The yeast Wickerhamomyces anomalus (Pichia anomala) inhabits the midgut and reproductive system of the Asian malaria vector Anopheles stephensi

Interkingdom interactions shape the fungal microbiome of mosquitoes

BACKGROUND

The mosquito microbiome is an important modulator of vector competence and vectoral capacity. Unlike the extensively studied bacterial microbiome, fungal communities in the mosquito microbiome (the mycobiome) remain largely unexplored. To work towards getting an improved understanding of the fungi associated with mosquitoes, we sequenced the mycobiome of three field-collected and laboratory-reared mosquito species (Aedes albopictus, Aedes aegypti, and Culex quinquefasciatus).

RESULTS

Our analysis showed both environment and host species were contributing to the diversity of the fungal microbiome of mosquitoes. When comparing species, Ae. albopictus possessed a higher number of diverse fungal taxa than Cx. quinquefasciatus, while strikingly less than 1% of reads from Ae. aegypti samples were fungal. Fungal reads from Ae. aegypti were < 1% even after inhibiting host amplification using a PNA blocker, indicating that this species lacked a significant fungal microbiome that was amplified using this sequencing approach. Using a mono-association mosquito infection model, we confirmed that mosquito-derived fungal isolates colonize Aedes mosquitoes and support growth and development at comparable rates to their bacterial counterparts. Strikingly, native bacterial taxa isolated from mosquitoes impeded the colonization of symbiotic fungi in Ae. aegypti suggesting interkingdom interactions shape fungal microbiome communities.

CONCLUSION

Collectively, this study adds to our understanding of the fungal microbiome of different mosquito species, that these fungal microbes support growth and development, and highlights that microbial interactions underpin fungal colonization of these medically relevent species.

Impact of the microbiome on mosquito-borne diseases

Mosquito-borne diseases present a significant threat to human health, with the possibility of outbreaks of new mosquito-borne diseases always looming. Unfortunately, current measures to combat these diseases such as vaccines and drugs are often either unavailable or ineffective. However, recent studies on microbiomes may reveal promising strategies to fight these diseases. In this review, we examine recent advances in our understanding of the effects of both the mosquito and vertebrate microbiomes on mosquito-borne diseases. We argue that the mosquito microbiome can have direct and indirect impacts on the transmission of these diseases, with mosquito symbiotic microorganisms, particularly Wolbachia bacteria, showing potential for controlling mosquito-borne diseases. Moreover, the skin microbiome of vertebrates plays a significant role in mosquito preferences, while the gut microbiome has an impact on the progression of mosquito-borne diseases in humans. As researchers continue to explore the role of microbiomes in mosquito-borne diseases, we highlight some promising future directions for this field. Ultimately, a better understanding of the interplay between mosquitoes, their hosts, pathogens, and the microbiomes of mosquitoes and hosts may hold the key to preventing and controlling mosquito-borne diseases.

Tripartite interactions comprising yeast-endobacteria systems in the gut of vector mosquitoes

It is shown that bacteria use yeast as a niche for survival in stressful conditions, therefore yeasts may act as temporary or permanent bacterial reservoirs. Endobacteria colonise the fungal vacuole of various osmotolerant yeasts which survive and multiply in sugar-rich sources such as plant nectars. Nectar-associated yeasts are present even in the digestive system of insects and often establish mutualistic symbioses with both hosts. Research on insect microbial symbioses is increasing but bacterial-fungal interactions are yet unexplored. Here, we have focused on the endobacteria of Wickerhamomyces anomalus (formerly Pichia anomala and Candida pelliculosa), an osmotolerant yeast associated with sugar sources and the insect gut. Symbiotic strains of W. anomalus influence larval development and contribute digestive processes in adults, in addition to exerting wide antimicrobial properties for host defence in diverse insects including mosquitoes. Antiplasmodial effects of W. anomalus have been shown in the gut of the female malaria vector mosquito Anopheles stephensi. This discovery highlights the potential of utilizing yeast as a promising tool for symbiotic control of mosquito-borne diseases. In the present study, we have carried out a large Next Generation Sequencing (NGS) metagenomics analysis including W. anomalus strains associated with vector mosquitoes Anopheles, Aedes and Culex, which has highlighted wide and heterogeneous EB communities in yeast. Furthermore, we have disclosed a Matryoshka-like association in the gut of A stephensi that comprises different EB in the strain of W. anomalus WaF17.12. Our investigations started with the localization of fast-moving bacteria-like bodies within the yeast vacuole of WaF17.12. Additional microscopy analyses have validated the presence of alive intravacuolar bacteria and 16S rDNA libraries from WaF17.12 have identified a few bacterial targets. Some of these EB have been isolated and tested for lytic properties and capability to re-infect the yeast cell. Moreover, a selective competence to enter yeast cell has been shown comparing different bacteria. We suggested possible tripartite interactions among EB, W. anomalus and the host, opening new knowledge on the vector biology.

Immune-related transcripts, microbiota and vector competence differ in dengue-2 virus-infected geographically distinct Aedes aegypti populations

BACKGROUND

Vector competence in Aedes aegypti is influenced by various factors. Crucial new control methods can be developed by recognizing which factors affect virus and mosquito interactions.

METHODS

In the present study we used three geographically distinct Ae. aegypti populations and compared their susceptibility to infection by dengue virus serotype 2 (DENV-2). To identify any differences among the three mosquito populations, we evaluated expression levels of immune-related genes and assessed the presence of microbiota that might contribute to the uniqueness in their vector competence.

RESULTS

Based on the results from the DENV-2 competence study, we categorized the three geographically distinct Ae. aegypti populations into a refractory population (Vilas do Atlântico), a susceptible population (Vero) and a susceptible but low transmission population (California). The immune-related transcripts were highly expressed in the California population but not in the refractory population. However, the Rel-1 gene was upregulated in the Vilas do Atlântico population following ingestion of a non-infectious blood meal, suggesting the gene's involvement in non-viral responses, such as response to microbiota. Screening of the bacteria, fungi and flaviviruses revealed differences between populations, and any of these could be one of the factors that interfere with the vector competence.

CONCLUSIONS

The results reveal potential factors that might impact the virus and mosquito interaction, as well as influence the Ae. aegypti refractory phenotype.

Long-term sperm storage in eusocial Hymenoptera

In internally fertilizing species, sperm transfer is not always immediately followed by egg fertilization, and female sperm storage (FSS) may occur. FSS is a phenomenon in which females store sperm in a specialized organ for periods lasting from a few hours to several years, depending on the species. Eusocial hymenopterans (ants, social bees, and social wasps) hold the record for FSS duration. In these species, mating takes place during a single nuptial flight that occurs early in adult life for both sexes; they never mate again. Males die quickly after copulation but survive posthumously as sperm stored in their mates' spermathecae. Reproductive females, also known as queens, have a much longer life expectancy, up to 20 years in some species. Here, we review what is currently known about the molecular adaptations underlying the remarkable FSS capacities in eusocial hymenopterans. Because sperm quality is crucial to the reproductive success of both sexes, we also discuss the mechanisms involved in sperm storage and preservation in the male seminal vesicles prior to ejaculation. Finally, we propose future research directions that should broaden our understanding of this unique biological phenomenon.

