Influences of a Prolific Gut Fungus (Zancudomyces culisetae) on Larval and Adult Mosquito (Aedes aegypti)-Associated Microbiota

A New Overview of Sex Bias in Fungal Infections

Fungal infections often disproportionately affect males over females. Since the NIH mandated in 2016 that researchers test their hypotheses in both biological sexes, numerous other fungal infections/colonizations have been found to exhibit sex-specific patterns. These patterns have been observed in various species, including mice, drosophila, cats, and bats, suggesting significant implications for understanding these diseases and developing treatments. Despite the recognition of this sex bias, primary research explaining its underlying causes or mechanisms remains limited. Current evidence suggests that potential causes might be linked to sex hormones, genetic expression, and evolutionary behaviors. This review consolidates recent data on sex bias in fungal infections or colonizations among different species and proposes future research directions to address existing gaps. Thus, this review advances the comprehension of the intricate relationships between biological sex, fungal infections, and broader health implications.

Microplastic ingestion perturbs the microbiome of Aedes albopictus (Diptera: Culicidae) and Aedes aegypti

Microplastics (MPs) are common environmental pollutants; however, little is known about their effects after ingestion by insects. Here we fed Aedes (Stegomyia) aegypti (L.) and Aedes (Stegomyia) albopictus (Skuse) mosquito larvae 1 µm polystyrene MPs and examined the impacts of ingestion on adult emergence rates, gut damage, and fungal and bacterial microbiota. Results show that MPs accumulate in the larval guts, resulting in gut damage. However, little impact on adult emergence rates was observed. MPs are also found in adult guts postemergence from the pupal stage, and adults expel MPs in their frass after obtaining sugar meals. Moreover, MPs effects on insect microbiomes need to be better defined. To address this knowledge gap, we investigated the relationship between MP ingestion and the microbial communities in Ae. albopictus and Ae. aegypti. The microbiota composition was altered by the ingestion of increasing concentrations of MPs. Amplicon sequence variants (ASVs) that contributed to differences in the bacterial and fungal microbiota composition between MP treatments were from the genera Elizabethkingia and Aspergillus, respectively. Furthermore, a decrease in the alpha diversity of the fungal and bacterial microbiota was observed in treatments where larvae ingested MPs. These results highlight the potential for the bacterial and fungal constituents in the mosquito microbiome to respond differently to the ingestion of MPs. Based on our findings and the effects of MP ingestion on the mosquito host micro- and mycobiome, MP pollution could impact the vector competence of important mosquito-transmitted viruses and parasites that cause human and animal diseases.

Microbiota in disease-transmitting vectors

Haematophagous arthropods, including mosquitoes, ticks, flies, triatomine bugs and lice (here referred to as vectors), are involved in the transmission of various pathogens to mammals on whom they blood feed. The diseases caused by these pathogens, collectively known as vector-borne diseases (VBDs), threaten the health of humans and animals. Although the vector arthropods differ in life histories, feeding behaviour as well as reproductive strategies, they all harbour symbiotic microorganisms, known as microbiota, on which they depend for completing essential aspects of their biology, such as development and reproduction. In this Review, we summarize the shared and unique key features of the symbiotic associations that have been characterized in the major vector taxa. We discuss the crosstalks between microbiota and their arthropod hosts that influence vector metabolism and immune responses relevant for pathogen transmission success, known as vector competence. Finally, we highlight how current knowledge on symbiotic associations is being explored to develop non-chemical-based alternative control methods that aim to reduce vector populations, or reduce vector competence. We conclude by highlighting the remaining knowledge gaps that stand to advance basic and translational aspects of vector-microbiota interactions.

Role of the Microbiome in Aedes spp. Vector Competence: What Do We Know?

Aedes aegypti and Aedes albopictus are the vectors of important arboviruses: dengue fever, chikungunya, Zika, and yellow fever. Female mosquitoes acquire arboviruses by feeding on the infected host blood, thus being able to transmit it to their offspring. The intrinsic ability of a vector to infect itself and transmit a pathogen is known as vector competence. Several factors influence the susceptibility of these females to be infected by these arboviruses, such as the activation of the innate immune system through the Toll, immunodeficiency (Imd), JAK-STAT pathways, and the interference of specific antiviral response pathways of RNAi. It is also believed that the presence of non-pathogenic microorganisms in the microbiota of these arthropods could influence this immune response, as it provides a baseline activation of the innate immune system, which may generate resistance against arboviruses. In addition, this microbiome has direct action against arboviruses, mainly due to the ability of Wolbachia spp. to block viral genome replication, added to the competition for resources within the mosquito organism. Despite major advances in the area, studies are still needed to evaluate the microbiota profiles of Aedes spp. and their vector competence, as well as further exploration of the individual roles of microbiome components in activating the innate immune system.

