道的侧壁附近聚集在最低或最高温度处。另一方面，在足够陡峭且温度范围较宽的梯度中，大肠杆菌聚集在中间温度处，最大的细胞浓度远离侧壁。我们在不含有大肠杆菌任何主要趋化剂的运动缓冲液中观察到这种中间温度聚集，这与一些先前的报告相矛盾，这些报告表明这种类型的聚集需要在培养基中存在至少一种主要趋化剂。更令人惊讶的是，聚集温度强烈依赖于梯度的陡峭程度，当陡峭程度从 27°C/mm 增加到 53°C/mm 时，聚集温度降低了约 10°C。我们的实验还突出了一个事实，即通过温度敏感荧光染料的荧光变化来评估温度梯度，需要考虑到染料的热泳。
