The composition and abundance of bacterial communities residing in the gut of Glossina palpalis palpalis captured in two sites of southern Cameroon

Background

A number of reports have demonstrated the role of insect bacterial flora on their host’s physiology and metabolism. The tsetse host and vector of trypanosomes responsible for human sleeping sickness (human African trypanosomiasis, HAT) and nagana in animals (African animal trypanosomiasis, AAT) carry bacteria that influence its diet and immune processes. However, the mechanisms involved in these processes remain poorly documented. This underscores the need for increased research into the bacterial flora composition and structure of tsetse flies. The aim of this study was to identify the diversity and relative abundance of bacterial genera in Glossina palpalis palpalis flies collected in two trypanosomiasis foci in Cameroon.

Methods

Samples of G. p. palpalis which were either negative or naturally trypanosome-positive were collected in two foci located in southern Cameroon (Campo and Bipindi). Using the V3V4 and V4 variable regions of the small subunit of the 16S ribosomal RNA gene, we analyzed the respective bacteriome of the flies’ midguts.

Results

We identified ten bacterial genera. In addition, we observed that the relative abundance of the obligate endosymbiont Wigglesworthia was highly prominent (around 99%), regardless of the analyzed region. The remaining genera represented approximately 1% of the bacterial flora, and were composed of Salmonella, Spiroplasma, Sphingomonas, Methylobacterium, Acidibacter, Tsukamurella, Serratia, Kluyvera and an unidentified bacterium. The genus Sodalis was present but with a very low abundance. Globally, no statistically significant difference was found between the bacterial compositions of flies from the two foci, and between positive and trypanosome-negative flies. However, Salmonella and Serratia were only described in trypanosome-negative flies, suggesting a potential role for these two bacteria in fly refractoriness to trypanosome infection. In addition, our study showed the V4 region of the small subunit of the 16S ribosomal RNA gene was more efficient than the V3V4 region at describing the totality of the bacterial diversity.

Conclusions

A very large diversity of bacteria was identified with the discovering of species reported to secrete anti-parasitic compounds or to modulate vector competence in other insects. For future studies, the analyses should be enlarged with larger sampling including foci from several countries.

Electronic supplementary material

The online version of this article (10.1186/s13071-019-3402-2) contains supplementary material, which is available to authorized users.

Modulation of trypanosome establishment in Glossina palpalis palpalis by its microbiome in the Campo sleeping sickness focus, Cameroon

The purpose of this study was to investigate factors involved in vector competence by analyzing whether the diversity and relative abundance of the different bacterial genera inhabiting the fly's gut could be associated with its trypanosome infection status. This was investigated on 160 randomly selected G. p. palpalis flies - 80 trypanosome-infected, 80 uninfected - collected in 5 villages of the Campo trypanosomiasis focus in South Cameroon. Trypanosome species were identified using specific primers, and the V4 region of the 16S rRNA gene of bacteria was targeted for metabarcoding analysis in order to identify the bacteria and determine microbiome composition. A total of 261 bacterial genera were identified of which only 114 crossed two barriers: a threshold of 0.01% relative abundance and the presence at least in 5 flies. The secondary symbiont Sodalis glossinidius was identified in 50% of the flies but it was not considered since its relative abundance was much lower than the 0.01% relative abundance threshold. The primary symbiont Wigglesworthia displayed 87% relative abundance, the remaining 13% were prominently constituted by the genera Spiroplasma, Tediphilus, Acinetobacter and Pseudomonas. Despite a large diversity in bacterial genera and in their abundance observed in micobiome composition, the statistical analyzes of the 160 tsetse flies showed an association with flies' infection status and the sampling sites. Furthermore, tsetse flies harboring Trypanosoma congolense Savanah type displayed a different composition of bacterial flora compared to uninfected flies. In addition, our study revealed that 36 bacterial genera were present only in uninfected flies, which could therefore suggest a possible involvement in flies' refractoriness; with the exception of Cupriavidus, they were however of low relative abundance. Some genera, including Acinetobacter, Cutibacterium, Pseudomonas and Tepidiphilus, although present both in infected and uninfected flies, were found to be associated with uninfected status of tsetse flies. Hence their effective role deserves to be further evaluated in order to determine whether some of them could become targets for tsetse control of fly vector competence and consequently for the control of the disease. Finally, when comparing the bacterial genera identified in tsetse flies collected during 4 epidemiological surveys, 39 genera were found to be common to flies from at least 2 sampling campaigns.

Prevalence of dermal trypanosomes in suspected and confirmed cases of gambiense human African trypanosomiasis in Guinea

The skin is an anatomical reservoir for African trypanosomes, yet the prevalence of extravascular parasite carriage in the population at risk of gambiense Human African Trypanosomiasis (gHAT) remains unclear. Here, we conducted a prospective observational cohort study in the HAT foci of Forecariah and Boffa, Republic of Guinea. Of the 18,916 subjects serologically screened for gHAT, 96 were enrolled into our study. At enrolment and follow-up visits, participants underwent a dermatological examination and had blood samples and superficial skin snip biopsies taken for examination by molecular and immuno-histological methods. In seropositive individuals, dermatological symptoms were significantly more frequent as compared to seronegative controls. Trypanosoma brucei DNA was detected in the blood of 67% of confirmed cases (22/33) and 9% of unconfirmed seropositive individuals (3/32). However, parasites were detected in the extravascular dermis of up to 71% of confirmed cases (25/35) and 41% of unconfirmed seropositive individuals (13/32) by PCR and/or immuno-histochemistry. Six to twelve months after treatment, trypanosome detection in the skin dropped to 17% of confirmed cases (5/30), whereas up to 25% of unconfirmed, hence untreated, seropositive individuals (4/16) were still found positive. Dermal trypanosomes were observed in subjects from both transmission foci, however, the occurrence of pruritus and the PCR positivity rates were significantly higher in unconfirmed seropositive individuals in Forecariah. The lower sensitivity of superficial skin snip biopsies appeared critical for detecting trypanosomes in the basal dermis. These results are discussed in the context of the planned elimination of gHAT.

Patterns of microbiome composition in tsetse fly Glossina palpalis palpalis during vector control using Tiny Targets in Campo, South Cameroon

Novel vector control tools against African trypanosomiases require a deep understanding of the factors driving tsetse vector fitness or population resilience in their ecosystems. Following evidence of microbiota-mediated host fitness or traits shaping, including insecticide resistance in arthropod populations, we undertook a comparative study of the microbiota in wild-caught tsetse flies during vector control with deltamethrin-impregnated traps called Tiny Targets. The bacterial microbiome composition of tsetse flies collected before and after 6, 12, and 18 months of vector control were characterized using high-throughput sequencing of the V3-V4 hypervariable region of the bacterial 16S rRNA gene and compared. Overall, 48 bacterial genera and five phyla were identified. The primary symbiont Wigglesworthia dominated almost all the samples with an overall relative abundance of 71.76%. A significant increase was observed in microbiome diversities over the vector control with new taxa identified. Interestingly, few genera, like Curvibacter for instance, displayed a regularly increasing abundance, from 0.57% to 0.65%, 4.73%, and 8.57% after 6, 12, and 18 months of tsetse control, respectively. This study provided preliminary for further investigation into the role and mechanism of action of microbiota in tsetse fly fitness under selective pressure like insecticides.IMPORTANCEThe interest in vector control in the fight against African trypanosomiases has been reinforced in recent years, with the development of small insecticide-impregnated screens, known as "Tiny Targets". As some tsetse biotopes are difficult to access for their installation, other tools are under consideration that involve using bacteria harbored by the tsetse vector to block the development of trypanosomes or impair the tsetse's fitness in its natural environment. Several bacterial symbionts were previously described as important for tsetse fly development, and some like Burkholderia and Citrobacter also found in tsetse flies were found associated with insecticide tolerance in other arthropods. In this research, we found the bacterial genera, Curvibacter and Acinetobacter, increased in abundance in tsetse flies during vector control. These bacteria deserve further attention to determine if they can interfere with insecticides used to control tsetse fly populations.

FLAgellum Member 8 modulates extravascular distribution of African trypanosomes

In the mammalian host, the biology of tissue-dwelling Trypanosoma brucei parasites is not completely understood, especially the mechanisms involved in their extravascular colonization. The trypanosome flagellum is an essential organelle in multiple aspects of the parasites' development. The flagellar protein termed FLAgellar Member 8 (FLAM8) acts as a docking platform for a pool of cyclic AMP response protein 3 (CARP3) that is involved in signaling. FLAM8 exhibits a stage-specific distribution suggesting specific functions in the mammalian and vector stages of the parasite. Analyses of knockdown and knockout trypanosomes in their mammalian forms demonstrated that FLAM8 is not essential in vitro for survival, growth, motility and stumpy differentiation. Functional investigations in experimental infections showed that FLAM8-deprived trypanosomes can establish and maintain an infection in the blood circulation and differentiate into insect transmissible forms. However, quantitative bioluminescence imaging and gene expression analysis revealed that FLAM8-null parasites exhibit a significantly impaired dissemination in the extravascular compartment, that is restored by the addition of a single rescue copy of FLAM8. In vitro trans-endothelial migration assays revealed significant defects in trypanosomes lacking FLAM8. FLAM8 is the first flagellar component shown to modulate T. brucei distribution in the host tissues, possibly through sensing functions, contributing to the maintenance of extravascular parasite populations in mammalian anatomical niches, especially in the skin.