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Table of Contents
RESEARCH ARTICLE
Year : 2021  |  Volume : 58  |  Issue : 1  |  Page : 74-84

Characterization of midgut microbiome of Anopheles stephensi Liston


1 ICMR-National Institute of Malaria Research, DHS Building, Campal, Panaji, Goa, India; Department of Zoology, Goa University, India
2 ICMR-National Institute of Malaria Research, DHS Building, Campal, Panaji, Goa, India
3 Department of Microbiology, Goa University, Goa, India
4 ICMR-Vector Control Research Centre (ICMR), Indira Nagar, Puducherry, India

Date of Submission23-Feb-2020
Date of Acceptance27-Feb-2020
Date of Web Publication18-Nov-2021

Correspondence Address:
Ashwani Kumar
ICMR-Vector Control Research Centre (ICMR), Indira Nagar, Puducherry
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0972-9062.289392

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  Abstract 

Background & objectives: Anopheles stephensi is an important vector of malaria in South East Asia. The abundance and diversity of gut microbiota in the disease vectors affect their development, digestion, metabolism and immunity. The immatures of An. stephensi engulf microbes from their aquatic environment. The present study investigates midgut microbiota of wild and laboratory populations and compares it with their habitat bacterial diversity to study transstadial transmissibility.
Methods: The gut microbes from immatures, adults and water samples were cultured at ambient conditions on different media. The colony and biochemical characteristics, and 16S rRNA gene sequencing of gut microbes were studied.
Results: Altogether, 298 bacterial isolates were characterized as 21 genera belonging to four major Phyla viz., Actinobacteria, Bacteroidetes, Firmicutes and Proteobacteria. In the field population-1, Proteobacteria and Firmicutes accounted for 49% and Actinobacteria constituted 51% of the bacterial isolates. In field population-2, Bacteroidetes and Firmicutes accounted for 99% of the isolates. In the laboratory populations, Firmicutes constituted 77%, while Proteobacteria 23% of the isolates. Additionally, 9 genera occurred in the breeding habitats, 13 in the larval midgut, 6 in pupal midgut, 9 in male midgut and 10 in the female midgut.
Interpretation & conclusion: This is a unique study on diversity of microbiota of An. stephensi from breeding water, developmental stages and adults. Different culture media used enhanced the isolation of diverse bacteria. The presence of Micrococcus and Leucobacter in different life stages indicates their adaptation in An. stephensi as symbionts which need further evaluation for their role in paratransgenesis.

Keywords: Mosquito; microbiota; diversity; midgut; transstadial


How to cite this article:
Pereira MH, Mohanty AK, Garg S, Tyagi S, Kumar A. Characterization of midgut microbiome of Anopheles stephensi Liston. J Vector Borne Dis 2021;58:74-84

How to cite this URL:
Pereira MH, Mohanty AK, Garg S, Tyagi S, Kumar A. Characterization of midgut microbiome of Anopheles stephensi Liston. J Vector Borne Dis [serial online] 2021 [cited 2021 Nov 27];58:74-84. Available from: https://www.jvbd.org/text.asp?2021/58/1/74/289392




  Introduction Top


Around 85% of malaria cases were reported in nineteen countries, i.e. in India and 18 African countries in 2018. Globally, 53% of Plasmodium vivax burden was in South East Asia region with 47% of cases reported in India[1]. Anopheles stephensi pose a major threat to human health as malaria vector, in urban areas globally and in several small, medium and metropolitan cities of India[2]. Some 0.84 million malaria cases and 194 deaths were reported by the National Vector Borne Disease Control Programme in 2017, with the highest number of cases reported from Odisha state[3].

An. stephensi prefers to breed in manmade habitats viz., wells, fountains, masonary tanks, overhead tanks, groundwater tanks, tyres, barrels, tins, intra domestic containers and curing water in construction sites[2],[4],[5]. A previous study has showed that 0.1 to 0.4 percent of all curing waters stagnant in the construction sites in Goa, India acted as breeding habitats for An. stephensi[6]. Breeding sites are rich source of microflora and their microbial content varies depending on the depth and type of water bodies and other abiotic factors. There is a probability of intake of these microorganisms by immatures from the breeding habitats during filter feeding and subsequently becoming a part of their gut micro flora[7].

Earlier investigations had demonstrated diverse influences of microbiota on the vector competence of mosquitoes. For instance, in Anopheles gambiae, removal of the gut microbiome increased its susceptibility to Plasmodium falciparum infection. On the contrary, Boissiere et al. demonstrate a positive correlation between the abundance of Enterobacteriaceae family in the midgut of mosquito and the Plasmodium infection status[8],[9]. Microorganisms like Bacillus and Staphylococcus help in maintaining the normal and high fecundity in Culex pipiens[10]. Similarly, Wolbachia is also known to play an important role in enhancing the rate of vertical transmission in the host by manipulating the Cytoplasmic incompatibility[11]. Also, microbiota has been found to play a significant role in determining the vectorial capacity of the Anophelines[8],[9],[12].

