Mathematics, University of Tennessee, 1403 Circle Dr, Knoxville, TN, 37996, USA.
Biochemistry and Cellular & Molecular Biology, University of Tennessee, 1311 Cumberland Ave, Knoxville, TN, 37996, USA.
BMC Microbiol. 2019 May 17;19(1):101. doi: 10.1186/s12866-019-1468-9.
Bacterial chemotaxis, the ability of motile bacteria to navigate gradients of chemicals, plays key roles in the establishment of various plant-microbe associations, including those that benefit plant growth and crop productivity. The motile soil bacterium Azospirillum brasilense colonizes the rhizosphere and promotes the growth of diverse plants across a range of environments. Aerotaxis, or the ability to navigate oxygen gradients, is a widespread behavior in bacteria. It is one of the strongest behavioral responses in A. brasilense and it is essential for successful colonization of the root surface. Oxygen is one of the limiting nutrients in the rhizosphere where density and activity of organisms are greatest. The aerotaxis response of A. brasilense is also characterized by high precision with motile cells able to detect narrow regions in a gradient where the oxygen concentration is low enough to support their microaerobic lifestyle and metabolism.
Here, we present a mathematical model for aerotaxis band formation that captures most critical features of aerotaxis in A. brasilense. Remarkably, this model recapitulates experimental observations of the formation of a stable aerotactic band within 2 minutes of exposure to the air gradient that were not captured in previous modeling efforts. Using experimentally determined parameters, the mathematical model reproduced an aerotactic band at a distance from the meniscus and with a width that matched the experimental observation.
Including experimentally determined parameter values allowed us to validate a mathematical model for aerotactic band formation in spatial gradients that recapitulates the spatiotemporal stability of the band and its position in the gradient as well as its overall width. This validated model also allowed us to capture the range of oxygen concentrations the bacteria prefer during aerotaxis, and to estimate the effect of parameter values (e.g. oxygen consumption rate), both of which are difficult to obtain in experiments.
细菌的趋化性，即运动细菌在化学物质梯度中导航的能力，在各种植物-微生物共生关系的建立中起着关键作用，包括那些有益于植物生长和作物生产力的共生关系。游动土壤细菌巴西固氮螺菌定殖于根际并促进各种植物在广泛的环境中生长。趋氧性，即沿着氧气梯度导航的能力，是细菌中的一种广泛存在的行为。它是巴西固氮螺菌最强烈的行为反应之一，对于成功定殖根表面至关重要。氧气是根际中限制营养物质之一，那里的生物密度和活性最大。巴西固氮螺菌的趋氧反应也具有高精度，游动细胞能够检测到梯度中氧气浓度足够低以支持其微需氧生活方式和代谢的狭窄区域。
在这里，我们提出了一个用于趋氧带形成的数学模型，该模型捕获了巴西固氮螺菌趋氧性的大多数关键特征。值得注意的是，该模型再现了在暴露于空气梯度后 2 分钟内形成稳定趋氧带的实验观察结果，而这些结果在以前的建模工作中并未捕获。使用实验确定的参数，数学模型再现了在离弯月面一定距离处的趋氧带，并且其宽度与实验观察结果相匹配。
包括实验确定的参数值使我们能够验证一个用于空间梯度中趋氧带形成的数学模型，该模型再现了带的时空稳定性及其在梯度中的位置以及其整体宽度。该经过验证的模型还允许我们捕获细菌在趋氧过程中偏好的氧气浓度范围，并估计参数值（例如氧气消耗率）的影响，这些在实验中都很难获得。
