Cooling towers are a major source of large community-associated outbreaks of Legionnaires’ disease, a severe pneumonia. This disease is contracted when inhaling aerosols that are contaminated with bacteria from the genus Legionella, most importantly Legionella pneumophila. How cooling towers support the growth of this bacterium is still not well understood. As Legionella species are intracellular parasites of protozoa, it is assumed that protozoan community in cooling towers play an important role in Legionella ecology and outbreaks. However, the exact mechanism of how the eukaryotic community contributes to Legionella ecology is still unclear. Therefore, we used 18S rRNA gene amplicon sequencing to characterize the eukaryotic communities of 18 different cooling towers. The data from the eukaryotic community was then analysed with the bacterial community of the same towers in order to understand how each community could affect Legionella spp. ecology in cooling towers. We identified several microbial groups in the cooling tower ecosystem associated with Legionella spp. that suggest the presence of a microbial loop in these systems. Dissolved organic carbon was shown to be a major factor in shaping the eukaryotic community and may be an important factor for Legionella ecology. Network analysis, based on co-occurrence, revealed that Legionella was correlated with a number of different organisms. Out of these, the bacterial genus Brevundimonas and the ciliate class Oligohymenophorea were shown, through in vitro experiments, to stimulate the growth of L. pneumophila through direct and indirect mechanisms. Our results suggest that Legionella ecology depends on the host community, including ciliates and on several groups of organisms that contribute to its survival and growth in the cooling tower ecosystem. These findings further support the idea that some cooling tower microbiomes may promote the survival and growth of Legionella better than others.

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揭示真核生物和细菌群落的重要性及其与军团菌的关系。冷却塔中的生态：一个复杂的网络

冷却塔是大规模社区爆发退伍军人病（一种严重肺炎）的主要来源。当吸入被军团菌属细菌（最重要的是嗜肺军团菌）污染的气溶胶时，就会感染这种疾病。冷却塔如何支持这种细菌的生长仍不清楚。由于军团菌是原生动物的细胞内寄生虫，因此推测冷却塔中的原生动物群落在军团菌生态和爆发中发挥着重要作用。然而，真核生物群落如何促进军团菌生态的确切机制仍不清楚。因此，我们利用18S rRNA基因扩增子测序来表征18个不同冷却塔的真核生物群落。然后将来自真核生物群落的数据与同一塔的细菌群落进行分析，以了解每个群落如何影响军团菌。冷却塔的生态学。我们确定了冷却塔生态系统中与军团菌相关的几个微生物群。这表明这些系统中存在微生物循环。溶解的有机碳被证明是塑造真核生物群落的主要因素，并且可能是军团菌生态的重要因素。基于共现的网络分析表明，军团菌与许多不同的生物体相关。其中，通过体外实验表明，短波单胞菌属和纤毛虫类寡膜藻可以通过直接和间接机制刺激嗜肺军团菌的生长。我们的结果表明，军团菌生态取决于宿主群落，包括纤毛虫和有助于其在冷却塔生态系统中生存和生长的几类生物体。这些发现进一步支持了这样的观点，即某些冷却塔微生物组可能比其他微生物组更好地促进军团菌的生存和生长。