The composition of the midgut microbiome may vary due to diverse factors and one of them which can directly influence the vector microflora diversity is the breeding habitat. Hence, understanding the correlation between the type of breeding habitat and its influence on midgut microbiome remains an exciting area of research. Paratransgenesis is an upcoming vector control strategy that uses mosquito associated microbiome for the control of vector borne diseases. Screening of isolates for paratransgenesis is routinely carried out using culture methods. For example, the introduction of Asaia, which induces the activation of mosquito immunity significantly, reduces the development of malaria parasite in An. stephensi[13]. This strategy could be exploited to combat malaria[14].

The present study focuses on the isolation and characterization of microflora using the cell culture techniques as has been previously attempted successfully[8],[15],[16],[17],[18]. Among all the techniques, routine culture method provides deeper insight into the different characteristics of the bacteria. Hence, this study will not only elucidate the influence of breeding habitats on vector competence, but will also provide information on whether there is a transstadial transmission of the gut microbiota across various stages of the mosquito life cycle. The deciphering of the gut microbiota would be a first preliminary step towards its manipulation as a tool in the integrated vector management and the control of vector borne diseases.


  Material & Methods Top


Study area and time

The wild larval populations were sampled from a construction site located in Ponda city of Goa, India. The larval collections were made from the puddle (FS-1) and Curing waters on cement slabs (FS-2). The sampling of the larvae was carried out from FS-1 in pre-monsoon and from FS-2 in post-monsoon period during May 2015 to August 2016. For a comparison, the laboratory reared larval population was obtained from the cyclic colony of An. stephensi maintained at National Institute of Malaria Research, Field Unit at Goa.

Collection and maintenance of An. stephensi immatures from the breeding habitats

The larvae of An. stephensi were separated and identified based on their morphology and distinct mobility pattern. The identity of the species was further confirmed taxonomically from adults reared from the pupae obtained from the sampling sites. The immatures were collected from the breeding sites in sterile containers using the dipping technique[19]. Similarly, water samples from the breeding site were also collected for the maintenance of immatures in the laboratory. The collected specimens along with breeding water were labelled, transported and reared in the insectary of National Institute of Malaria Research, field unit at Goa, India.

Maintenance of An. stephensi immatures in the laboratory conditions

The third and the fourth instar larvae were separated from the population collected from the breeding sites and reared in separate sterile bowls that were suitably labelled. The rearing of immatures was done in the field collected water brought from their respective breeding habitat (FS-1 & FS-2).

Pupae were collected from the breeding sites in a 500 ml plastic bowls containing 200 ml sterilised water and were placed inside a closed cage measuring 10 cubic feet for the emergence and identification. The immatures were reared in the plastic trays under laboratory conditions at 27±2°C, 70 ± 5% relative humidity and 12h light and 12h dark photo period cycle. A pinch of sterilised food, a combination of CerelacTM powder (Nestle) and fish food in a ratio of 1:1, was given to the larvae once a day until pupal stage. On emergence, the species of adult mosquitoes were identified using standard taxonomical keys[20]. Freshly emerged adults were offered 10% sterilised glucose solution soaked in a sterilised cotton pad for the first 24 hours.

Surface sterilisation and midgut isolation from immatures and adults

For the isolation of microbiota, the larvae, pupae, adults (male and female) were surface sterilized by immersing into Sodium hypochlorite solution (10%) for a minute followed by immersion into alcohol (70%) for five minutes. They were washed with Phosphate Buffer Saline (PBS) four times to remove all traces of alcohol. The sterile immatures and adults were placed on a sterile slide containing a drop of normal saline for dissection. The dissected midguts isolated from the larvae, pupae, adults (male and female) of An. stephensi from different sites (lab and the two field samples) were then collected in the sterile Eppendorf tubes containing PBS (using modified protocol)[15],[19]. The numbers of midguts dissected from each stage are given as supplementary information S1.[Additional file 1]

Isolation of microbiota from the midgut of immatures and adults of An. stephensi

The dissected midguts were completely homogenised with the sterile pestle and Eppendorf tubes in PBS and the extracts were serially diluted up to 10-5 dilution. Each dilution was spread onto five different media i.e. Blood Agar (BA), Brain Heart Agar (BHA), Nutrient Agar (NA), Luria Berthani Agar (LBA) and Yeast Extract Agar (YEA) plates[9],[15],[19],[21]. The breeding water sample was also diluted and spread onto these media. The culture plates were incubated for 24–48 hr at 30°C. After incubation Total Viable Count (TVC) was recorded and the isolates were picked up based on their differences in colony morphology. The isolates were re-streaked onto respective agar medium to obtain the pure cultures. The purified colonies were maintained at 4°C.

