Microbial metabolomics: essential definitions and the importance of cultivation conditions for utilizing Bacillus species as bionematicides

Root-knot nematodes are destructive phytopathogens that damage agricultural crops globally, and there is growing interest in the use of biocontrol based on rhizobacteria such as Bacillus to combat Meloidogyne species. It is hypothesized that nematicidal activity of Bacillus can be attributed to the production of secondary metabolites and hydrolytic enzymes. Yet, few studies have characterized these metabolites and their identities remain unknown. Others are speculative or fail to elaborate on how secondary metabolites were detected or distinguished from primary metabolites. Metabolites can be classified based on their origin as either intracellular or extracellular and based on their function, as either primary or secondary. Although this classification is in general use, the boundaries are not always well defined. An understanding of the secondary metabolite and hydrolytic enzyme classification of Bacillus species will facilitate investigations aimed at bionematicide development. This review summarizes the significance of Bacillus hydrolytic enzymes and secondary metabolites in bionematicide research and provides an overview of known classifications. The importance of appropriate cultivation conditions for optimum metabolite and enzyme production is also discussed. Finally, the use of metabolomics for the detection and identification of nematicidal compounds is considered.