Mass lysis of predatory bacteria drives the enrichment of antibiotic resistance in soil microbial communities

Several studies have investigated the effects of antibiotics on the evolution and maintenance of antimicrobial resistance (AMR). However, the impact of microbial interactions in antibiotic-free environments on resistance in natural communities remains unclear. Here, we investigated whether the predatory bacterium M. xanthus, which can produce antimicrobials as a prey-killing mechanisms, influences the abundance of antibiotic-resistant bacteria in its local environment, regardless of active predation. We observed an association between the presence of M. xanthus in soil and the frequency of antibiotic-resistant bacteria. Additionally, culture-based and metagenomic analysis showed that co-culturing M. xanthus with soil-derived communities in liquid cultures enriched AMR among non-myxobacterial isolates. This is because the lysis of M. xanthus, triggered during the starvation phase of the co-culture experiments, releases diffusible growth inhibitory compounds that enrich pre-existing resistant bacteria. Furthermore, our results show that death during multicellular fruiting body formation, a starvation-induced stress response in M. xanthus that results in over 90% cell death, also releases growth inhibitory molecules that enrich resistant bacteria. Hence, the higher abundance of resistant bacteria in the soil communities, where M. xanthus can be detected, was because of the diffusible growth-inhibitory substances that were released due to the death of M. xanthus cells during fruiting body formation. Together, our findings demonstrate how the death of M. xanthus, an important aspect of its life cycle, can impact antibiotic resistomes in natural soil communities without the anthropogenic influx of antibiotics.已有多项研究探讨了抗生素对抗菌素耐药性（antimicrobial resistance, AMR）的演化与维持的影响。然而，无抗生素环境中的微生物互作对自然群落耐药性的影响仍不明晰。本研究旨在探究可通过产生抗菌物质作为猎物杀伤机制的捕食性细菌M. xanthus（Myxococcus xanthus），是否可在不依赖主动捕食的前提下，影响其局部环境中耐药菌的丰度。我们观察到土壤中M. xanthus的存在与耐药菌的检出频率存在关联。此外，基于培养的实验与宏基因组学分析（metagenomic analysis）显示，将M. xanthus与土壤来源群落共培养于液体培养基时，非黏球菌分离株的抗生素耐药性丰度得到富集。其背后机制为：共培养实验饥饿阶段触发的M. xanthus裂解，会释放可扩散的生长抑制性化合物，从而富集预先存在的耐药菌。进一步研究发现，M. xanthus受饥饿诱导的应激反应——多细胞子实体（fruiting body）形成过程中发生的细胞死亡，可导致超过90%的细胞死亡——同样会释放生长抑制性分子，进而富集耐药菌。因此，在可检出M. xanthus的土壤群落中，耐药菌丰度更高的原因，正是由于M. xanthus细胞在子实体形成过程中死亡所释放的可扩散生长抑制物质。综上，本研究结果阐明了M. xanthus生命周期中的重要环节——细胞死亡——如何在无抗生素人为输入的情况下，影响自然土壤群落中的抗生素抗性组（antibiotic resistomes）。