Alteration of Manure Antibiotic Resistance Genes via Soil Fauna Is Associated with the Intestinal Microbiome

Livestock wastes contain high levels of antibiotic resistance genes (ARGs) and a variety of human-related pathogens. Bioconversion of livestock manure using larvae of the beetle Protaetia brevitarsis is an effective technique for waste reduction and value creation; however, the fate of manure ARGs during gut passage and interaction with the gut microbiome of P. brevitarsis remains unclear. To investigate this, we fed P. brevitarsis with dry chicken manure for 6 days and measured bacterial community dynamics and ARG abundance and diversity along the P. brevitarsis gut tract using high-throughput quantitative PCR and metagenomics approaches. The diversity of ARGs was significantly lower in larval midgut, hindgut, and frass than in raw chicken manure, and around 80% of pathogenicity-related genes (PRGs) exhibited reduced abundance. Network analysis demonstrated that Bacteroidetes and Firmicutes were the key bacterial phyla associated with ARG reduction. Metagenomic analysis further indicated that ARGs, mobile genetic elements (MGEs), and PRGs were simultaneously attenuated in the hindgut, implicating a decreased likelihood for horizontal gene transfer (HGT) of ARGs among bacteria and pathogens during manure bioconversion. Our findings demonstrated that the attenuation of ARGs is strongly associated with the variation of the gut microbiome of P. brevitarsis, providing insights into mechanisms of risk mitigation of ARG dissemination during manure bioconversion. IMPORTANCE Saprophagous fauna like the oriental edible beetle (P. brevitarsis) plays a fundamental role in converting organic wastes into biofertilizer. Accumulating evidence has shown that soil fauna can reduce the abundance of ARGs, although the underlying mechanism of ARG reduction is still unclear. In our previous research, we found a large reduction of ARGs in vegetable roots and leaves from frass compared with raw manure, providing a promising biofertilizer for soil-vegetable systems. Therefore, in this study, temporal dynamic changes in the microbiomes of the donor (chicken manure) and host (P. brevitarsis) were investigated, and we found a close association between the gut microbiome and the alteration of ARGs. These results shed new light on how the insect gut microbiome can mitigate manure-borne ARGs and provide insights into the bioconversion process via a typical member of the saprophagous fauna, P. brevitarsis.

Re-Analysis of 16S rRNA Gene Sequence Data Sets Uncovers Disparate Laboratory-Specific Microbiomes Associated with the Yellow Fever Mosquito (Aedes aegypti)

Host-microbiome dynamics occurring in the yellow fever mosquito (Aedes aegypti) contribute to host life history traits, and particular bacterial taxa are proposed to comprise a "core" microbiota that influences host physiology. Laboratory-based studies are frequently performed to investigate these processes; however, experimental results are often presumed to be generalizable across laboratories, and few efforts have been made to independently reproduce and replicate significant findings. A recent study by Muturi et al. (FEMS Microbiol Ecol 95 (1):213, 2019) demonstrated the food source imbibed by laboratory-reared adult female mosquitoes significantly impacted the host-associated microbiota-a foundational finding in the field of mosquito biology worthy of independent evaluation. Here, we coalesce these data with two additional mosquito-derived 16S rRNA gene sequence data sets using a unifying bioinformatics pipeline to reproduce the characterization of these microbiota, test for a significant food source effect when independent samples were added to the analyses, assess whether similarly fed mosquito microbiomes were comparable across laboratories, and identify conserved bacterial taxa. Our pipeline characterized similar microbiome composition and structure from the data published previously, and a significant food source effect was detected with the addition of independent samples, increasing the robustness of this previously discovered component of mosquito biology. However, distinct microbial communities were identified from similarly fed but independently reared mosquitoes, and surveys across all samples did not identify conserved bacterial taxa. These findings demonstrated that while the main effect of the food source was supported, laboratory-specific conditions may produce inherently differential microbiomes across independent laboratory environments.

