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.

Mutual exclusion of Asaia and Wolbachia in the reproductive organs of mosquito vectors

Background

Wolbachia is a group of intracellular maternally inherited bacteria infecting a high number of arthropod species. Their presence in different mosquito species has been largely described, but Aedes aegypti, the main vector of Dengue virus, has never been found naturally infected by Wolbachia. Similarly, malaria vectors and other anophelines are normally negative to Wolbachia, with the exception of an African population where these bacteria have recently been detected. Asaia is an acetic acid bacterium stably associated with several mosquito species, found as a dominant microorganism of the mosquito microbiota. Asaia has been described in gut, salivary glands and in reproductive organs of adult mosquitoes in Ae. aegypti and in anophelines. It has recently been shown that Asaia may impede vertical transmission of Wolbachia in Anopheles mosquitoes. Here we present an experimental study, aimed at determining whether there is a negative interference between Asaia and Wolbachia, for the gonad niche in mosquitoes.

Methods

Different methods (PCR and qPCR, monoclonal antibody staining and FISH) have been used to address the question of the co-localization and the relative presence/abundance of the two symbionts. PCR and qPCR were performed to qualitatively and quantitatively verify the distribution of Asaia and Wolbachia in different mosquito species/organs. Monoclonal antibody staining and FISH were performed to localize the symbionts in different mosquito species.

Results

Here we provide evidence that, in Anopheles and in other mosquitoes, there is a reciprocal negative interference between Asaia and Wolbachia symbionts, in terms of the colonization of the gonads. In particular, we have shown that in some mosquito species the presence of one of the symbionts prevented the establishment of the second, while in other systems the symbionts were co-localized, although at reduced densities.

Conclusions

A mutual exclusion or a competition between Asaia and Wolbachia may contribute to explain the inability of Wolbachia to colonize the female reproductive organs of anophelines, inhibiting its vertical transmission and explaining the absence of Wolbachia infection in Ae. aegypti and in the majority of natural populations of Anopheles mosquitoes.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-015-0888-0) contains supplementary material, which is available to authorized users.

Interactions between Asaia, Plasmodium and Anopheles: new insights into mosquito symbiosis and implications in malaria symbiotic control

Background

Malaria represents one of the most devastating infectious diseases. The lack of an effective vaccine and the emergence of drug resistance make necessary the development of new effective control methods. The recent identification of bacteria of the genus Asaia, associated with larvae and adults of malaria vectors, designates them as suitable candidates for malaria paratransgenic control.

To better characterize the interactions between Asaia, Plasmodium and the mosquito immune system we performed an integrated experimental approach.

Methods

Quantitative PCR analysis of the amount of native Asaia was performed on individual Anopheles stephensi specimens. Mosquito infection was carried out with the strain PbGFP_CON and the number of parasites in the midgut was counted by fluorescent microscopy.

The colonisation of infected mosquitoes was achieved using GFP or DsRed tagged-Asaia strains.

Reverse transcriptase-PCR analysis, growth and phagocytosis tests were performed using An. stephensi and Drosophila melanogaster haemocyte cultures and DsRed tagged-Asaia and Escherichia coli strains.

Results

Using quantitative PCR we have quantified the relative amount of Asaia in infected and uninfected mosquitoes, showing that the parasite does not interfere with bacterial blooming. The correlation curves have confirmed the active replication of Asaia, while at the same time, the intense decrease of the parasite.

The ‘in vitro’ immunological studies have shown that Asaia induces the expression of antimicrobial peptides, however, the growth curves in conditioned medium as well as a phagocytosis test, indicated that the bacterium is not an immune-target.

Using fluorescent strains of Asaia and Plasmodium we defined their co-localisation in the mosquito midgut and salivary glands.

Conclusions

We have provided important information about the relationship of Asaia with both Plasmodium and Anopheles. First, physiological changes in the midgut following an infected or uninfected blood meal do not negatively affect the residing Asaia population that seems to benefit from this condition. Second, Asaia can act as an immune-modulator activating antimicrobial peptide expression and seems to be adapted to the host immune response. Last, the co-localization of Asaia and Plasmodium highlights the possibility of reducing vectorial competence using bacterial recombinant strains capable of releasing anti-parasite molecules.

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

While symbiosis between bacteria and insects has been thoroughly investigated in the last two decades, investments on the study of yeasts associated with insects have been limited. Insect-associated yeasts are placed on different branches of the phylogenetic tree of fungi, indicating that these associations evolved independently on several occasions. Isolation of yeasts is frequently reported from insect habitats, and in some cases yeasts have been detected in the insect gut and in other organs/tissues. Here we show that the yeast Wickerhamomyces anomalus, previously known as Pichia anomala, is stably associated with the mosquito Anopheles stephensi, a main vector of malaria in Asia. Wickerhamomyces anomalus colonized pre-adult stages (larvae L(1)-L(4) and pupae) and adults of different sex and age and could be isolated in pure culture. By a combination of transmission electron microscopy and fluorescent in situ hybridization techniques, W. anomalus was shown to localize in the midgut and in both the male and female reproductive systems, suggesting multiple transmission patterns.

