Interactions of Free-Living Nematodes and Associated Microorganisms with Plant-Parasitic Nematodes

Soil nematode communities comprise different trophic groups, including herbivores, bacterivores, fungivores, omnivores, and predators. Aim of this review is to discuss the relationship linking free-living and plant-parasitic nematodes with their associated soil bacteria. Nematodes and bacteria mutual relationships in food webs include pathogenic bacteria transmission, pest control, role in sustainable agriculture and healthy soil, and the regulation of harmful nematodes. Plant-parasitic and free-living nematodes (bacterivores and fungivores) are the most influential groups observed in most agricultural systems. Free-living species are essential in soil nutrient cycling through mineralization and are a resource in sustainable agriculture. They spread and transmit bacteria, and some of them, such as Steinernema and Heterorhabditis, bear symbiont species with pest control capabilities. Metabarcoding analyses showed a complex of bacteria (e.g., Pedobacter) associated with free-living bacterivores nematodes. The association of the free-living nematodes with bacteria can impact the reproduction of plant-parasitic nematodes in soil. Increasing the soil microbial component through decomposition and natural plants in uncultivated soil is associated to increasing numbers of free-living nematodes. However, entomopathogenic species may suppress plant-parasitic nematodes or have no effect on their density in the agricultural system. Free-living bacterivores (e.g., Panagrellus redivivus), omnivores (e.g., Aporcelaimellus), and fungivores (e.g., Aphelenchus avenae) are considered as indicators of soil health in the various natural and agricultural systems. In conclusion, free-living nematodes and their associated microbiome play a critical role in nutrient cycling with an indirect potential in the regulation of plant-parasitic nematodes.