The Survival Strategy And Mechanism Of Nocardia Farcinica Responses To Caenorhabditis Elegants Predation

In the natural environment,bacteria,as one of the largest functional group,build up the fundamental ground for almost all known ecosystems.Bacterial life history is full of various stresses from the living environment.Of these stresses,predation is a major pressure for the survival of bacteria.Nematodes,one of the most important predators of bacteria,drive bacteria to develop a variety of mechanisms to reduce predation stress through coevolution process,including;infection mechanisms,secretion of toxins,and proteases.Recently,our laboratory has revealed that food bacteria reduce the predation pressure indirectly by secreting signaling molecules into the environment,mobilizing nematodes' natural predators to reduced predation stress.Soil bacteria and nematodes share the same niche.The study of the interaction between soil bacteria and nematodes is mainly focused on pathogenic bacteria,especially human pathogens.The defense mechanism of less pathogenic soil bacteria have used in reduction of predation stress remains unknown.In this study,we have found Nocardia farcinica leave the life spans of adult C.elegans unaffected.Specifically,the p38 MAPK pathway through its downstream transcription factor,SKN-1 mediates the tolerance of adult nematode to N.farcinica.The protection effects of p38 MAPK pathway able nematodes to utilize N.farcinica as normal diet for its entire life.Interestingly,eggs laid by adult nematode on N.farcinica unable to hatch.We found the products were secreted by N.farcinica after inducing by predation stress effectively inhibits nematodes' eggs from hatch.Induced product not only acts on down regulation of two mitochondrial unfold protein response chaperone hsp-6 and hsp-60 expression level,in turn cause mitochondria dysfunction,but also down-regulated the expression level of phase II detoxification gene gst-4 and autophagy flux.Mitochondria dysfunction and down regulation of gst-4 expression have caused ROS level increase sharply,in turn mediates lysosome expansion and dysfunction.Antioxidant vitamin D3 and a-tocopherol effectively preventing the rise of ROS,consequently the lysosome function have restored while treated with induced product.Correspondingly,the hatch rate of the nematodes' eggs has also recovered.In this study,we have proved the increase of ROS level mediates the lysosomes dysfunction,in turn to affect the hatch ability of nematodes' eggs.N.farcinica inhibits the hatch process of nematodes' eggs,as the result,the expansion of nematodes' population stopped since there is no offspring,in turn to efficiently lower the predation stress.This study reveals a novel mechanism which used by less pathogenic soil bacteria(N.farcinica)to conquer predation stress in nature.