The Effect of Phylogenetically Different Bacteria on the Fitness of Pseudomonas fluorescens in Sand Microcosms

In most environments many microorganisms live in close vicinity and can interact in various ways. Recent studies suggest that bacteria are able to sense and respond to the presence of neighbouring bacteria in the environment and alter their response accordingly. This ability might be an important strategy in complex habitats such as soils, with great implications for shaping the microbial community structure. Here, we used a sand microcosm approach to investigate how Pseudomonas fluorescens Pf0-1 responds to the presence of monocultures or mixtures of two phylogenetically different bacteria, a Gram-negative (Pedobacter sp. V48) and a Gram-positive (Bacillus sp. V102) under two nutrient conditions. Results revealed that under both nutrient poor and nutrient rich conditions confrontation with the Gram-positive Bacillus sp. V102 strain led to significant lower cell numbers of Pseudomonas fluorescens Pf0-1, whereas confrontation with the Gram-negative Pedobacter sp. V48 strain did not affect the growth of Pseudomonas fluorescens Pf0-1. However, when Pseudomonas fluorescens Pf0-1 was confronted with the mixture of both strains, no significant effect on the growth of Pseudomonas fluorescens Pf0-1 was observed. Quantitative real-time PCR data showed up-regulation of genes involved in the production of a broad-spectrum antibiotic in Pseudomonas fluorescens Pf0-1 when confronted with Pedobacter sp. V48, but not in the presence of Bacillus sp. V102. The results provide evidence that the performance of bacteria in soil depends strongly on the identity of neighbouring bacteria and that inter-specific interactions are an important factor in determining microbial community structure.在绝大多数自然环境中，大量微生物紧密毗邻生存，并可通过多种方式发生相互作用。近期研究表明，细菌能够感知环境中邻近其他细菌的存在，并据此调整自身的响应策略。该能力在土壤等复杂生境中或为关键生存策略，对微生物群落结构的塑造具有重要意义。本研究采用砂基微宇宙（sand microcosm）实验体系，探究在两种营养条件下，荧光假单胞菌Pf0-1（Pseudomonas fluorescens Pf0-1）如何响应两种系统发育迥异的细菌——革兰氏阴性（Gram-negative）的Pedobacter sp. V48与革兰氏阳性（Gram-positive）的Bacillus sp. V102——的单一菌株培养物或混合培养物的存在。实验结果显示，在贫营养与富营养两种条件下，当荧光假单胞菌Pf0-1与革兰氏阳性的Bacillus sp. V102菌株共培养时，其细胞数量均显著下降；而与革兰氏阴性的Pedobacter sp. V48菌株共培养时，其生长未受显著影响。不过，当荧光假单胞菌Pf0-1与两种菌株的混合培养物共培养时，其生长未出现显著变化。实时定量PCR（Quantitative real-time PCR）数据显示，当荧光假单胞菌Pf0-1与Pedobacter sp. V48共培养时，其参与广谱抗生素合成的基因呈现上调表达；而在与Bacillus sp. V102共培养时则无此现象。本研究结果证实，土壤中细菌的生存与生长状况极大程度上取决于邻近细菌的种类，且种间相互作用是决定微生物群落结构的关键因素之一。