Assessment of antibiotic treatment on Anopheles darlingi survival and susceptibility to Plasmodium vivax

Antibiotic treatment has been used to enhance anopheline susceptibility to Plasmodium infection, because bacterial microbiota play a fundamental role in modulating the vector competence of mosquitoes that transmit Plasmodium parasites. However, few studies have examined the impact of antibiotic treatments on Plasmodium vivax sporogonic development in neotropical anopheline mosquitoes. Herein, we assessed the impact of antibiotic treatment on P. vivax development and survival in Anopheles darlingi, the main vector of malaria in the Amazon region. Female mosquitoes were treated continuously with antibiotics to impact the gut bacterial load and then tested for prevalence, infection intensity, and survival in comparison with untreated mosquitoes. Antibiotic-fed mosquitoes had not dramatic impact on P. vivax development previously observed in P. falciparum. However, antibiotic treatment increases mosquito survival, which is known to increase vectorial capacity. These findings raise questions about the effect of antibiotics on P. vivax development and survival in An. darlingi.

Contemporary exploitation of natural products for arthropod-borne pathogen transmission-blocking interventions

An integrated approach to innovatively counter the transmission of various arthropod-borne diseases to humans would benefit from strategies that sustainably limit onward passage of infective life cycle stages of pathogens and parasites to the insect vectors and vice versa. Aiming to accelerate the impetus towards a disease-free world amid the challenges posed by climate change, discovery, mindful exploitation and integration of active natural products in design of pathogen transmission-blocking interventions is of high priority. Herein, we provide a review of natural compounds endowed with blockade potential against transmissible forms of human pathogens reported in the last 2 decades from 2000 to 2021. Finally, we propose various translational strategies that can exploit these pathogen transmission-blocking natural products into design of novel and sustainable disease control interventions. In summary, tapping these compounds will potentially aid in integrated combat mission to reduce disease transmission trends.

Malaria-Transmitting Vectors Microbiota: Overview and Interactions With Anopheles Mosquito Biology

Malaria remains a vector-borne infectious disease that is still a major public health concern worldwide, especially in tropical regions. Malaria is caused by a protozoan parasite of the genus Plasmodium and transmitted through the bite of infected female Anopheles mosquitoes. The control interventions targeting mosquito vectors have achieved significant success during the last two decades and rely mainly on the use of chemical insecticides through the insecticide-treated nets (ITNs) and indoor residual spraying (IRS). Unfortunately, resistance to conventional insecticides currently being used in public health is spreading in the natural mosquito populations, hampering the long-term success of the current vector control strategies. Thus, to achieve the goal of malaria elimination, it appears necessary to improve vector control approaches through the development of novel environment-friendly tools. Mosquito microbiota has by now given rise to the expansion of innovative control tools, such as the use of endosymbionts to target insect vectors, known as "symbiotic control." In this review, we will present the viral, fungal and bacterial diversity of Anopheles mosquitoes, including the bacteriophages. This review discusses the likely interactions between the vector microbiota and its fitness and resistance to insecticides.

Formulation and Safety Tests of a Wickerhamomyces anomalus-Based Product: Potential Use of Killer Toxins of a Mosquito Symbiotic Yeast to Limit Malaria Transmission

Wickerhamomyces anomalus strain WaF17.12 is a yeast with an antiplasmodial property based on the production of a killer toxin. For its symbiotic association with Anopheles mosquitoes, it has been proposed for the control of malaria. In an applied view, we evaluated the yeast formulation by freeze-drying WaF17.12. The study was carried out by comparing yeast preparations stored at room temperature for different periods, demonstrating that lyophilization is a useful method to obtain a stable product in terms of cell growth reactivation and maintenance of the killer toxin antimicrobial activity. Moreover, cytotoxic assays on human cells were performed, showing no effects on the cell viability and the proinflammatory response. The post-formulation effectiveness of the killer toxin and the safety tests indicate that WaF17.12 is a promising bioreagent able to impair the malaria parasite in vector mosquitoes.

Diversity and Functions of Yeast Communities Associated with Insects

Following the concept of the holobiont, insect-microbiota interactions play an important role in insect biology. Many examples of host-associated microorganisms have been reported to drastically influence insect biological processes such as development, physiology, nutrition, survival, immunity, or even vector competence. While a huge number of studies on insect-associated microbiota have focused on bacteria, other microbial partners including fungi have been comparatively neglected. Yeasts, which establish mostly commensal or symbiotic relationships with their host, can dominate the mycobiota of certain insects. This review presents key advances and progress in the research field highlighting the diversity of yeast communities associated with insects, as well as their impact on insect life-history traits, immunity, and behavior.

The Axenic and Gnotobiotic Mosquito: Emerging Models for Microbiome Host Interactions

The increasing availability of modern research tools has enabled a revolution in studies of non-model organisms. Yet, one aspect that remains difficult or impossible to control in many model and most non-model organisms is the presence and composition of the host-associated microbiota or the microbiome. In this review, we explore the development of axenic (microbe-free) mosquito models and what these systems reveal about the role of the microbiome in mosquito biology. Additionally, the axenic host is a blank template on which a microbiome of known composition can be introduced, also known as a gnotobiotic organism. Finally, we identify a "most wanted" list of common mosquito microbiome members that show the greatest potential to influence host phenotypes. We propose that these are high-value targets to be employed in future gnotobiotic studies. The use of axenic and gnotobiotic organisms will transition the microbiome into another experimental variable that can be manipulated and controlled. Through these efforts, the mosquito will be a true model for examining host microbiome interactions.

Comprehensive Ecological and Geographic Characterization of Eukaryotic and Prokaryotic Microbiomes in African Anopheles

Exposure of mosquitoes to numerous eukaryotic and prokaryotic microbes in their associated microbiomes has probably helped drive the evolution of the innate immune system. To our knowledge, a metagenomic catalog of the eukaryotic microbiome has not been reported from any insect. Here we employ a novel approach to preferentially deplete host 18S ribosomal RNA gene amplicons to reveal the composition of the eukaryotic microbial communities of Anopheles larvae sampled in Kenya, Burkina Faso and Republic of Guinea (Conakry). We identified 453 eukaryotic operational taxonomic units (OTUs) associated with Anopheles larvae in nature, but an average of 45% of the 18S rRNA sequences clustered into OTUs that lacked a taxonomic assignment in the Silva database. Thus, the Anopheles microbiome contains a striking proportion of novel eukaryotic taxa. Using sequence similarity matching and de novo phylogenetic placement, the fraction of unassigned sequences was reduced to an average of 4%, and many unclassified OTUs were assigned as relatives of known taxa. A novel taxon of the genus Ophryocystis in the phylum Apicomplexa (which also includes Plasmodium) is widespread in Anopheles larvae from East and West Africa. Notably, Ophryocystis is present at fluctuating abundance among larval breeding sites, consistent with the expected pattern of an epidemic pathogen. Species richness of the eukaryotic microbiome was not significantly different across sites from East to West Africa, while species richness of the prokaryotic microbiome was significantly lower in West Africa. Laboratory colonies of Anopheles coluzzii harbor 26 eukaryotic OTUs, of which 38% (n = 10) are shared with wild populations, while 16 OTUs are unique to the laboratory colonies. Genetically distinct An. coluzzii colonies co-housed in the same facility maintain different prokaryotic microbiome profiles, suggesting a persistent host genetic influence on microbiome composition. These results provide a foundation to understand the role of the Anopheles eukaryotic microbiome in vector immunity and pathogen transmission. We hypothesize that prevalent apicomplexans such as Ophryocystis associated with Anopheles could induce interference or competition against Plasmodium within the vector. This and other members of the eukaryotic microbiome may offer candidates for new vector control tools.

