Thermal stress responses of <i>Sodalis glossinidius</i>, an indigenous bacterial symbiont of hematophagous tsetse flies

Tsetse flies (Diptera: Glossinidae) house a taxonomically diverse microbiota that includes environmentally acquired bacteria, maternally transmitted symbiotic bacteria, and pathogenic African trypanosomes. Sodalis glossinidius, which is a facultative symbiont that resides intra and extracellularly within multiple tsetse tissues, has been implicated as a mediator of trypanosome infection establishment in the fly’s gut. Tsetse’s gut-associated population of Sodalis are subjected to marked temperature fluctuations each time their ectothermic fly host imbibes vertebrate blood. The molecular mechanisms that Sodalis employs to deal with this heat stress are unknown. In this study, we examined the thermal tolerance and heat shock response of Sodalis. When grown on BHI agar plates, the bacterium exhibited the most prolific growth at 25oC, and did not grow at temperatures above 30oC. Growth on BHI agar plates at 31°C was dependent on either the addition of blood to the agar or reduction in oxygen levels. Sodalis was viable in liquid cultures for 24 hours at 30oC, but began to die upon further exposure. The rate of death increased with increased temperature. Similarly, Sodalis was able to survive for 48 hours within tsetse flies housed at 30oC, while a higher temperature (37oC) was lethal. Sodalis’ genome contains homologues of the heat shock chaperone protein-encoding genes dnaK, dnaJ, and grpE, and their expression was up-regulated in thermally stressed Sodalis, both in vitro and in vivo within tsetse fly midguts. Arrested growth of E. coli dnaK, dnaJ, or grpE mutants under thermal stress was reversed when the cells were transformed with a low copy plasmid that encoded the Sodalis homologues of these genes. The information contained in this study provides insight into how arthropod vector enteric commensals, many of which mediate their host’s ability to transmit pathogens, mitigate heat shock associated with the ingestion of a blood meal.舌蝇（双翅目：舌蝇科）宿主内拥有分类学多样性丰富的微生物群，涵盖环境获取的细菌、母系传递的共生细菌以及致病性非洲锥虫。索氏血杆菌（Sodalis glossinidius）是一种兼性共生菌，可在舌蝇多种组织的胞内与胞外定殖，被认为可介导锥虫在蝇类肠道中的感染定殖。与舌蝇肠道相关的索氏血杆菌种群，在其变温宿主摄取脊椎动物血液时，会经历显著的温度波动。目前学界尚未明确索氏血杆菌应对此类热胁迫的分子机制。本研究针对索氏血杆菌的耐热性与热休克反应展开了探究。当在脑心浸液（BHI）琼脂平板上培养时，该菌在25℃下生长最为旺盛，而在30℃以上温度下无法生长；在31℃下于BHI琼脂平板上培养时，该菌的生长依赖于向琼脂中添加血液，或是降低培养环境的氧含量。索氏血杆菌在30℃的液体培养基中可存活24小时，进一步暴露于该温度下则会开始死亡，且死亡率随温度升高而上升。类似地，在饲养于30℃环境下的舌蝇体内，索氏血杆菌可存活48小时，而更高的温度（37℃）则对其具有致死性。索氏血杆菌的基因组中存在热休克伴侣蛋白编码基因dnaK、dnaJ与grpE的同源基因；在体外培养以及舌蝇中肠体内的热胁迫条件下，这些同源基因的表达均会上调。将编码索氏血杆菌上述同源基因的低拷贝质粒转化至大肠杆菌（E. coli）dnaK、dnaJ或grpE突变株后，突变株在热胁迫下停滞的生长状态得以恢复。本研究的结果为节肢动物媒介肠道共生菌（其中多数可介导宿主传播病原体）如何缓解因摄取血液而产生的热胁迫提供了新的认知视角。