Bacterial ectosymbionts in cuticular organs chemically protect a beetle during molting stages

With a little help from my friends: the roles of microbial symbionts in insect populations and communities

To understand insect abundance, distribution and dynamics, we need to understand the relevant drivers of their populations and communities. While microbial symbionts are known to strongly affect many aspects of insect biology, we lack data on their effects on populations or community processes, or on insects' evolutionary responses at different timescales. How these effects change as the anthropogenic effects on ecosystems intensify is an area of intense research. Recent developments in sequencing and bioinformatics permit cost-effective microbial diversity surveys, tracking symbiont transmission, and identification of functions across insect populations and multi-species communities. In this review, we explore how different functional categories of symbionts can influence insect life-history traits, how these effects could affect insect populations and their interactions with other species, and how they may affect processes and patterns at the level of entire communities. We argue that insect-associated microbes should be considered important drivers of insect response and adaptation to environmental challenges and opportunities. We also outline the emerging approaches for surveying and characterizing insect-associated microbiota at population and community scales. This article is part of the theme issue 'Towards a toolkit for global insect biodiversity monitoring'.

The insect microbiome is a vast source of bioactive small molecules

Covering: September 1964 to June 2023Bacteria and fungi living in symbiosis with insects have been studied over the last sixty years and found to be important sources of bioactive natural products. Not only classic producers of secondary metabolites such as Streptomyces and other members of the phylum Actinobacteria but also numerous bacteria from the phyla Proteobacteria and Firmicutes and an impressive array of fungi (usually pathogenic) serve as the source of a structurally diverse number of small molecules with important biological activities including antimicrobial, cytotoxic, antiparasitic and specific enzyme inhibitors. The insect niche is often the exclusive provider of microbes producing unique types of biologically active compounds such as gerumycins, pederin, dinactin, and formicamycins. However, numerous insects still have not been described taxonomically, and in most cases, the study of their microbiota is completely unexplored. In this review, we present a comprehensive survey of 553 natural products produced by microorganisms isolated from insects by collating and classifying all the data according to the type of compound (rather than the insect or microbial source). The analysis of the correlations among the metadata related to insects, microbial partners, and their produced compounds provides valuable insights into the intricate dynamics between insects and their symbionts as well as the impact of their metabolites on these relationships. Herein, we focus on the chemical structure, biosynthesis, and biological activities of the most relevant compounds.

Antimicrobial resistance in the wild: Insights from epigenetics

Spreading of bacterial and fungal strains that are resistant to antimicrobials poses a serious threat to the well-being of humans, animals, and plants. Antimicrobial resistance has been mainly investigated in clinical settings. However, throughout their evolutionary history microorganisms in the wild have encountered antimicrobial substances, forcing them to evolve strategies to combat antimicrobial action. It is well known that many of these strategies are based on genetic mechanisms, but these do not fully explain important aspects of the antimicrobial response such as the rapid development of resistance, reversible phenotypes, and hetero-resistance. Consequently, attention has turned toward epigenetic pathways that may offer additional insights into antimicrobial mechanisms. The aim of this review is to explore the epigenetic mechanisms that confer antimicrobial resistance, focusing on those that might be relevant for resistance in the wild. First, we examine the presence of antimicrobials in natural settings. Then we describe the documented epigenetic mechanisms in bacteria and fungi associated with antimicrobial resistance and discuss innovative epigenetic editing techniques to establish causality in this context. Finally, we discuss the relevance of these epigenetic mechanisms on the evolutionary dynamics of antimicrobial resistance in the wild, emphasizing the critical role of priming in the adaptation process. We underscore the necessity of incorporating non-genetic mechanisms into our understanding of antimicrobial resistance evolution. These mechanisms offer invaluable insights into the dynamics of antimicrobial adaptation within natural ecosystems.

Fungal infection of insects: molecular insights and prospects

Entomopathogenic fungi (EPF) distribute in different fungal phyla with variable host ranges and play essential role in regulating insect populations by infecting hosts via cuticle penetration. The representative ascomycete EPF of Metarhizium and Beauveria species have been widely used in mechanistic investigations of fungus-insect interactions and as ecofriendly mycoinsecticides. Here, we review the function of diverse genes, pathways, and secondary metabolites associated with EPF stepwise infections. In particular, emerging evidence has shown that EPF have to outcompete insect ectomicrobiotas prior to penetrating cuticles, and subvert or evade host antifungal immunity by using effector-like proteins and chemicals like plant pathogens. Future prospects are discussed for a better understanding of fungal pathobiology, which will provide novel insights into microbe-animal interactions.

