Inhibition of symbiote fungus of the leaf cutter ant Atta sexdens by secondary metabolites from the bacterium Xenorhabdus szentirmaii associated with entomopathogenic nematodes

ABSTRACT: Leaf-cutter ants (Hymenoptera: Formicidae) have evolved as dominant herbivores on the American continent. These social insects remove the leaves of economically important plant species to maintain their colony’s food reserves, the symbiotic fungus Leucocoprinus gongylophorus, a basidiomycete. Such fungus can be used for applications of fungicide molecules from metabolites generated by symbiont bacteria (Xenorhabdus and Photorhabdus) from entomopathogenic nematodes (Steinernema and Heterorhabditis). Through isolation and multiplication in tryptic soy broth (TSB) medium of the bacteria Xenorhabdus szentirmaii isolated PAM 25, we conducted laboratorial tests using treatments with 10, 25, and 50% of the metabolites obtained in the sixth day of cultivation. The treatments were centrifuged and filtered to generate a supernatant, which was diluted in potato + dextrose + agar (PDA), to verify the consequences of exposure to the fungus L. gongylophorus in Petri dishes. To confirm metabolite efficiency, the control treatments in PDA only and mixed (PDA+TSB) media were conducted simultaneously for 14 days. We observed total inhibition of the symbiont fungus in both the 25 and 50% dilutions during the first days of the tests. Our results support that these metabolites have inhibitory effect on the development of symbiont fungus of leaf-cutter ants.

摘要：切叶蚁（Leaf-cutter ants）隶属于膜翅目（Hymenoptera）蚁科（Formicidae），已演化成为美洲大陆的优势植食性类群。这类社会性昆虫会采集具有经济价值的植物叶片，用以维系其蚁群的食物储备——共生真菌（Leucocoprinus gongylophorus），该真菌隶属于担子菌（basidiomycete）。此类真菌可被用于依托昆虫病原线虫（Steinernema与Heterorhabditis）的共生细菌致病杆菌属（Xenorhabdus）与光杆状菌属（Photorhabdus）所产生的代谢物开发杀菌剂分子的相关研究与应用。本研究通过在胰蛋白胨大豆肉汤（TSB）培养基中分离培养菌株PAM 25——即致病杆菌Xenorhabdus szentirmaii——，设置10%、25%与50%三个梯度的培养第6天所得代谢物处理组开展室内实验。将处理组样本经离心、过滤后获取上清液，以马铃薯葡萄糖琼脂（PDA）进行稀释，随后在培养皿（Petri dishes）中接种共生真菌L. gongylophorus，以观测该真菌在代谢物暴露条件下的生长变化。为验证代谢物的抑菌效能，本研究同步设置仅含PDA的空白对照组与PDA+TSB混合培养基对照组，所有组别均培养14天。实验初期即可观测到25%与50%稀释梯度的处理组对该共生真菌实现了完全抑制。本研究结果证实，上述代谢物对切叶蚁共生真菌的生长具有显著抑制作用。