Dependent population dynamics between chironomids (nonbiting midges) and Vibrio cholerae

The Role of Nutrients and Nutritional Signals in the Pathogenesis of Vibrio cholerae

Vibrio cholerae, the agent of cholera, is a natural inhabitant of aquatic environments. Over the past decades, the importance of specific nutrients and micronutrients in the environmental survival, host colonization, and pathogenesis of this species has become increasingly clear. For instance, V. cholerae has evolved ingenious mechanisms that allow the bacterium to colonize and establish a niche in the intestine of human hosts, where it competes with commensals (gut microbiota) and other pathogenic bacteria for available nutrients. Here, we discuss the carbon and energy sources utilized by V. cholerae and what is known about the role of nutrition in V. cholerae colonization. We examine how nutritional signals affect virulence gene regulation and how interactions with intestinal commensal species can affect intestinal colonization.

Comparative Microbiota Composition Across Developmental Stages of Natural and Laboratory-Reared Chironomus circumdatus Populations From India

Chironomids are aquatic insects that undergo a complete metamorphosis of four life stages. Here we studied, for the first time, the microbiota composition of Chironomus circumdatus, a tropical midge species, both from the Mula and Mutha Rivers in Pune, India and as a laboratory-reared culture. We generated a comparative microbial profile of the eggs, larvae and pupae, the three aquatic life stages of C. circumdatus. Non-metric multidimensional scaling analysis (NMDS) demonstrated that the developmental stage had a more prominent effect on the microbiota composition compared to the sampling location. Notably, the microbiota composition of the egg masses from the different sampling points clustered together and differed from laboratory culture larvae. Proteobacteria was the dominant phylum in all the environmental and laboratory-reared egg masses and pupal samples, and in the laboratory-reared larvae, while Fusobacteria was the dominant phylum in the larvae collected from the field environment. The most abundant genera were Cetobacterium, Aeromonas, Dysgonomonas, Vibrio, and Flavobacterium. The ten amplicon sequence variants (ASVs) that most significantly contributed to differences in microbiota composition between the three sampled locations were: Burkholderiaceae (ASVs 04 and 37), C39 (Rhodocyclaceae, ASV 14), Vibrio (ASV 07), Arcobacter (ASV 21), Sphaerotilus (ASV 22), Bacteroidia (ASVs 12 and 28), Flavobacterium (ASV 29), and Gottschalkia (ASV 10). No significant differences were found in the microbial richness (Chao1) or diversity (Shannon H’) of the three sampled locations. In contrast, significant differences were found between the microbial richness of the three life stages. Studying the microbiota of this Chironomus species may contribute to a better understanding of the association of C. circumdatus and its microbial inhabitants.

Identification of chironomid species as natural reservoirs of toxigenic Vibrio cholerae strains with pandemic potential

Vibrio cholerae causes the fatal cholera diarrhea. Chironomids (Diptera; Chironomidae) are abundant in freshwater aquatic habitats and estuaries and are natural reservoirs of V. cholerae. Until now, only the non-O1/O139 serogroups of V. cholerae were identified in chironomids. Here, we explored whether chironomids are natural reservoirs of V. cholerae O1/O139 serogroups, which are associated with cholera endemics and pandemics. All four life stages of chironomids were sampled from two rivers, and a laboratory culture in Pune, India, and from a pond in Israel. In total, we analyzed 223 chironomid samples. The presence of V. cholerae O1/O139 serogroups was verified using molecular tools. Nine chironomid species were identified; of them, