Nocardia Farcinica Can Produce Short Branched-Chain Fatty Acids To Inhibit The Hatching Of Eggs In C.elegans

In the natural environment,bacteria and Caenorhabditis elegans,as important parts of soil organisms,can produce a variety of interactions.C.elegans can feed on different kinds of bacteria.Meanwhile,these food bacteria adapt different ways to cope with the pressure of prey.For instance,our previous study has reavealed that certain bacteria can mobilize nematode-trapping fungi to reduce predation pressure.Previous studies in our laboratory showed that the C.elegans could normally grow fed Nocardia farcinica.However,the eggs laid on a plate containing N.farcinica did not hatch as normal.N.farcinica induced by C.elegans could produce some secondary metabolite,which could inhibit egg hatching.However,the active ingredients from the induced products are still unknown.In this study,we isolated and identified active compounds that inhibited egg hatching.We found that the pH of the culture medium induced by the C.elegans was significantly lowered.By changing the pH of the fermentation broth induced by C.elegans,the inhibitory activity of egg hatching was decreased with the increase of pH value.It was presumed that the active compounds were probably acidic substances.By analyzing the transcriptome of N.farcinica,we found that the expression of multiple metabolic pathway genes in N.farcinica subjected to the predation pressure by C.elegans.GO and KEGG analysis revealed that multiple pathways related to fatty acid and amino acid metabolism were significantly up-regulated.Then,we used LC-MS to trace the active compounds in the induced products.These components of N.farcinica were significantly changed after induction by C.elegans,especially short branched-chain fatty acids.Of these fatty acids,isobutyric acid,3-methylbutyric acid,and 2-methylbutyric acid were capable of inhibiting egg hatching.However,the inhibitory effects of one of single fatty acids were less than that of the induced products.Then we determined the contents of these three fatty acids.We found that the mixure of isobutyric acid,3-methylbutyric acid and 2-methylbutyric acid according to their ratios in the induced products could more strongly inhibited egg hatching than one of single fatty acids.Finally,treatment of these fatty acids significantly induced the production of reactive oxygen species(ROS),while the expression level of gst-4,which was an important antioxidant and detoxicification gene was markedly decreased.These results indicate that egg non-hatching caused by these fatty acids is probably related to increased oxidative stress.In conclusion,our results reveal a bacterial defense model against predation pressure induced by worms.