From phyllosphere to insect cuticles: silkworms gather antifungal bacteria from mulberry leaves to battle fungal parasite attacks

BACKGROUND

Bacterial transfers from plants to insect herbivore guts have been well investigated. However, bacterial exchanges between plant phyllospheres and insect cuticles remain unclear, as does their related biological function.

RESULTS

Here, we report that the cuticular bacterial loads of silkworm larvae quickly increased after molting and feeding on the white mulberry (Morus alba) leaves. The isolation and examination of silkworm cuticular bacteria identified one bacterium Mammaliicoccus sciuri that could completely inhibit the spore germination of fungal entomopathogens Metarhizium robertsii and Beauveria bassiana. Interestingly, Ma. sciuri was evident originally from mulberry leaves, which could produce a secreted chitinolytic lysozyme (termed Msp1) to damage fungal cell walls. In consistency, the deletion of Msp1 substantially impaired bacterial antifungal activity. Pretreating silkworm larvae with Ma. sciuri cells followed by fungal topical infections revealed that this bacterium could help defend silkworms against fungal infections. Unsurprisingly, the protective efficacy of ΔMsp1 was considerably reduced when compared with that of wild-type bacterium. Administration of bacterium-treated diets had no negative effect on silkworm development; instead, bacterial supplementation could protect the artificial diet from Aspergillus contamination.

CONCLUSIONS

The results of this study evidence that the cross-kingdom transfer of bacteria from plant phyllospheres to insect herbivore cuticles can help protect insects against fungal parasite attacks. Video Abstract.

Developmental Shifts in the Microbiome of a Cosmopolitan Pest: Unraveling the Role of Wolbachia and Dominant Bacteria

Wolbachia bacteria (phylum Proteobacteria) are ubiquitous intracellular parasites of diverse invertebrates. In insects, coevolution has forged mutualistic associations with Wolbachia species, influencing reproduction, immunity, development, pathogen resistance, and overall fitness. However, the impact of Wolbachia on other microbial associates within the insect microbiome, which are crucial for host fitness, remains less explored. The diamondback moth (Plutella xylostella), a major pest of cruciferous vegetables worldwide, harbors the dominant Wolbachia strain plutWB1, known to distort its sex ratio. This study investigated the bacterial community diversity and dynamics across different developmental life stages and Wolbachia infection states in P. xylostella using high-throughput 16S rDNA amplicon sequencing. Proteobacteria and Firmicutes dominated the P. xylostella microbiome regardless of life stage or Wolbachia infection. However, the relative abundance of dominant genera, including an unclassified genus of Enterobacteriaceae, Wolbachia, Carnobacterium, and Delftia tsuruhatensis, displayed significant stage-specific variations. While significant differences in bacterial diversity and composition were observed across life stages, Wolbachia infection had no substantial impact on overall diversity. Nonetheless, relative abundances of specific genera differed between infection states. Notably, Wolbachia exhibited a stable, high relative abundance across all stages and negatively correlated with an unclassified genus of Enterobacteriaceae, Delftia tsuruhatensis, and Carnobacterium. Our findings provide a foundational understanding of the complex interplay between the host, Wolbachia, and the associated microbiome in P. xylostella, paving the way for a deeper understanding of their complex interactions and potential implications for pest control strategies.

Fights on the surface prior to fungal invasion of insects

Entomopathogenic fungi (EPF) infect insects by landing on and penetrating cuticles. Emerging evidence has shown that, prior to the invasion of insects, fungal cells have to battle and overcome diverse challenges, including the host behavioral defenses, colonization resistance mediated by ectomicrobiotas, host recognition, and generation of enough penetration pressure. The ascomycete EPF such as Metarhizium and Beauveria can thus produce adhesive proteins and/or the exopolysaccharide mucilage to tightly glue fungal cells on cuticles. Producing antimicrobial peptides and chemical compounds can enable EPF to outcompete cuticular defensive microbes. The use of divergent membrane receptors, accumulation, and quick degradation of lipid droplets in conidial cells can help EPF recognize proper hosts and build up cellular turgor to breach cuticles for systematic invasion. Further investigations are still required to unveil the multifaceted and intricate relationships between EPF and insect hosts.