Morphological and biochemical characterization of the isolates

The isolates were screened based on their colony characteristics including size, shape, colour, elevation and texture. The cell morphology of the isolate was determined using Gram staining. Microbial isolates were further characterised based on biochemical tests namely Glucose utilization, Citrate, utilisation, Catalase, Methyl Red, Vogues Proskauer, Nitrate Reductase and Gelatinase production[15]. These tests were conducted for the preliminary identification of the isolates following the Bergey’s Manual. Initial grouping and clustering of the isolates were based on their colony morphology, gram characteristics and biochemical methods[15],[16],[19].

Extraction of genomic DNA and 16S rRNA gene amplification of the isolates for molecular identification

The 298 purified bacterial isolates were grown for 24 hr, in respective media at 30°C. DNA was extracted using the protocol as described[22]. Gene sequences of 16S rRNA was amplified using the universal primers, (Forward: S-D-Bact-0011-a-5-17:5`-GTTTGATCCTGGCTCAG-3 & Reverse: S-*-Univ- 1392-b-A-15: 5`-ACGGGCG-GTGTGTNC-3`). Amplification of 1.5 kilo base (kb) of the 16S rRNA gene was carried out from all of the DNA samples extracted from individual bacterial isolates. The PCR conditions were set as an initial denaturation at 94°C for 3 min, followed by 35 cycles of denaturation at 94°C for 1 min, annealing at 60°C for 1 min, and extension at 72°C for 2 mins, followed by a final extension at 72°C for 5 min. All successfully amplified 16S rRNA amplicons were purified using QIA quick PCR purification kit and QIAquick Gel extraction kit. The 16s rRNA were then sequenced using illumina Miseq platform.

Sequencing and phylogenetic analysis

In order to identify the bacterial species, sequences obtained in the study were compared with the Genbank database using the BLAST algorithm and were submitted to the GenBank under the accession numbers as given in (Supplementary information S2). Multiple sequence alignment of partial 16s rRNA gene sequence was performed and the phylogenetic relatedness of 298 sequences were analysed by using Molecular Evolutionary Genetic Analysis (MEGA 10.0.05). Phylogenetic analysis was performed using Maximum Likelihood method and Tamura-Nei model, with a bootstrap analysis of 1000 replicates. Sulpholobus solfataricus (Accession no. X03235) was selected as an out-group[15].

Statistical analysis

In order to account for the number of genera present in the different stages of the midgut and breeding water, the diversity indices like Shannon diversity, Simpson’s diversity and Shannon Evenness was determined. These indices assist in assessing bacterial generic diversity obtained from the culture based method. Species diversity and evenness were calculated for all the isolates at different developmental stages. Simpson’s Diversity was calculated using the formula given by Mohlmann[23]. The presence of each isolate on the same plate was considered for calculation of the indices (Supplementary information S2). Simpson’s Diversity reflects the probability that two individuals taken randomly from the dataset are not the same species. Values for Simpson’s diversity range between 0 and 1, with larger values representing greater diversity. The Shannon Diversity is based on the uncertainty that an individual taken at random from the dataset is predicted correctly as a certain species. Larger values represented larger uncertainty, thus greater diversity. Shannon evenness was also calculated, the values ranged between 0 and 1; where 1 is complete evenness, i.e. all species being equally abundant[23].


  Results Top


Microbial diversity and abundance was calculated for different stages of Anopheles stephensi along with breeding water of both laboratory reared and wild population. Overall a total of 340 midguts were dissected from the larvae, pupae, male and female of both laboratory reared and wild population of An. stephensi. Total no. of colonies isolated on each medium plate for different stages were used to determine the total viable count (TVC). A total number of 4744 isolated colonies were obtained from all the stages and different sites, which were then characterised using biochemical and molecular tools.