Honey Bee Larval and Adult Microbiome Life Stages Are Effectively Decoupled with Vertical Transmission Overcoming Early Life Perturbations

Microbiomes provide a range of benefits to their hosts which can lead to the coevolution of a joint ecological niche. However, holometabolous insects, some of the most successful organisms on Earth, occupy different niches throughout development, with larvae and adults being physiologically and morphologically highly distinct. Furthermore, transition between the stages usually involves the loss of the gut microbiome since the gut is remodeled during pupation. Most eusocial organisms appear to have evolved a workaround to this problem by sharing their communal microbiome across generations. However, whether this vertical microbiome transmission can overcome perturbations of the larval microbiome remains untested. Honey bees have a relatively simple, conserved, coevolved adult microbiome which is socially transmitted and affects many aspects of their biology. In contrast, larval microbiomes are more variable, with less clear roles. Here, we manipulated the gut microbiome of in vitro-reared larvae, and after pupation of the larvae, we inoculated the emerged bees with adult microbiome to test whether adult and larval microbiome stages may be coupled (e.g., through immune priming). Larval treatments differed in bacterial composition and abundance, depending on diet, which also drove larval gene expression. Nonetheless, adults converged on the typical core taxa and showed limited gene expression variation. This work demonstrates that honey bee adult and larval stages are effectively microbiologically decoupled, and the core adult microbiome is remarkably stable to early developmental perturbations. Combined with the transmission of the microbiome in early adulthood, this allows the formation of long-term host-microbiome associations. IMPORTANCE This work investigated host-microbiome interactions during a crucial developmental stage-the transition from larvae to adults, which is a challenge to both, the insect host and its microbiome. Using the honey bee as a tractable model system, we showed that microbiome transfer after emergence overrides any variation in the larvae, indicating that larval and adult microbiome stages are effectively decoupled. Together with the reliable vertical transfer in the eusocial system, this decoupling ensures that the adults are colonized with a consistent and derived microbiome after eclosion. Taken all together, our data provide additional support that the evolution of sociality, at least in the honey bee system tested here, is linked with host-microbiome relationships.

Analyzing gut microbiota composition in individual Anopheles mosquitoes after experimental treatment

The microbiota of Anopheles mosquitoes influences malaria transmission. Antibiotics ingested during a blood meal impact the mosquito microbiome and malaria transmission, with substantial differences between drugs. Here, we assessed if amoxicillin affects the gut mosquito microbiota. We collected Anopheles larvae in Burkina Faso, kept them in semi-field conditions, and offered a blood meal to adult females. We tested the impact of blood supplementation with two alternative amoxicillin preparations on microbiota composition, determined by high-throughput sequencing in individual gut samples. Our analysis detected four major genera, Elizabethkingia, Wigglesworthia, Asaia, and Serratia. The antibiotic treatment significantly affected overall microbiota composition, with a specific decrease in the relative abundance of Elizabethkingia and Asaia during blood digestion. Besides its interest on the influence of amoxicillin on the mosquito microbiota, our study proposes a thorough approach to report negative-control data of high-throughput sequencing studies on samples with a reduced microbial load.

Stage correlation of symbiotic bacterial community and function in the development of litchi bugs (Hemiptera: Tessaratomidae)

Bacterial symbionts of insects have been shown to play important roles in host fitness. However, little is known about the bacterial community of Tessaratoma papillosa which is one of the most destructive pests of the well-known fruits Litchi chinensis Sonn and Dimocarpus longan Lour in Oriental Region, especially in South-east Asia and adjacent areas. In this study, we surveyed the bacterial community diversity and dynamics of T. papillosa in all developmental stages with both culture-dependent and culture-independent methods by the third-generation sequencing technology. Five bacterial phyla were identified in seven developmental stages of T. papillosa. Proteobacteria was the dominant phylum and Pantoea was the dominant genus of T. papillosa. The results of alpha and beta diversity analyses showed that egg stage had the most complex bacterial community. Some of different developmental stages showed similarities, which were clustered into three phases: (1) egg stage, (2) early nymph stages (instars 1-3), and (3) late nymph stages (instars 4-5) and adult stage. Functional prediction indicated that the bacterial community played different roles in these three phases. Furthermore, 109 different bacterial strains were isolated and identified from various developmental stages. This study revealed the relationship between the symbiotic bacteria and the development of T. papillosa, and may thus contribute to the biological control techniques of T. papillosa in the future.