Wickerhamomyces anomalus in Mosquitoes: A Promising Yeast-Based Tool for the "Symbiotic Control" of Mosquito-Borne Diseases

The ascomycete yeast Wickerhamomyces anomalus is a mutualistic symbiont of different insects, including diptera vectors of diseases. Although fungal symbioses have been so far poorly characterized, the topic is gaining attention as yeast-insect interactions can provide pivotal information on insect biology, such as their environmental adaptation or vectorial capability. We review the symbiosis between W. anomalus and mosquitoes, which implies nutritional and protective functions. Furthermore, we focus on antiplasmodial effects of W. anomalus in malaria vectors and discuss the yeast potential for the “symbiotic control” (SC) of mosquito-borne diseases (MBDs).

Mosquito Mycobiota: An Overview of Non-Entomopathogenic Fungal Interactions

The growing expansion of mosquito vectors leads to the emergence of vector-borne diseases in new geographic areas and causes major public health concerns. In the absence of effective preventive treatments against most pathogens transmitted, vector control remains one of the most suitable strategies to prevent mosquito-borne diseases. Insecticide overuse raises mosquito resistance and deleterious impacts on the environment and non-target species. Growing knowledge of mosquito biology has allowed the development of alternative control methods. Following the concept of holobiont, mosquito-microbiota interactions play an important role in mosquito biology. Associated microbiota is known to influence many aspects of mosquito biology such as development, survival, immunity or even vector competence. Mosquito-associated microbiota is composed of bacteria, fungi, protists, viruses and nematodes. While an increasing number of studies have focused on bacteria, other microbial partners like fungi have been largely neglected despite their huge diversity. A better knowledge of mosquito-mycobiota interactions offers new opportunities to develop innovative mosquito control strategies. Here, we review the recent advances concerning the impact of mosquito-associated fungi, and particularly nonpathogenic fungi, on life-history traits (development, survival, reproduction), vector competence and behavior of mosquitoes by focusing on Culex, Aedes and Anopheles species.

Mosquito Microbiota and Implications for Disease Control

Mosquito-transmitted diseases account for about 500 000 deaths every year. Blocking these pathogens in the mosquito vector before they are transmitted to humans is an effective strategy to prevent mosquito-borne diseases. Like most higher organisms, mosquitoes harbor a highly diverse and dynamic microbial flora that can be explored for prevention of pathogen transmission. Here we review the structure and function of the mosquito microbiota, including bacteria, fungi, and viruses, and discuss the potential of using components of the microbiota to thwart pathogen transmission.

Identification of a Killer Toxin from Wickerhamomyces anomalus with β-Glucanase Activity

The yeast Wickerhamomyces anomalus has several applications in the food industry due to its antimicrobial potential and wide range of biotechnological properties. In particular, a specific strain of Wickerhamomyces anomalus isolated from the malaria mosquito Anopheles stephensi, namely WaF17.12, was reported to secrete a killer toxin with strong anti-plasmodial effect on different developmental stages of Plasmodium berghei; therefore, we propose its use in the symbiotic control of malaria. In this study, we focused on the identification/characterization of the protein toxin responsible for the observed antimicrobial activity of the yeast. For this purpose, the culture medium of the killer yeast strain WaF17.12 was processed by means of lateral flow filtration, anion exchange and gel filtration chromatography, immunometric methods, and eventually analyzed by liquid chromatography-tandem mass spectrometry (LC–MS/MS). Based on this concerted approach, we identified a protein with a molecular weight of approximately 140 kDa and limited electrophoretic mobility, corresponding to a high molecular weight β-glucosidase, as confirmed by activity tests in the presence of specific inhibitors.

Effects of a Resident Yeast from the Honeybee Gut on Immunity, Microbiota, and Nosema Disease

The western honeybee (Apis mellifera) has a core bacterial microbiota that is well described and important for health. Honeybees also host a yeast community that is poorly understood with respect to host nutrition and immunity, and also the symbiotic bacterial microbiota. In this work, we present two studies focusing on the consequences of dysbiosis when honeybees were control-fed a yeast that was isolated from a honeybee midgut, Wickerhamomyces anomalus. Yeast augmentation for bees with developed microbiota appeared immunomodulatory (lowered immunity and hormone-related gene expression) and affected the microbial community, while yeast augmentation for newly emerged bees without an established bacterial background did not lead to decreased immunity— and hormone—related gene expression. In newly emerged bees that had a naturally occurring baseline level of W. anomalus, we observed that the addition of N. ceranae led to a decrease in yeast levels. Overall, we show that yeasts can affect the microbiome, immunity, and physiology.

Killer yeasts exert anti-plasmodial activities against the malaria parasite Plasmodium berghei in the vector mosquito Anopheles stephensi and in mice

BACKGROUND

Wickerhamomyces anomalus is a yeast associated with different insects including mosquitoes, where it is proposed to be involved in symbiotic relationships with hosts. Different symbiotic strains of W. anomalus display a killer phenotype mediated by protein toxins with broad-spectrum antimicrobial activities. In particular, a killer toxin purified from a W. anomalus strain (WaF17.12), previously isolated from the malaria vector mosquito Anopheles stephensi, has shown strong in vitro anti-plasmodial activity against early sporogonic stages of the murine malaria parasite Plasmodium berghei.

RESULTS

Here, we provide evidence that WaF17.12 cultures, properly stimulated to induce the expression of the killer toxin, can directly affect in vitro P. berghei early sporogonic stages, causing membrane damage and parasite death. Moreover, we demonstrated by in vivo studies that mosquito dietary supplementation with activated WaF17.12 cells interfere with ookinete development in the midgut of An. stephensi. Besides the anti-sporogonic action of WaF17.12, an inhibitory effect of purified WaF17.12-killer toxin was observed on erythrocytic stages of P. berghei, with a consequent reduction of parasitaemia in mice. The preliminary safety tests on murine cell lines showed no side effects.