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.

Mosquito-bacteria symbiosis: the case of Anopheles gambiae and Asaia

The symbiotic relationship between Asaia, an α-proteobacterium belonging to the family Acetobacteriaceae, and mosquitoes has been studied mainly in the Asian malaria vector Anopheles stephensi. Thus, we have investigated the nature of the association between Asaia and the major Afro-tropical malaria vector Anopheles gambiae. We have isolated Asaia from different wild and laboratory reared colonies of A. gambiae, and it was detected by PCR in all the developmental stages of the mosquito and in all the specimens analyzed. Additionally, we have shown that it localizes in the midgut, salivary glands and reproductive organs. Using recombinant strains of Asaia expressing fluorescent proteins, we have demonstrated the ability of the bacterium to colonize A. gambiae mosquitoes with a pattern similar to that described for A. stephensi. Finally, fluorescent in situ hybridization on the reproductive tract of females of A. gambiae showed a concentration of Asaia at the very periphery of the eggs, suggesting that transmission of Asaia from mother to offspring is likely mediated by a mechanism of egg-smearing. We suggest that Asaia has potential for use in the paratransgenic control of malaria transmitted by A. gambiae.

Bacteria of the genus Asaia stably associate with Anopheles stephensi, an Asian malarial mosquito vector

Here, we show that an alpha-proteobacterium of the genus Asaia is stably associated with larvae and adults of Anopheles stephensi, an important mosquito vector of Plasmodium vivax, a main malaria agent in Asia. Asaia bacteria dominate mosquito-associated microbiota, as shown by 16S rRNA gene abundance, quantitative PCR, transmission electron microscopy and in situ-hybridization of 16S rRNA genes. In adult mosquitoes, Asaia sp. is present in high population density in the female gut and in the male reproductive tract. Asaia sp. from An. stephensi has been cultured in cell-free media and then transformed with foreign DNA. A green fluorescent protein-tagged Asaia sp. strain effectively lodged in the female gut and salivary glands, sites that are crucial for Plasmodium sp. development and transmission. The larval gut and the male reproductive system were also colonized by the transformed Asaia sp. strain. As an efficient inducible colonizer of mosquitoes that transmit Plasmodium sp., Asaia sp. may be a candidate for malaria control.

Lyme disease and babesiosis: preliminary findings on the transmission risk in highly frequented areas of the Monti Sibillini National Park (Central Italy)

The Monti Sibillini National Park is a recently borne protected area in the central Apennines that has become an important tourist destination. Mountain grasslands and woods have made it also a suitable habitat for sheep and bovine cattle, as well as for wild animals such as the wild bore and the roe deer. Therefore, a preliminary investigation was conducted to assess the risk of transmission of tick-borne zoonoses, such as Lyme disease and Babesiosis, by actively looking for both the arthropod vector and the causative agent. For two consecutive years, ambushing ticks were collected in four distinct geographical areas, comprehensive of many highly frequented tourist places. The tick fauna, tick habitats and the seasonal distribution of the different tick life stages collected with the method of "flagging and dragging" have been reported. Almost all the collected specimens belonged to the species Haemaphysalis punctata (Canestrini and Fanzago) (Acari: Ixodidae), which was found moderately infected with Babesia spp. Only a few Ixodes ricinus (Linnaeus) ticks, the most competent vector of Lyme disease, were found and on PCR examination all of them resulted negative as far as the infectious agent Borrelia burgdorferi sensu lato (Johnson) is concerned.

An estimation of Toxocara canis prevalence in dogs, environmental egg contamination and risk of human infection in the Marche region of Italy

The human risk of infection with larvae of Toxocara canis was estimated in people from the Marche region of Italy. This region includes both urban and rural areas and its inhabitants frequently keep dogs for company, hunting, as guardians or shepherds. T. canis infection was diagnosed in 33.6% out of 295 dogs examined. Nearly half of the dogs (48.4%) living in rural areas were found T. canis positive, compared to about one-quarter of the dogs (26.2%) from urban areas. Analysis by provenance and role revealed the highest infection rate in rural hunting dogs (64.7%) and the lowest in urban companion dogs (22.1%). According to questionnaire data, the peridomestic environment, i.e. gardens and dog pens, is the most important defecation site in both rural and urban areas. Since over 40% of the dogs who defecate in dog pens are infected and 24% of urban and 47% of rural dogs who leave their droppings in the house surroundings harbour the parasite, it is clear that these environments may constitute sites of zoonotic risk. Our analysis of soil samples from 60 farms confirmed the high contamination level, revealing positive soil samples in more than half of the farms. Substantial egg contamination was also found in urban areas, as 3/6 parks examined were Toxocara spp. positive. Finally, our serological findings indicate that human infection actually occurs in the area: 7 out of 428 adults examined (1.6%) had very high levels of antibodies to T. canis antigen, suggesting a previous contact with the Larva migrans of the nematode.