A life-and-death struggle: interaction of insects with entomopathogenic fungi across various infection stages

Insects constitute approximately 75% of the world's recognized fauna, with the majority of species considered as pests. Entomopathogenic fungi (EPF) are parasitic microorganisms capable of efficiently infecting insects, rendering them potent biopesticides. In response to infections, insects have evolved diverse defense mechanisms, prompting EPF to develop a variety of strategies to overcome or circumvent host defenses. While the interaction mechanisms between EPF and insects is well established, recent findings underscore that their interplay is more intricate than previously thought, especially evident across different stages of EPF infection. This review primarily focuses on the interplay between EPF and the insect defense strategies, centered around three infection stages: (1) Early infection stage: involving the pre-contact detection and avoidance behavior of EPF in insects, along with the induction of behavioral responses upon contact with the host cuticle; (2) Penetration and intra-hemolymph growth stage: involving the initiation of intricate cellular and humoral immune functions in insects, while symbiotic microbes can further contribute to host resistance; (3) Host insect's death stage: involving the ultimate confrontation between pathogens and insects. Infected insects strive to separate themselves from the healthy population, while pathogens rely on the infected insects to spread to new hosts. Also, we discuss a novel pest management strategy underlying the cooperation between EPF infection and disturbing the insect immune system. By enhancing our understanding of the intricate interplay between EPF and the insect, this review provides novel perspectives for EPF-mediated pest management and developing effective fungal insecticides.

Extracellular symbiont colonizes insect during embryo development

Insects typically acquire their beneficial microbes early in development. Endosymbionts housed intracellularly are commonly integrated during oogenesis or embryogenesis, whereas extracellular microbes are only known to be acquired after hatching by immature instars such as larvae or nymphs. Here, however, we report on an extracellular symbiont that colonizes its host during embryo development. Tortoise beetles (Chrysomelidae: Cassidinae) host their digestive bacterial symbiont Stammera extracellularly within foregut symbiotic organs and in ovary-associated glands to ensure its vertical transmission. We outline the initial stages of symbiont colonization and observe that although the foregut symbiotic organs develop 3 days prior to larval emergence, they remain empty until the final 24 h of embryo development. Infection by Stammera occurs during that timeframe and prior to hatching. By experimentally manipulating symbiont availability to embryos in the egg, we describe a 12-h developmental window governing colonization by Stammera. Symbiotic organs form normally in aposymbiotic larvae, demonstrating that these Stammera-bearing structures develop autonomously. In adults, the foregut symbiotic organs are already colonized following metamorphosis and host a stable Stammera population to facilitate folivory. The ovary-associated glands, however, initially lack Stammera. Symbiont abundance subsequently increases within these transmission organs, thereby ensuring sufficient titers at the onset of oviposition ~29 days following metamorphosis. Collectively, our findings reveal that Stammera colonization precedes larval emergence, where its proliferation is eventually decoupled in adult beetles to match the nutritional and reproductive requirements of its host.

Interplay Between Antimicrobial Resistance and Global Environmental Change

Antibiotic resistance genes predate the therapeutic uses of antibiotics. However, the current antimicrobial resistance crisis stems from our extensive use of antibiotics and the generation of environmental stressors that impose new selective pressure on microbes and drive the evolution of resistant pathogens that now threaten human health. Similar to climate change, this global threat results from human activities that change habitats and natural microbiomes, which in turn interact with human-associated ecosystems and lead to adverse impacts on human health. Human activities that alter our planet at global scales exacerbate the current resistance crisis and exemplify our central role in large-scale changes in which we are both protagonists and architects of our success but also casualties of unanticipated collateral outcomes. As cognizant participants in this ongoing planetary experiment, we are driven to understand and find strategies to curb the ongoing crises of resistance and climate change.

Impact of intraspecific variation in insect microbiomes on host phenotype and evolution

Microbes can be an important source of phenotypic plasticity in insects. Insect physiology, behaviour, and ecology are influenced by individual variation in the microbial communities held within the insect gut, reproductive organs, bacteriome, and other tissues. It is becoming increasingly clear how important the insect microbiome is for insect fitness, expansion into novel ecological niches, and novel environments. These investigations have garnered heightened interest recently, yet a comprehensive understanding of how intraspecific variation in the assembly and function of these insect-associated microbial communities can shape the plasticity of insects is still lacking. Most research focuses on the core microbiome associated with a species of interest and ignores intraspecific variation. We argue that microbiome variation among insects can be an important driver of evolution, and we provide examples showing how such variation can influence fitness and health of insects, insect invasions, their persistence in new environments, and their responses to global environmental changes. A and B are two stages of an individual or a population of the same species. The drivers lead to a shift in the insect associated microbial community, which has consequences for the host. The complex interplay of those consequences affects insect adaptation and evolution and influences insect population resilience or invasion.

Fungus-insect interactions beyond bilateral regimes: the importance and strategy to outcompete host ectomicrobiomes by fungal parasites

Fungus-insect interactions have long been investigated at the bilateral level to unveil the factors involved in mediating fungal entomopathogenicity and insect antifungal immunity. Emerging evidence has shown that insect cuticles are inhabited by different bacteria that can delay and deter fungal parasite infections. Entomopathogenic fungi (EPF), however, have evolved strategies to combat the colonization resistance mediated by insect ectomicrobiomes by producing antimicrobial peptides or antibiotic compounds. Deprivation of micronutrients may also be employed by EPF to counteract the ectomicrobiome antagonism. Further investigations of insect ectomicrobiome assemblage and fungal factors involved in outcompeting cuticular microbiomes may benefit the development of cost-effective mycoinsecticides while protecting ecologically and economically important insect species.