CONCLUSIONS

Our findings demonstrate the anti-plasmodial activity of WaF17.12 against different developmental stages of P. berghei. New studies on P. falciparum are needed to evaluate the use of killer yeasts as innovative tools in the symbiotic control of malaria.

Microorganism-Based Larval Diets Affect Mosquito Development, Size and Nutritional Reserves in the Yellow Fever Mosquito Aedes aegypti (Diptera: Culicidae)

Background

Mosquito larvae feed on organic detritus from the environment, particularly microorganisms comprising bacteria, protozoa, and algae as well as crustaceans, plant debris, and insect exuviae. Little attention has been paid to nutritional studies in Aedes aegypti larvae.

Objectives

We investigated the effects of yeast, bacteria and microalgae diets on larval development, pupation time, adult size, emergence, survivorship, lifespan, and wing morphology.

Materials and Methods

Microorganisms (or Tetramin^® as control) were offered as the only source of food to recently hatched first instar larvae and their development was followed until the adult stage. Protein, carbohydrate, glycogen, and lipid were analyzed in single larvae to correlate energetic reserve accumulation by larva with the developmental rates and nutritional content observed. FITC-labeled microorganisms were offered to fourth instar larvae, and its ingestion was recorded by fluorescence microscopy and quantitation.

Results and Discussion

Immature stages developed in all diets, however, larvae fed with bacteria and microalgae showed a severe delay in development rates, pupation time, adult emergence and low survivorship. Adult males emerged earlier as expected and had longer survival than females. Diets with better nutritional quality resulted in adults with bigger wings. Asaia sp. and Escherichia coli resulted in better nutrition and developmental parameters and seemed to be the best bacterial candidates to future studies using symbiont-based control. The diet quality was measured and presented different protein and carbohydrate amounts. Bacteria had the lowest protein and carbohydrate rates, yeasts had the highest carbohydrate amount and microalgae showed the highest protein content. Larvae fed with microalgae seem not to be able to process and store these diets properly. Larvae were shown to be able to process yeast cells and store their energetic components efficiently.

Conclusion

Together, our results point that Ae. aegypti larvae show high plasticity to feed, being able to develop under different microorganism-based diets. The important role of Ae. aegypti in the spread of infectious diseases requires further biological studies in order to understand the vector physiology and thus to manage the larval natural breeding sites aiming a better mosquito control.

Isolation and identification of Asaia sp. in Anopheles spp. mosquitoes collected from Iranian malaria settings: steps toward applying paratransgenic tools against malaria

BACKGROUND

In recent years, the genus Asaia (Rhodospirillales: Acetobacteraceae) has been isolated from different Anopheles species and presented as a promising tool to combat malaria. This bacterium has unique features such as presence in different organs of mosquitoes (midgut, salivary glands and reproductive organs) of female and male mosquitoes and vertical and horizontal transmission. These specifications lead to the possibility of introducing Asaia as a robust candidate for malaria vector control via paratransgenesis technology. Several studies have been performed on the microbiota of Anopheles mosquitoes (Diptera: Culicidae) in Iran and the Middle East to find a suitable candidate for controlling the malaria based on paratransgenesis approaches. The present study is the first report of isolation, biochemical and molecular characterization of the genus Asaia within five different Anopheles species which originated from different zoogeographical zones in the south, east, and north of Iran.

METHODS

Mosquitoes originated from field-collected and laboratory-reared colonies of five Anopheles spp. Adult mosquitoes were anesthetized; their midguts were isolated by dissection, followed by grinding the midgut contents which were then cultured in enrichment broth media and later in CaCO₃ agar plates separately. Morphological, biochemical and physiological characterization were carried out after the appearance of colonies. For molecular confirmation, selected colonies were cultured, their DNAs were extracted and PCR was performed on the 16S ribosomal RNA gene using specific newly designed primers.

RESULTS

Morphological, biochemical, physiological and molecular results indicated that all isolates are members of the genus Asaia.

CONCLUSIONS

Contrary to previous opinions, our findings show that Asaia bacteria are present in both insectary-reared colonies and field-collected mosquitoes and can be isolated by simple and specific methods. Furthermore, with respect to the fact that we isolated Asaia within the different Anopheles specimens from distinct climatic and zoogeographical regions, it is promising and may be concluded that species of this genus can tolerate the complicated environmental conditions of the vector-borne diseases endemic regions. Therefore, it can be considered as a promising target in paratransgenesis and vector control programs. However, we suggest that introducing the new technologies such as next generation sequencing and robust in silico approaches may pave the way to find a unique biomarker for rapid and reliable differentiation of the Asaia species.

Yeast-insect associations: It takes guts

Insects interact with microorganisms in several situations, ranging from the accidental interaction to locate attractive food or the acquisition of essential nutrients missing in the main food source. Despite a wealth of studies recently focused on bacteria, the interactions between insects and yeasts have relevant implications for both of the parties involved. The insect intestine shows several structural and physiological differences among species, but it is generally a hostile environment for many microorganisms, selecting against the most sensitive and at the same time guaranteeing a less competitive environment to resistant ones. An intensive characterization of the interactions between yeasts and insects has highlighted their relevance not only for attraction to food but also for the insect's development and behaviour. Conversely, some yeasts have been shown to benefit from interactions with insects, in some cases by being carried among different environments. In addition, the insect intestine may provide a place to reside for prolonged periods and possibly mate or generate sexual forms able to mate once back in the external environments. YEA-May-17-0084.R3.

The tripartite interactions between the mosquito, its microbiota and Plasmodium

The microbiota of Anopheles mosquitoes interferes with mosquito infection by Plasmodium and influences mosquito fitness, therefore affecting vectorial capacity. This natural barrier to malaria transmission has been regarded with growing interest in the last 20 years, as it may be a source of new transmission-blocking strategies. The last decade has seen tremendous progress in the functional characterisation of the tripartite interactions between the mosquito, its microbiota and Plasmodium parasites. In this review, we provide insights into the effects of the mosquito microbiota on Plasmodium infection and on mosquito physiology, and on how these aspects together influence vectorial capacity. We also discuss three current challenges in the field, namely the need for a more relevant microbiota composition in experimental mosquitoes involved in vector biology studies, for a better characterisation of the non-bacterial microbiota, and for further functional studies of the microbiota present outside the gut.

The mycobiota of the sand fly Phlebotomus perniciosus: Involvement of yeast symbionts in uric acid metabolism

The knowledge of the fungal mycobiota of arthropods, including the vectors of human and animal diseases, is still limited. Here, the mycobiota associated with the sand fly Phlebotomus perniciosus, the main vector of leishmaniasis in the western Mediterranean area, by a culture-dependent approach (microbiological analyses and sequencing of the 26S rRNA gene), internal transcribed spacer (ITS) rRNA amplicon-based next-generation sequencing, fluorescence in situ hybridisation (FISH), and genome sequencing of the dominant yeast species was investigated. The dominant species was Meyerozyma guilliermondii, known for its biotechnological applications. The focus was on this yeast and its prevalence in adults, pupae and larvae of reared sand flies (overall prevalence: 57.5%) and of field-collected individuals (overall prevalence: 9%) was investigated. Using whole-mount FISH and microscopic examination, it was further showed that M. guilliermondii colonizes the midgut of females, males and larvae and the distal part of Malpighian tubules of female sand flies, suggesting a possible role in urate degradation. Finally, the sequencing and analysis of the genome of M. guilliermondii allowed predicting the complete uric acid degradation pathway, suggesting that the yeast could contribute to the removal of the excess of nitrogenous wastes after the blood meal of the insect host.