A Neurotoxic Insecticide Promotes Fungal Infection in Aedes aegypti Larvae by Altering the Bacterial Community

Symbiotic bacteria have a significant impact on the formation of defensive mechanisms against fungal pathogens and insecticides. The microbiome of the mosquito Aedes aegypti has been well studied; however, there are no data on the influence of insecticides and pathogenic fungi on its structure. The fungus Metarhizium robertsii and a neurotoxic insecticide (avermectin complex) interact synergistically, and the colonization of larvae with hyphal bodies is observed after fungal and combined (conidia + avermectins) treatments. The changes in the bacterial communities (16S rRNA) of Ae. aegypti larvae under the influence of fungal infection, avermectin toxicosis, and their combination were studied. In addition, we studied the interactions between the fungus and the predominant cultivable bacteria in vitro and in vivo after the coinfection of the larvae. Avermectins increased the total bacterial load and diversity. The fungus decreased the diversity and insignificantly increased the bacterial load. Importantly, avermectins reduced the relative abundance of Microbacterium (Actinobacteria), which exhibited a strong antagonistic effect towards the fungus in in vitro and in vivo assays. The avermectin treatment led to an increased abundance of Chryseobacterium (Flavobacteria), which exerted a neutral effect on mycosis development. In addition, avermectin treatment led to an elevation of some subdominant bacteria (Pseudomonas) that interacted synergistically with the fungus. We suggest that avermectins change the bacterial community to favor the development of fungal infection.

Exploring Changes in the Microbiota of Aedes albopictus: Comparison Among Breeding Site Water, Larvae, and Adults

The mosquito body hosts highly diverse microbes, which influence different physiological traits of both larvae and adults. The composition of adult mosquito microbiota is tightly linked to that of larvae, which are aquatic and feed on organic detritus, algae and prokaryotic microorganisms present in their breeding sites. Unraveling the ecological features of larval habitats that shape the structure of bacterial communities and their interactions with the mosquito host is still a poorly investigated topic in the Asian tiger mosquito Aedes albopictus, a highly invasive species that is vector of numerous arboviruses, including Dengue, Chikungunya, and Zika viruses. In this study, we investigated the composition of the bacterial community present in the water from a natural larval breeding site in which we separately reared wild-collected larvae and hatched eggs of the Foshan reference laboratory strain. Using sequence analysis of bacterial 16S rRNA gene amplicons, we comparatively analyzed the microbiota of the larvae and that of adult mosquitoes, deriving information about the relative impact of the breeding site water on shaping mosquito microbiota. We observed a higher bacterial diversity in breeding site water than in larvae or adults, irrespective of the origin of the sample. Moreover, larvae displayed a significantly different and most diversified microbial community than newly emerged adults, which appeared to be dominated by Proteobacteria. The microbiota of breeding site water significantly increased its diversity over time, suggesting the presence of a dynamic interaction among bacterial communities, breeding sites and mosquito hosts. The analysis of Wolbachia prevalence in adults from Foshan and five additional strains with different geographic origins confirmed the described pattern of dual wAlbA and wAlbB strain infection. However, differences in Wolbachia prevalence were detected, with one strain from La Reunion Island showing up to 18% uninfected individuals. These findings contribute in further understanding the dynamic interactions between the ecology of larval habitats and the structure of host microbiota, as well as providing additional information relative to the patterns of Wolbachia infection.

Shifts in the microbiota associated with male mosquitoes (Aedes aegypti) exposed to an obligate gut fungal symbiont (Zancudomyces culisetae)

Research characterizing arthropod-associated microbiota has revealed that microbial dynamics can have an important impact on host phenotypic traits. The influence of fungi on these interactions are emerging as targets for research, especially in organisms associated with global human health. A recent study demonstrated colonization of a widespread gut fungus (Zancudomyces culisetae) in a larval mosquito (Aedes aegypti) digestive tract affected microbiomes in larvae and newly emerged adult females (Frankel-Bricker et al. Appl Environ Microbiol, 2020. 10.1128/AEM.02334-19) but did not investigate these processes in males. The objective of the study presented here was to assess fungal influences on adult male mosquito microbiomes to enable a more complete assessment of fungal–bacterial–host interactions in the A. aegypti–Z. culisetae system. Sequencing of 16S rRNA gene amplicons from microbiomes harbored in adult males directly after emerging from pupae revealed larval fungal exposure significantly decreased overall microbial community diversity, altered microbiome composition and structure, and decreased within-group microbiome variation across individuals. Further, bacteria in the family Burkholderiaceae were present in high abundance in fungal-exposed males, likely contributing to the disparate microbiota between treatment groups. Comparisons between male and the female microbiomes analyzed in Frankel-Bricker et al. (2020), showed distinct shifts in bacterial communities incurred by larval exposure to fungi, potentially revealing sex-specific fungal–bacterial–host dynamics in A. aegypti. These findings highlight the complex role a gut fungus can play in influencing the microbial communities harbored in an important insect and emphasize the significance of accounting for an organism’s sex when studying fungal–bacterial–host dynamics.