Symbiotic Bacteria Regulating Insect–Insect/Fungus/Virus Mutualism

SIMPLE SUMMARY: Interactions between insects and symbiotic bacteria affect the behavior or biological characteristics of host insects, which in turn affect the intraspecific and interspecific relationships of insects. In this paper, we review multi-trophic interactions among symbiotic bacteria, insects, and their mutualistic partners, and the known or suspected mechanisms that modulate these interactions. The nutritional-, antifungal- and semiochemical-producing functions of symbiotic bacteria in several mutualism systems, as well as the modes of transmission of these bacteria, are summarized in detail. Finally, we also try to provide insights into future research directions. ABSTRACT: Bacteria associated with insects potentially provide many beneficial services and have been well documented. Mutualism that relates to insects is widespread in ecosystems. However, the interrelation between “symbiotic bacteria” and “mutualism” has rarely been studied. We introduce three systems of mutualism that relate to insects (ants and honeydew-producing Hemiptera, fungus-growing insects and fungi, and plant persistent viruses and vector insects) and review the species of symbiotic bacteria in host insects, as well as their functions in host insects and the mechanisms underlying mutualism regulation. A deeper understanding of the molecular mechanisms and role of symbiotic bacteria, based on metagenomics, transcriptomics, proteomics, metabolomics, and microbiology, will be required for describing the entire interaction network.