Candidacidal Activity of a Novel Killer Toxin from Wickerhamomyces anomalus against Fluconazole-Susceptible and -Resistant Strains

The isolation and characterization from the sand fly Phlebotomus perniciosus of a Wickerhamomyces anomalus yeast strain (Wa1F1) displaying the killer phenotype was recently reported. In the present work, the killer toxin (KT) produced by Wa1F1 was purified and characterized, and its antimicrobial activity in vitro was investigated against fluconazole- susceptible and -resistant clinical isolates and laboratory strains of Candida albicans and C. glabrata displaying known mutations. Wa1F1-KT showed a differential killing ability against different mutant strains of the same species. The results may be useful for the design of therapeutic molecules based on Wa1F1-KT and the study of yeast resistance mechanisms.

Mosquitoes can harbour yeasts of clinical significance and contribute to their environmental dissemination

There is still a lack of studies on fungal microbiota in mosquitoes, compared with the number available on bacterial microbiota. This study reports the identification of yeasts of clinical significance in laboratory mosquito species: Anopheles gambiae, Anopheles stephensi, Culex quinquefasciatus, Aedes albopictus and Aedes aegypti. Among the yeasts isolated, they focused on the opportunistic pathogen Candida parapsilosis, since there is a need to better understand breakthrough candidaemia with resistance to the usual antifungals, which requires careful consideration in the broad-spectrum therapy, as documented in many clinical reports. C. parapsilosis occurs widely and has been isolated from diverse sources, including insects, which may contribute to its dissemination. In this study, it was isolated from the gut of An. gambiae and its presence in developmental stages and organs of different mosquito species was studied. Our results indicated that there was a stable association between C. parapsilosis and reared mosquitoes during the entire life cycle, and in adult male and female gut and gonads. A wide occurrence of C. parapsilosis was also documented in several populations of wild mosquitoes. Based on these findings, it can be said that mosquitoes might participate in the spreading of this opportunistic pathogen, not only as a carrier.

Enzyme activity and expression pattern of intra- and extracellular chitinase and β-1,3-glucanase of Wickerhamomyces anomalus EG2 using glycol chitin and glucan-containing high polymer complex

We investigated cell growth and activity of intra- and extracellular chitinase, β-1,3-glucanase, and chitin deacetylase with SDS-PAGE by incubating W. anomalus EG2 in PDB and YPD media for 24h in presence of different concentrations (0%, 0.1%, 0.3%, and 0.5%) of colloidal chitin. Maximum cell growth was observed in both PDB and YPD media without colloidal chitin. In the absence of colloidal chitin, maximum extracellular β-1,3-glucanase activity of 32.96 and 47.28 units/mL was reported at 18h in PDB medium and 6h in YPD medium, respectively. In addition, extracellular chitinase was unaffected by various concentrations of carboxymethyl chitin in both PDB and YPD media. In the absence of colloidal chitin, maximum intracellular chitinase activity was indicated to be 9.82 and 9.86 units/mg protein in PDB and YPD media, respectively. Maximum intracellular β-1,3-glucanase activity reported was 17.34 units/mg protein in PDB medium containing 0.5% colloidal chitin and 15.0 units/mg protein in YPD medium containing 0.3% colloidal chitin. Five major isozymes, GN1, GN2, GN3, GN4, and GN5, of intracellular β-1,3-glucanase were detected with glucan-containing high polymer complex as a substrate with or without colloidal chitin.

Microbial control of arthropod-borne disease

Arthropods harbor a diverse array of microbes that profoundly influence many aspects of host biology, including vector competence. Additionally, symbionts can be engineered to produce molecules that inhibit pathogens. Due to their intimate association with the host, microbes have developed strategies that facilitate their transmission, either horizontally or vertically, to conspecifics. These attributes make microbes attractive agents for applied strategies to control arthropod-borne disease. Here we discuss the recent advances in microbial control approaches to reduce the burden of pathogens such as Zika, Dengue and Chikungunya viruses, and Trypanosome and Plasmodium parasites. We also highlight where further investigation is warranted.

A natural Anopheles-associated Penicillium chrysogenum enhances mosquito susceptibility to Plasmodium infection

Whereas studies have extensively examined the ability of bacteria to influence Plasmodium infection in the mosquito, the tripartite interactions between non-entomopathogenic fungi, mosquitoes, and Plasmodium parasites remain largely uncharacterized. Here we report the isolation of a common mosquito-associated ascomycete fungus, Penicillium chrysogenum, from the midgut of field-caught Anopheles mosquitoes. Although the presence of Pe. chrysogenum in the Anopheles gambiae midgut does not affect mosquito survival, it renders the mosquito significantly more susceptible to Plasmodium infection through a secreted heat-stable factor. We further provide evidence that the mechanism of the fungus-mediated modulation of mosquito susceptibility to Plasmodium involves an upregulation of the insect’s ornithine decarboxylase gene, which sequesters arginine for polyamine biosynthesis. Arginine plays an important role in the mosquito’s anti-Plasmodium defense as a substrate of nitric oxide production, and its availability therefore has a direct impact on the mosquito’s susceptibility to the parasite. While this type of immunomodulatory mechanism has already been demonstrated in other host-pathogen interaction systems, this is the first report of a mosquito-associated fungus that can suppress the mosquito’s innate immune system in a way that would favor Plasmodium infection and possibly malaria transmission.

Midgut fungal and bacterial microbiota of Aedes triseriatus and Aedes japonicus shift in response to La Crosse virus infection

Understanding how midgut microbial communities of field-collected mosquitoes interact with pathogens is critical for controlling vector infection and disease. We used 16S rRNA and internal transcribed spacer sequencing to characterize the midgut bacterial and fungal communities of adult females of Aedes triseriatus and Aedes japonicus collected as pupae in tree holes, plastic bins and waste tires and their response to La Crosse virus (LACV) infection. For both mosquito species and across all habitat and virus treatments, a total of 62 bacterial operational taxonomic units (OTUs) from six phyla and 21 fungal OTUs from two phyla were identified. The majority of bacterial (92%) and fungal (71%) OTUs were shared between the mosquito species; however, several OTUs were unique to each species. Bacterial and fungal communities of individuals that took either infectious or noninfectious bloodmeals were less diverse and more homogeneous compared to those of newly emerged adults. Interestingly, LACV-infected A. triseriatus and A. japonicus had higher bacterial richness and lower fungal richness compared to individuals that took a noninfectious bloodmeal, suggesting that viral infection was associated with an increase in bacterial OTUs and a decrease in fungal OTUs. For both mosquito species, several OTUs were identified that had both high fidelity and specificity to mosquito midguts that were infected with LACV. Overall, these findings demonstrate that bacterial and fungal communities that reside in mosquito midguts respond to host diet and viral infection and could play a role in modulating vector susceptibility to LACV.