Total viable count (bacterial abundance)

The midgut homogenate was spread plate onto five different media, the TVC was calculated for each stage on all five media. The highest bacterial count of 2.40 × 105cfu/midgut was obtained from the larval midgut of FS-2 sample on BA. The least bacterial count of 2.38 × 102cfu/midgut was obtained from the midgut of laboratory reared larvae on YEA media. The midguts isolated from the male of laboratory population of An. stephensi showed, the highest bacterial count followed by female, pupae and larvae on LBA, BHA and YEA media respectively as shown in [Figure 1]. In case of NA and BA the highest count was seen in the male midgut followed by female, larvae and pupae.
Figure 1: Comparative Total Viable Count of midgut of different life stages of Anopheles stephensi in the laboratory population on five different media

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In case of water collection site (FS-1 sample) population, the highest bacterial count of 15 × 105cfu/midgut was obtained from the pupal midguts on BA as shown in [Figure 2]. The least count of 2.75 × 102cfu/midgut was obtained from the midguts of male on BA. In case of LBA and YEA, the larvae showed the highest count followed by the pupae. On LBA medium, both male and female mid-guts showed the similar bacterial count. Bacterial count on YEA was higher in male than in female. On NA and BA media, the pupae midgut showed highest TVC followed by larvae with male and female adults showed similar counts. Bacterial count obtained from the female midgut was greater than in male on BA. On BHA medium, the female midgut showed the highest TVC followed by pupae, larvae and male.
Figure 2: Comparative Total Viable Count of midgut of different life stages of Anopheles stephensi in FS-1 sample population on five different media

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In case of second breeding site (FS-2: curing water), a highest count of 2.4 × 105cfu/midgut was obtained from the midgut of larvae on the BA medium and the least was in the case of male midgut on YEA medium (7.82 × 102cfu/midgut) as shown in [Figure 3]. In the case of BA, LBA and NA media, the highest bacterial count was obtained from the larval midgut followed by female, pupae and male midgut. In BHA, the highest count was found in the larvae midgut followed by male, female and pupae. On YEA medium, the highest count was found in the larvae midgut followed by midgut of pupae, female and male. Different media used showed diverse viability counts, suggesting the significance of using multiple media in isolating the microflora from the different stages of the vector species and the breeding sites.
Figure 3: Comparative Total Viable Count of midgut of different life stages of Anopheles stephensi from FS-2 population on five different media

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Determination of midgut microbiome associated with different life cycle stages

Total no. of colonies isolated on each medium plate from different stages were used to determine the Total viable count. A total of 4744 isolates were obtained from all the stages among different sites. Altogether 298 isolates-from 4744 were selected for characterization based on biochemical and 16S rRNA sequencing method. The characterization of gut flora in An. stephensi from three different populations using culture dependent method led to the identification of 21 bacterial genera. These were isolated from different media and mainly belonged to four major phyla of bacteria, namely Actinobacteria,Bacteroidetes, Firmicutes and Proteobacteria. The detailed information of every genus at each developmental stage and their accession numbers are given in the phylogenetic tree shown in [Figure S3],[Figure S4],[Figure S5]. In FS-1 sample, Phyla Proteobacteria and Firmicutes accounted for 49% of the species, while Actinobacteria constituted about 50% of the total bacterial isolates. In case of FS-2 sample, Bacteroidetes and Firmicutes accounted for 99% of the isolates. While, in the Laboratory population, Firmicutes constituted about 77% of total isolates with 23% of Proteobacteria. A total of 9 genera were observed from the breeding habitat of An. stephensi, 13 from the larval midgut, 6 from the pupal midgut, 9 from the male midgut and 10 from the female midgut. Overall, the most predominant Phylum was found to be Firmicutes, which was recorded in all the stages of mosquitoes of both laboratory and wild population. All the different isolates were clustered (grouped) in the phylogenetic tree analysis as shown in [Figure S3].

Microbiota of breeding habitats

As for bacterial isolates from water samples, four bacterial genera were detected in FS-1 sample, six in FS-2 sample and two in the laboratory water sample as shown in [Figure 4],[Figure 5],[Figure 6]. Among all the isolates obtained from the breeding habitat, Bacillus was isolated from all three sites. The least count was found in FS-1 water sample, followed by FS-2 sample and Laboratory sample. Exiguobacterium was found in FS-1 (67%) and FS-2 (33%) samples. Micrococcus and Kocuria were present in the water sample from FS-1 site. While Leucobacter, Lysinibacillus, Microbacterium and Pedobacter were identified only from the breeding habitat water of the FS-2, with an exception of Kocuria which was reported in FS-1 site only.
Figure 4: Relative abundance of the bacterial genera within mosquito midguts of larval, pupal, adult stages and from the water sample of breeding site (FS-1).

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Figure 5: Relative abundance of the bacterial genera within mosquito midguts of larval, pupal, adult stages and from the curing water breeding habitat sample (FS-2).

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Figure 6: Relative abundance of the bacterial genera in midguts of larval, pupal, adult stages and water used for rearing An. stephensi in the laboratory.

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Midgut microbiota of the larvae of An. stephensi

The larvae midgut showed presence of genera Bacillus and Micrococcus in all three sampling sites. Bacillus was identified with highest percentage (81%) in LAB population followed by FS-1sample (39%) and FS-2 sample (6%) respectively. Leucobacter growth was seen higher in FS-2 in comparison to FS-1. While Lysinibacillus was found more dominant in FS-2 (5%) followed by lab population.

Pseudoxanthomonas, Staphylococcus, and Luteimicrobium were exclusively found in FS-1 samples. Also, Pedobacter was exclusive to FS-2 larval midguts. The genera present only in the laboratory population included Pseudomonas, Brevibacterium, Delftia and Chryseobacterium.

Midgut Microbiota of the pupae of An. stephensi

The pupal midgut showed Bacillus and Leucobacter in all three samples. Besides these, Pedobacter (6%) and Micrococcus (47%) were present only in the pupal midgut of FS-1 population. Exiguobacterium (6%) and Enterococcus (32%) were recorded only in the pupal population of FS-2 sample.

Midgut microbiota of adult male An. stephensi

In case of the male midgut, Bacillus and Staphylococcus were the only two genera isolated from FS-2 and lab population. Genera reported from FS-1 male midgut included, Aeromonas, Kocuria and Luteimicrobium. In the FS-2 sample population, Staphylococcus, Enterococcus, and Exiguobacterium were observed. Similarly, Comamonas, Pseudomonas and Chryseobacterium were found in the male midgut of the lab population of An. stephensi.

Midgut microbiota of adult female An. stephensi

In the female midgut, the only genus recorded from all three samples was Bacillus. It was predominant in lab population, followed by FS-1 and FS-2 samples. Micrococcus was reported from the female midgut of FS-1 and FS-2 populations. While Exiguobacterium, Dermacoccus, Micrococcus, Staphylococcus and Yimella were found in the female midgut of FS-1 sample. Leucobacter was the only genus reported from FS-2 sample population. The female midgut from lab population showed abundance of Enterococcus, Pseudomonas and Elizabethkingia.

Overall, a multitude of culturable midgut isolates were reported for the first time in An. stephensi and these included Delftia, Leucobacter, Luteimicrobium, Micrococcus, Pedobacter and Staphylococcus. All these were isolated from the larval midgut of An. stephensi. While, Leucobacter, Micrococcus and Pedobacter were isolated from the pupal midgut of An. stephensi, Aeromonas, Comamonas, Exiguobacterium, Luteimicrobium, Pseudomonas and Kocuria were reported from the midgut of male An. stephensi. In the female midgut, the bacterial isolates like Elizabethkingia, Microbacterium and Staphylococcus were reported for the first time. Similarly, Exiguobacterium, Kocuria, Micrococcus and Microbacterium were the isolates reported for the first time from the mosquito breeding habitat.

Luteimicrobium is isolated for the first time from the midgut of mosquito; previously, it was identified from the long horned beetle[24]. To the best of our knowledge, this is the first study conducted on the pupal stage of An. stephensi, earlier study on pupae in Anophelines were conducted on An. gambiae[17],[25].

Microbiota common to breeding habitat and life stages of An. stephensi in FS-1 population

Some isolates were seen only in one stage, while some were recorded in more than two developmental stages. As seen [Figure 7], there were only two isolates common in breeding water, larvae, pupae and female of wild An. stephensi population (FS-1) stage viz., Bacillus and Micrococcus. Kocuria was found in both breeding habitat water and in the midgut of male. Exiguobacterium was obtained from the breeding habitat water and the midgut of the female. Leucobacter was a common isolate in the midgut of larvae and pupae. Luteimicrobium was found common in the midgut of the adult male and larvae. Staphylococcus was common isolate in the midguts of larvae and adult female. The isolate specifically found in larvae, pupae and male midgut was Pseudoxanthomonas, Pedobacterand Aeromonas. Dermacoccus and Yimella were isolated from the female midgut of FS-1 sample population.
Figure 7: Venn diagram presenting distinctive and common microbial species between the larval, pupal and adult stages of Anopheles stephensi isolated from the wild population sample of field site 1.

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Microbiota common to breeding water and life stages of An. stephensi in FS-2 sample population

In case of FS-2, Bacillus was reported from all three life cycle stages and also in the breeding habitats as seen in [Figure 8]. Exiguobacterium was reported in the breeding water, larvae, pupae and the male midgut. Similarly, leucobacter genus was reported in the breeding water, larvae, pupae and the female midgut. Pedobacter and lysinibacillus are two genera common in breeding water and the larvae. Micrococcus was seen in both larvae and female midgut and Enterococcus was common isolate reported in male and pupae midgut. Both Microbacterium and Staphylococcus were individually found in the breeding water and the male midgut respectively.
Figure 8: Venn diagram presenting distinctive and common microbial species between the larval, pupal and adult stages of Anopheles stephensi isolated from the wild population sample of field site 2.

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Microbiota common to breeding water and life stages of An. stephensi in lab population

In case of lab sample, Bacillus was reported from the breeding water, larvae, pupae, male and the female mid-gut [Figure 9]. Pseudomonas was reported from the larvae, male and female population. Micrococcus was observed in the larvae and the breeding water. Chryseobacterium was found common to larvae and the male midgut. Brevibacterium, Lysinibacillus and Delftia are the genera obtained from the larvae midgut while, Leucobacter is the only isolate obtained from the pupal midgut. Comamonas and Staphylococcus were obtained from the male midgut while Elizabethkingia and Enterococcus were reported from the female midgut. Bacillus, Exiguobacterium, Micrococcus and Leucobacter were found in different developmental stages as well as in the breeding water and hence could be considered as potential candidates for further investigations on transstadial transmission.
Figure 9: Venn diagram presenting distinctive and common microbial species between the larval, pupal and adult stages of Anopheles stephensi isolated from the Laboratory population sample.

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Besides the characterization of the microbiota, estimation of their abundance remains very important. The Simpson diversity, Shannon diversity and Shannon Evenness were estimated from all three sites for each developmental stage and the breeding water as summarised in [Table 1].
Table 1: Diversity indices of midgut bacterial isolates of different life stages of An. stephensi and the breeding habitat

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Diversity indices of midgut bacterial isolates of different life stages and the breeding habitat

In the present study, the value of Simpson diversity index ranged from 0 to 0.36 among breeding habitat, 0.34 to 0.64 in larvae midgut, 0.11 to 0.63 in the pupae midgut, 0.29 to 0.45 in the male midgut and 0.26 to 0.75 in the female midgut. As observed, breeding habitat may affect the diversity of microbiota in pupae and female midguts, while larval and adult male stages have resulted in a less or more consistent observed diversity suggesting stage-specific preference of microbial communities.

With regards to site variables (FS-1, FS-2 and LAB), generic diversity was generally higher in most stages of the mosquitoes collected from FS-1 site (outdoors) and generally lower in stages of mosquitoes reared on tap water in the laboratory-reared populations; with the exception of adult males which showed higher generic diversity in Lab rather than FS-1 collected. The male midgut of the lab population showed five bacterial genera out of which Bacillus, Comamonas, Pseudomonas and Chryseobacterium showed the highest total viable count. While in the case of FS-2 population very low Total Viable Count was shown by the bacterial genera present. Thus, resulting in the change in Simpson’s diversity. Despite higher sensitivity of Shannon diversity, calculations revealed no observable difference with respect to Simpson’s diversity; hence similar trends.

With regards to the evenness of microbial communities, the value of Shannon evenness ranged from 0.03 to 0.53 in the breeding habitat. In the case of larval midgut, the value ranged from 0.37 to 0.66, 0.32 to 0.82 in the pupal midgut, 0.31 to 0.64 in the male midgut and 0.34 to 0.87 in the female midgut. In the case of breeding water, larvae and pupae the highest Shannon evenness were noticed in FS-1 and the least in LAB population. In the case of male midgut from LAB population showed more evenness and least was observed in FS-2 population. In case of female midgut, highest diversity was observed in FS-2 and the least in the LAB population.

The high fluctuation of Shannon evenness in the lab population, suggests the uneven distribution of genera, while in the case of FS-2 the genera are well distributed. For instance, though there are three genera reported in FS-2 female in comparison to four genera in the lab population. In FS-2 population all the genera present have similar total viable count as reflected in its evenness [Table 1]. However, in case of Lab population higher relative abundance of Bacillus was observed compared to other genus resulting in uneven distribution, with an exception in male population.


  Discussion Top


The elucidation of mosquito gut microbiota is a growing field of investigation because of its potential role in vector competence to transmit disease pathogens. A protective role of bacterial microbiota of Anopheles sp. against Plasmodium infections has already been demonstrated[9],[26],[27]. The present study provides an overview of the midgut microbiome isolated from different life stages and breeding environment of An. stephensi. This is the possibly the first study where the analysis of midgut flora has been carried out in mosquito immatures and adults of An. stephensi obtained from breeding habitats, water collection area and curing water site,and compared with the laboratory population. In several earlier studies on Indian and Iranian An. stephensi microbiota of larvae and adult stages of mosquitoes, transstadial transmission of some bacterial genera such as Acinetobacter, Bacillus, Enterobacter, Staphylococcus, Pseudomonas, Chryseobacterium and Serratia sp. has been suspected[15],[16],[17],[19],[28]. In the present study, with 16S rRNA gene sequencing, a total of 298 midgut isolates were obtained from the different developmental stages and breeding habitat of the vector. The composition of isolates in this study showed that gram- positive bacteria of phylum Firmicutes and Actinobacteria dominate the midgut flora of both laboratory and field Anopheles mosquito species (Supporting Information S2 & S6). In contrast, the earlier culture-based studies from different geographical regions on An. stephensi, An. maculipennis, and Anopheles gambiae, revealed the supremacy of Gram-negative bacteria[29]. A total of 12 genera were identified from one field sample (FS-1), 9 from another (FS-2) and 11from the midguts of different life stages of the laboratory populations. The total viable count (TVC) was also calculated for the midgut homogenate of each stage on different media. In order to capture maximum diversity, five different media having different composition were utilized in the present study[15],[19],[21]. The genera Bacillus, Kocuria, Micrococcus, Pseudomonas and Pseudoxanthomonas were found on all the five media in the present study thereby showing culture adaptability. Other genera viz., Brevibacterium, Comamonas, Delftia and Yimella are reported for the first time in the adult mid-gut of An. stephensi. The highest bacterial count of 2.4×105 cfu/midgut and 15×105cfu/midgut were obtained from the midgut of larvae and pupae of wild population on Blood Agar respectively. Similarly in another study, four media were utilized and among which Brain Heart agar and Nutrient Agar supported the maximum genera[19]. Dong et al., had observed highest TVC in the female midgut of Anopheles gambiae with an average of 40,000 cfu/midgut using Luria berthaniagar[9]. Several studies have been undertaken on midgut microbiota of Anopheles using different media which identified a few bacterial species as compared to the current study[15],[19],[21],[30],[31]. Recovery of novel and additional bacterial genera using more number of media, for the first time in the current study, signifies the necessity to use multiple media in the identification of microflora of the midgut of An. stephensi using culture based technique.

Although, Aeromonas, Brevibacterium, Lysinibacillus, Pseudomonas, Staphylococcus and Kocuria have already been reported in An. stephensi but in different developmental stages and locations unlike here[4],[15],[16],[28],[30]. In the present study, apart from those already reported, several new genera viz., Delftia,Pedobacter and Micrococcus were reported for the first time in Anopheles stephensi. However, these have been previously reported in Aedes sp., Culex sp. and An. gambiae which suggest that microbiota of different mosquito species inhabiting similar environmentscould be common as has been earlier suggested[9],[12],[15],[27],[32],[33],[34]. Out of the 21 genera reported in the current study, genus Luteimicrobium was identified for the first time in the mosquito gut. So far the presence of Luteimicrobiumis reported from long horn beetle[24]. In the present study, Micrococcus and Bacillus were the most frequently isolated bacteria from the two field populations of An. stephensi. In case of lab population, except Bacillus, microbiota of all immature and adult stages and breeding water was distinct indicating its high variability. On the other hand, the presence of Bacillus and Micrococcus in breeding water, larvae, pupae and adult stages of An. stephensi in the two field populations signifies their transstadial transmission potential. The presence of different isolates of Micrococcus in a single clade of Phylogenetic tree further corroborates the above hypothesis [Figure S5]. However, an experimental validation using markers such as green fluorescent (GFP) is necessary to confirm this fact as has been done in case of Asaia species found in Anopheles gut[26]. Similarly, it is reported previously that due to an incomplete sterilization of An. stephensi gut, Bacillus could be retained in different developmental stages[15].

The presence of Micrococcus of Actinobacteria phylum in the wild population has been previously reported in several mosquito species such as Culex quinquefasciatus, Aedes albopictus and Aedes aegypti[4],[32],[34],[35]. In contrast to the hypothesis of metamorphosis, which alters the classes of bacteria in immatures and adult mosquitoes[36],[37], the presence of Microcoocus in all developmental stages provides an evidence that the metamorphosis, from one stage to another, does not completely eliminate gut microbiota. Similar observation was made in Culex tarsalis and Culex pipiens mosquitoes where some bacterial communities from the larval stages were shown to be sequestered in meconial peritrophic membrane and passed on to the adult stages[37],[38]. In addition to passing through the different stages, Microcoocus also managed to escape the immune response of Aedes aegypti where it was melanised but not phagocytised[39].

Another bacterial genus Leucobacter was found to be present in all the developmental stages of populations of both the field samples. This presence of different Leucobacter isolates in different developmental stages in the same clade of phylogenetic tree with high similarity [Figure S4],[Figure S5] provides further evidence to the possibility of transstadial transmission. However, experimental evidence is necessary to validate the hypothesis. It is noteworthy that in the earlier studies too, the genus Leucobacter was reported from the distinct stages of different mosquito species like in adult female of Culex quinquefasciatus and Aedes albopictus[32],[40], larvae of Aedes. aegypti and Ae. albopictus[34],[41] and adult males of Ae. albopictus[34].

Vector breeding site plays a significant role in micro-biome acquisition in the midgut of mosquitoes[42]. In the present study, the microbiota of the three breeding sites of Anopheles stephensi, a major malaria vector in India, was also explored. While 15 genera were observed in field population, eleven were reported in the laboratory population. Additionally, the genera observed in the two field populations were found almost common (e.g. Bacillus, Exiguobacterium, Leucobacter, Micrococcus and Pedobacter), while different genera (Brevibacterium., Delftia, Chryseobacterium, Pseudomonas, Elizabethkingia and Comamonas) were observed in the laboratory population, the reason for which cannot be explained without more data and further experimental evidence is needed to corroborate this phenomenon of distinctiveness of microbiota between the field and laboratory cultures of An. stephensi immatures. A higher bacterial diversity was found in immatures reared in the breeding water than the laboratory populations reared in the tap water, suggesting that microbiota of the adults emerging from these two populations would also likely to be distinct. Similar observations have previously been made in An. stephensi population of Haryana, India[15]. It is pertinent to mention here that, all the protocols and conditions for maintaining the mosquito life cycle stages of both field and lab mosquitoes were similar. Differences in the water utilized for the rearing of mosquitoes can possibly be an important factor for the observed variation in the diversity of microbiota in field and lab population. A reduction of the relative abundance of microbiota in the tap water has also been previously demonstrated in An. gambiae in Africa[43].

There is significant evidence that the presence of microbiota in the mosquito population depends on several factors but their living environment plays a key role in shaping their microbiome[44]. The presence of different bacterial genera in different breeding waters has been observed in the present study as has also been observed in the earlier studies carried out across different states of India. However, the genera reported in present study from Goa are different from those reported from Haryana, Mangalore, Chennai and Bangalore populations except Bacillus and Pseudomonas[15],[28]. The ubiquitous presence of Bacillus across different geographical location indicates its versatility and diverse ecological adaptation ranging from saprophytic nature in the soil to symbiotic relationship with different insects[45]. This is mainly accomplished because many Bacillus species display varied physiological abilities which help them to thrive in almost every natural environment[46]. The statistical quantification of the bacterial microbiome would indicate their significance in the host. The diversity index quantifies bacterial diversity in the community and describes it numerically. In present study, both Simpsons diversity index and Shannon evenness were found to be low in one of the field breeding habitats (FS-2) of An. stephensi, even though 6 genera were present. This suggests uneven distribution of a single genus mainly due to the abundance of a single isolate. In contrast, high Shannon diversity was found in the female midgut of immatures of the other field sample (FS-2). In the laboratory and FS-2 populations, Simpson’s diversity was found to be low in the breeding habitat, larvae, pupae and female midgut except in the male midgut. This was due to the presence of larger number of genera in the male midgut. Shannon evenness is used to record how well the species are distributed among the individuals[23]. Interestingly, the Shannon evenness was found to be uniform in all the developmental stages of the field population obtained from FS1 sample, which further suggests that the transstadial transmission of some of the genera occurs. On the contrary, the Shannon evenness was highly fluctuating in the lab population, suggesting the uneven distribution of bacterial genera in the midgut of immatures from the lab samples.


  Conclusion Top


In conclusion, to the best of our knowledge, this is the first approach in studying the culturable microbiome of the breeding habitat as well as population of immatures and adults of An. stephensi. The study emphasises the need for using multiple media to determine maximum diversity in gut microbiome. By analysing the phylogenetic tree and statistical data, it was concluded that diversity of microbiota is high in field populations as compared to laboratory population of An. stephensi. In this study, some bacterial isolates already reported in different mosquito species and beetle genera were identified for the first time in An. stephensi. Interestingly, the presence of identical Micrococcus and Leucobacter isolates in the field populations and different stages suggest that these genera might have adapted themselves to An. stephensi as symbionts. Further analysis is necessary to validate the above hypothesis. Further, elucidating the role of bacterial genera like Bacillus, Micrococcus and Leucobacter in An. stephensi will help in identifying suitable candidates for paratransgenesis. There is also need for further studies to identify the candidate microbes with stable transstadial traits so that they can be studied for their role in vector competence.

Conflict of interest: None

 
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