Colonization dynamics of a defensive insect ectosymbiont

Beneficial symbionts are horizontally or vertically transmitted to offspring, relying on host- or microbe-mediated mechanisms for colonization. While multiple studies on symbionts transmitted internally or by feeding highlight host adaptations and dynamics of symbiont colonization, less is known for beneficial microbes colonizing host external surfaces, such as the insect cuticle. Here, we investigate the colonization dynamics of a bacterial symbiont that protects eggs and larvae of Lagria villosa beetles against pathogens. After maternal application to the egg surface, symbionts colonize specialized cuticular invaginations on the dorsal surface of larvae. We assessed the colonization time point and investigated the involvement of the host during this process. Symbionts remain on the egg surface before hatching, providing protection. Immediately after hatching, cells from the egg surface colonize the larvae and horizontal acquisition can occur, yet efficiency decreases with increasing larval age. Additionally, passive or host-aided translocation likely supports colonization of the larval symbiotic organs. This may be especially important for the dominant non-motile symbiont strain, while motility of additional strains in the symbiont community might also play a role. Our findings provide insights into the colonization dynamics of cuticle-associated defensive symbionts and suggest alternate or complementary strategies used by different strains for colonization.

"Ectomosphere": Insects and Microorganism Interactions

This study focuses on interacting with insects and their ectosymbiont (lato sensu) microorganisms for environmentally safe plant production and protection. Some cases help compare ectosymbiont microorganisms that are insect-borne, -driven, or -spread relevant to endosymbionts' behaviour. Ectosymbiotic bacteria can interact with insects by allowing them to improve the value of their pabula. In addition, some bacteria are essential for creating ecological niches that can host the development of pests. Insect-borne plant pathogens include bacteria, viruses, and fungi. These pathogens interact with their vectors to enhance reciprocal fitness. Knowing vector-phoront interaction could considerably increase chances for outbreak management, notably when sustained by quarantine vector ectosymbiont pathogens, such as the actual Xylella fastidiosa Mediterranean invasion episode. Insect pathogenic viruses have a close evolutionary relationship with their hosts, also being highly specific and obligate parasites. Sixteen virus families have been reported to infect insects and may be involved in the biological control of specific pests, including some economic weevils. Insects and fungi are among the most widespread organisms in nature and interact with each other, establishing symbiotic relationships ranging from mutualism to antagonism. The associations can influence the extent to which interacting organisms can exert their effects on plants and the proper management practices. Sustainable pest management also relies on entomopathogenic fungi; research on these species starts from their isolation from insect carcasses, followed by identification using conventional light or electron microscopy techniques. Thanks to the development of omics sciences, it is possible to identify entomopathogenic fungi with evolutionary histories that are less-shared with the target insect and can be proposed as pest antagonists. Many interesting omics can help detect the presence of entomopathogens in different natural matrices, such as soil or plants. The same techniques will help localize ectosymbionts, localization of recesses, or specialized morphological adaptation, greatly supporting the robust interpretation of the symbiont role. The manipulation and modulation of ectosymbionts could be a more promising way to counteract pests and borne pathogens, mitigating the impact of formulates and reducing food insecurity due to the lesser impact of direct damage and diseases. The promise has a preventive intent for more manageable and broader implications for pests, comparing what we can obtain using simpler, less-specific techniques and a less comprehensive approach to Integrated Pest Management (IPM).

Evolution and ontogeny of bacteriocytes in insects

The ontogenetic origins of the bacteriocytes, which are cells that harbour bacterial intracellular endosymbionts in multicellular animals, are unknown. During embryonic development, a series of morphological and transcriptional changes determine the fate of distinct cell types. The ontogeny of bacteriocytes is intimately linked with the evolutionary transition of endosymbionts from an extracellular to an intracellular environment, which in turn is linked to the diet of the host insect. Here we review the evolution and development of bacteriocytes in insects. We first classify the endosymbiotic occupants of bacteriocytes, highlighting the complex challenges they pose to the host. Then, we recall the historical account of the discovery of bacteriocytes. We then summarize the molecular interactions between the endosymbiont and the host. In addition, we illustrate the genetic contexts in which the bacteriocytes develop, with examples of the genetic changes in the hosts and endosymbionts, during specific endosymbiotic associations. We finally address the evolutionary origin as well as the putative ontogenetic or developmental source of bacteriocytes in insects.