More than one rabbit out of the hat: Radiation, transgenic and symbiont-based approaches for sustainable management of mosquito and tsetse fly populations

Mosquitoes (Diptera: Culicidae) and tsetse flies (Diptera: Glossinidae) are bloodsucking vectors of human and animal pathogens. Mosquito-borne diseases (malaria, filariasis, dengue, zika, and chikungunya) cause severe mortality and morbidity annually, and tsetse fly-borne diseases (African trypanosomes causing sleeping sickness in humans and nagana in livestock) cost Sub-Saharan Africa an estimated US$ 4750 million annually. Current reliance on insecticides for vector control is unsustainable: due to increasing insecticide resistance and growing concerns about health and environmental impacts of chemical control there is a growing need for novel, effective and safe biologically-based methods that are more sustainable. The integration of the sterile insect technique has proven successful to manage crop pests and disease vectors, particularly tsetse flies, and is likely to prove effective against mosquito vectors, particularly once sex-separation methods are improved. Transgenic and symbiont-based approaches are in development, and more advanced in (particularly Aedes) mosquitoes than in tsetse flies; however, issues around stability, sustainability and biosecurity have to be addressed, especially when considering population replacement approaches. Regulatory issues and those relating to intellectual property and economic cost of application must also be overcome. Standardised methods to assess insect quality are required to compare and predict efficacy of the different approaches. Different combinations of these three approaches could be integrated to maximise their benefits, and all have the potential to be used in tsetse and mosquito area-wide integrated pest management programmes.

Culex pipiens Development Is Greatly Influenced by Native Bacteria and Exogenous Yeast

Culex pipiens is the most cosmopolitan mosquito of the Pipiens Assemblage. By studying the nature of interactions between this species and microorganisms common to its breeding environment we can unravel important pitfalls encountered during development. We tested the survival rate of larval stages, pupae and adults of a Cx. pipiens colony exposed to a variety of microorganisms in laboratory conditions and assessed the transmission to offspring (F1) by those organisms that secured development up to adulthood. Three complementary experiments were designed to: 1) explore the nutritional value of yeasts and other microorganisms during Cx. pipiens development; 2) elucidate the transstadial transmission of yeast to the host offspring; and 3) to examine the relevance of all these microorganisms in female choice for oviposition-substratum. The yeast Saccharomyces cerevisiae proved to be the most nutritional diet, but despite showing the highest survival rates, vertical transmission to F1 was never confirmed. In addition, during the oviposition trials, none of the gravid females was attracted to the yeast substratum. Notably, the two native bacterial strains, Klebsiella sp. and Aeromonas sp., were the preferred oviposition media, the same two bacteria that managed to feed neonates until molting into 2^nd instar larvae. Our results not only suggest that Klebsiella sp. or Aeromonas sp. serve as attractants for oviposition habitat selection, but also nurture the most fragile instar, L1, to assure molting into a more resilient stage, L2, while yeast proves to be the most supportive diet for completing development. These experiments unearthed survival traits that might be considered in the future development of strategies of Cx. pipiens control. These studies can be extended to other members of the Pipiens Assemblage.

Yeasts Associated with Culex pipiens and Culex theileri Mosquito Larvae and the Effect of Selected Yeast Strains on the Ontogeny of Culex pipiens

The success of mosquitoes in nature has been linked to their microbiota and bacteria in particular. Yet, knowledge on their symbioses with yeasts is lacking. To explore possible associations, culturable yeasts were isolated from wild larvae of Culex pipiens and Culex theileri. These yeasts were classified using restriction fragment length polymorphism (RFLP) analyses and identified by sequencing the D1/D2 region of the 26S rRNA gene. Representative strains of Candida, Cryptococcus, Galactomyces, Hannaella, Meyerozyma, Pichia, Rhodosporidium, Rhodotorula, Trichosporon and Wickerhamomyces were isolated. Our results provide, to our knowledge, the first records of the yeast microbiota from wild mosquito larvae and show that they may harbour potential clinically relevant yeast species, including the well-known opportunistic human pathogen Candida albicans. Also, diminished numbers of yeast isolates originating from adults, compared to larvae, support the hypothesis of microbial reduction/elimination during adult emergence and extend it to include yeasts. In addition, strains of Candida albicans, Candida glabrata, Candida pseudolambica, Cryptococcus gattii, Metschnikowia bicuspidata, Saccharomyces cerevisiae and Wickerhamomyces anomalus were tested as sole feed during a 21-day feeding experiment wherein cumulative larval growth, survival and pupation of Cx. pipiens were recorded. Although most yeasts supported larval growth in a similar manner to the positive control S. cerevisiae strain, the different yeast strains impacted differently on Culex pipiens ontogeny. Notably, survival and pupation of larvae were negatively impacted by a representative strain of the primary pathogen C. gattii - signifying some yeasts to be natural antagonists of mosquitoes.

Isolation of a Wickerhamomyces anomalus yeast strain from the sandfly Phlebotomus perniciosus, displaying the killer phenotype

The yeast Wickerhamomyces anomalus has been studied for its wide biotechnological potential, mainly for applications in the food industry. Different strains of W. anomalus have been isolated from diverse habitats and recently from insects, including mosquitoes of medical importance. This paper reports the isolation and phylogenetic characterization of W. anomalus from laboratory-reared adults and larvae of Phlebotomus perniciosus (Diptera: Psychodidae), a main phlebotomine vector of human and canine leishmaniasis. Of 65 yeast strains isolated from P. perniciosus, 15 strains were identified as W. anomalus; one of these was tested for the killer phenotype and demonstrated inhibitory activity against four yeast sensitive strains, as reported for mosquito-isolated strains. The association between P. perniciosus and W. anomalus deserves further investigation in order to explore the possibility that this yeast may exert inhibitory/killing activity against Leishmania spp.

A yeast strain associated to Anopheles mosquitoes produces a toxin able to kill malaria parasites

Background

Malaria control strategies are focusing on new approaches, such as the symbiotic control, which consists in the use of microbial symbionts to prevent parasite development in the mosquito gut and to block the transmission of the infection to humans. Several microbes, bacteria and fungi, have been proposed for malaria or other mosquito-borne diseases control strategies. Among these, the yeast Wickerhamomyces anomalus has been recently isolated from the gut of Anopheles mosquitoes, where it releases a natural antimicrobial toxin. Interestingly, many environmental strains of W. anomalus exert a wide anti-bacterial/fungal activity and some of these ‘killer’ yeasts are already used in industrial applications as food and feed bio-preservation agents. Since a few studies showed that W. anomalus killer strains have antimicrobial effects also against protozoan parasites, the possible anti-plasmodial activity of the yeast was investigated.

Methods

A yeast killer toxin (KT), purified through combined chromatographic techniques from a W. anomalus strain isolated from the malaria vector Anopheles stephensi, was tested as an effector molecule to target the sporogonic stages of the rodent malaria parasite Plasmodium berghei, in vitro. Giemsa staining was used to detect morphological damages in zygotes/ookinetes after treatment with the KT. Furthermore, the possible mechanism of action of the KT was investigated pre-incubating the protein with castanospermine, an inhibitor of β-glucanase activity.

Results

A strong anti-plasmodial effect was observed when the P. berghei sporogonic stages were treated with KT, obtaining an inhibition percentage up to around 90 %. Microscopy analysis revealed several ookinete alterations at morphological and structural level, suggesting the direct implication of the KT-enzymatic activity. Moreover, evidences of the reduction of KT activity upon treatment with castanospermine propose a β-glucanase-mediated activity.

Conclusion

The results showed the in vitro killing efficacy of a protein produced by a mosquito strain of W. anomalus against malaria parasites. Further studies are required to test the KT activity against the sporogonic stages in vivo, nevertheless this work opens new perspectives for the possible use of killer strains in innovative strategies to impede the development of the malaria parasite in mosquito vectors by the means of microbial symbionts.

Electronic supplementary material

The online version of this article (doi:10.1186/s12936-015-1059-7) contains supplementary material, which is available to authorized users.

Plasmodium falciparum suppresses the host immune response by inducing the synthesis of insulin-like peptides (ILPs) in the mosquito Anopheles stephensi

The insulin-like peptides (ILPs) and their respective signaling and regulatory pathways are highly conserved across phyla. In invertebrates, ILPs regulate diverse physiological processes, including metabolism, reproduction, behavior, and immunity. We previously reported that blood feeding alone induced minimal changes in ILP expression in Anopheles stephensi. However, ingestion of a blood meal containing human insulin or Plasmodium falciparum, which can mimic insulin signaling, leads to significant increases in ILP expression in the head and midgut, suggesting a potential role for AsILPs in the regulation of P. falciparum sporogonic development. Here, we show that soluble P. falciparum products, but not LPS or zymosan, directly induced AsILP expression in immortalized A. stephensi cells in vitro. Further, AsILP expression is dependent on signaling by the mitogen-activated protein kinase kinase/extracellular signal-regulated kinase (MEK/ERK) and phosphatidylinositol 3'-kinase (PI3K)/Akt branches of the insulin/insulin-like growth factor signaling (IIS) pathway. Inhibition of P. falciparum-induced ILPs in vivo decreased parasite development through kinetically distinct effects on mosquito innate immune responses. Specifically, knockdown of AsILP4 induced early expression of immune effector genes (1-6 h after infection), a pattern associated with significantly reduced parasite abundance prior to invasion of the midgut epithelium. In contrast, knockdown of AsILP3 increased later expression of the same genes (24 h after infection), a pattern that was associated with significantly reduced oocyst development. These data suggest that P. falciparum parasites alter the expression of mosquito AsILPs to dampen the immune response and facilitate their development in the mosquito vector.

Genitalia-associated microbes in insects

In sexual reproduction different types of symbiotic relationships between insects and microbes have become established. For example, some bacteria have evolved almost exclusive vertical transmission and even define the compatibility of insect mating partners. Many strictly sexually transmitted diseases have also been described in insects. Apart from such rather specific relationships the role of opportunistic infections in the reproductive process has been widely neglected. Opportunistic microbes transmitted passively during mating might impose an energetic cost, as the immune system will need to be alert and will use resources to fight potential intruders. Through mating wounds and contaminated reproductive organs opportunistic microbes might be transferred to mating partners and even enter the body cavity. Females as the "receiving" sex are particularly likely to have evolved adaptations to avoid or reduce opportunistic infections. Males of several species show highly complex seminal fluids, which as well as containing components that influence a males' fertilization success, also possess antimicrobial substances. The role of antimicrobials in the reproductive process is not well understood. Some evidence hints at the protection of sperm against microbes, indicating a role for natural selection in shaping the evolution of reproductive traits. By highlighting the potential importance of microbes in sexual selection and their role in reproduction in general I will make a case for studies in sexual selection, especially the ones investigating postcopulatory processes, that should incorporate environmental, as well as genotypic variation, in reproductive traits.

The insect microbiome modulates vector competence for arboviruses

Diseases caused by arthropod-borne viruses (arboviruses), such as Dengue, West Nile, and Chikungunya, constitute a major global health burden and are increasing in incidence and geographic range. The natural microbiota of insect vectors influences various aspects of host biology, such as nutrition, reproduction, metabolism, and immunity, and recent studies have highlighted the ability of insect-associated bacteria to reduce vector competence for arboviruses and other pathogens. This reduction can occur through mechanisms, such as immune response activation, resource competition, or the production of anti-viral molecules. Studying the interactions between insect vectors and their microbiota is an important step toward developing alternative strategies for arbovirus transmission control.

A Wickerhamomyces anomalus killer strain in the malaria vector Anopheles stephensi

The yeast Wickerhamomyces anomalus has been investigated for several years for its wide biotechnological potential, especially for applications in the food industry. Specifically, the antimicrobial activity of this yeast, associated with the production of Killer Toxins (KTs), has attracted a great deal of attention. The strains of W. anomalus able to produce KTs, called “killer” yeasts, have been shown to be highly competitive in the environment. Different W. anomalus strains have been isolated from diverse habitats and recently even from insects. In the malaria mosquito vector Anopheles stephensi these yeasts have been detected in the midgut and gonads. Here we show that the strain of W. anomalus isolated from An. stephensi, namely WaF17.12, is a killer yeast able to produce a KT in a cell-free medium (in vitro) as well as in the mosquito body (in vivo). We showed a constant production of WaF17.12-KT over time, after stimulation of toxin secretion in yeast cultures and reintroduction of the activated cells into the mosquito through the diet. Furthermore, the antimicrobial activity of WaF17.12-KT has been demonstrated in vitro against sensitive microbes, showing that strain WaF17.12 releases a functional toxin. The mosquito-associated yeast WaF17.12 thus possesses an antimicrobial activity, which makes this yeast worthy of further investigations, in view of its potential as an agent for the symbiotic control of malaria.

Genetic approaches to interfere with malaria transmission by vector mosquitoes

Malaria remains one of the most devastating diseases worldwide, causing over 1 million deaths every year. The most vulnerable stages of Plasmodium development in the vector mosquito occur in the midgut lumen, making the midgut a prime target for intervention. Mosquito transgenesis and paratransgenesis are two novel strategies that aim at rendering the vector incapable of sustaining Plasmodium development. Mosquito transgenesis involves direct genetic engineering of the mosquito itself for delivery of anti-Plasmodium effector molecules. Conversely, paratransgenesis involves the genetic modification of mosquito symbionts for expression of anti-pathogen effector molecules. Here we consider both genetic manipulation strategies for rendering mosquitoes refractory to Plasmodium infection, and discuss challenges for the translation of laboratory findings to field applications.

Symbiotic control of mosquito borne disease

It is well accepted that the symbiotic relationships insects have established with several microorganisms have had a key role in their evolutionary success. Bacterial symbiosis is also prevalent in insects that are efficient disease vectors, and numerous studies have sought to decrypt the basic mechanisms of the host-symbiont relationships and develop ways to control vector borne diseases. 'Symbiotic control', a new multifaceted approach that uses symbiotic microorganisms to control insect pests or reduce vector competence, seems particularly promising. Three such approaches currently at the cutting edge are: (1) the disruption of microbial symbionts required by insect pests; (2) the manipulation of symbionts that can express anti-pathogen molecules within the host; and (3) the introduction of endogenous microbes that affect life-span and vector capacity of the new hosts in insect populations. This work reviews current knowledge on microbial symbiosis in mosquitoes that holds promise for development of symbiotic control for mosquito borne diseases.

Mosquito/microbiota interactions: from complex relationships to biotechnological perspectives

To date around 3500 different species of mosquito have been described, several tens of which are vectors of pathogens of remarkable interest in public health. Mosquitoes are present all around the world showing a great ability to adapt to very different types of habitats where they play relevant ecological roles. It is very likely that components of the mosquito microbiota have given the mosquito a great capacity to adapt to different environments. Current advances in understanding the mosquito-microbiota relationships may have a great impact in a better understanding of some traits of mosquito biology and in the development of innovative mosquito-borne disease-control strategies aimed to reduce mosquito vectorial capacity and/or inhibiting pathogen transmission.

Dissemination of wine-related yeasts by migratory birds

The present work was undertaken to evaluate the contribution of migratory birds in the environmental dissemination of yeasts. Four sites (Mazara del Vallo, Lampedusa, Ustica and Linosa), representing the main stop-over points in Sicily, were analysed during spring and autumnal bird migration and 349 birds (belonging to 10 families) were ringed and analysed for yeast presence. A total of 125 yeasts were isolated and identified by a multiple genotypic approach, consisting of restriction fragment length polymorphism (RFLP) of 5.8S rRNA gene and 26S rRNA and sequencing of D1/D2 domain of the 26S rRNA gene, which resulted in the recognition of 18 species, including the technological relevant Saccharomyces cerevisiae which were characterized at strain level applying three techniques (interdelta analysis, minisatellite analysis based on the separate amplification of three genes and microsatellite multiplex PCR of polymorphic microsatellite loci). The evaluation of the persistence of living S. cerevisiae in birds for about 12 h from ingestion of inoculated feed allowed the conclusion that yeasts with technological potential are disseminated during migration.

Microbial symbionts: a resource for the management of insect-related problems

Microorganisms establish with their animal hosts close interactions. They are involved in many aspects of the host life, physiology and evolution, including nutrition, reproduction, immune homeostasis, defence and speciation. Thus, the manipulation and the exploitation the microbiota could result in important practical applications for the development of strategies for the management of insect-related problems. This approach, defined as 'Microbial Resource Management' (MRM), has been applied successfully in various environments and ecosystems, as wastewater treatments, prebiotics in humans, anaerobic digestion and so on. MRM foresees the proper management of the microbial resource present in a given ecosystem in order to solve practical problems through the use of microorganisms. In this review we present an interesting field for application for MRM concept, i.e. the microbial communities associated with arthropods and nematodes. Several examples related to this field of applications are presented. Insect microbiota can be manipulated: (i) to control insect pests for agriculture; (ii) to control pathogens transmitted by insects to humans, animals and plants; (iii) to protect beneficial insects from diseases and stresses. Besides, we prospect further studies aimed to verify, improve and apply MRM by using the insect-symbiont ecosystem as a model.

Identification of yeast associated with the planthopper, Perkinsiella saccharicida: potential applications for Fiji leaf gall control

Yeasts associate with numerous insects, and they can assist the metabolic processes within their hosts. Two distinct yeasts were identified by PCR within the planthopper Perkinsiella saccharicida, the vector of Fiji disease virus to sugarcane. The utility of both microbes for potential paratransgenic approaches to control Fiji leaf gall (FLG) was assessed. Phylogenetic analysis showed one of the microbes is related to yeast-like symbionts from the planthoppers: Laodelphax striatellus, Nilaparvata lugens, and Sogetella furcifera. The second yeast was a member of the Candida genus, a group that has been identified in beetles and recently described in planthoppers. Microscopy revealed the presence of yeast in the fat body of P. saccharicida. The Candida yeast was cultured, and transformation was accomplished by electroporation of Candida albicans codon optimized plasmids, designed to integrate into the genome via homologous recombination. Transgenic lines conferred resistance to the antibiotic nourseothricin and expression of green fluorescent protein was observed in a proportion of the yeast cells. Stably transformed yeast lines could not be isolated as the integrative plasmids presumably replicated within the yeast without integration into the genome. If stable transformation can be achieved, then this yeast may be useful as an agent for a paratransgenic control of FLG.

Different mosquito species host Wickerhamomyces anomalus (Pichia anomala): perspectives on vector-borne diseases symbiotic control

The genetic manipulation of the microbial community associated with hematophagus insects is particularly relevant for public health applications. Within mosquito populations, this relationship has been overlooked until recently. New advances in molecular biotechnology propose the genetic manipulation of mosquito symbionts to prevent the transmission of pathogens to humans by interfering with the obligatory life cycle stages within the insect through the use of effector molecules. This approach, defined as 'paratransgenesis', has opened the way for the investigation and characterization of microbes residing in the mosquito body, particularly those localised within the gut. Some interesting bacteria have been identified as candidates for genetic modification, however, endosymbiotic yeasts remain largely unexplored with little information on the symbiotic relationships to date. Here we review the recent report of symbiotic relationship between Wickerhamomyces anomalus (Pichia anomala) and several mosquito vector species as promising methods to implement control of mosquito-borne diseases.

Pichia anomala: cell physiology and biotechnology relative to other yeasts

Pichia anomala is a most interesting yeast species, from a number of environmental, industrial and medical aspects. This yeast has been isolated from very diverse natural habitats (e.g. in foods, insects, wastewaters etc.) and it also exhibits wide metabolic and physiological diversity. Some of the activities of P. anomala, particularly its antimicrobial action, make it a very attractive organism for biological control applications in the agri-food sectors of industry. Being a 'robust' organism, it additionally has potential to be exploited in bioremediation of environmental pollutants. This paper provides an overview of cell physiological characteristics (growth, metabolism, stress responses) and biotechnological potential (e.g. as a novel biocontrol agent) of P. anomala and compares such properties with other yeast species, notably Saccharomyces cerevisiae, which remains the most exploited industrial microorganism. We await further basic knowledge of P. anomala cell physiology and genetics prior to its fuller commercial exploitation, but the exciting biotechnological potential of this yeast is highlighted in this paper.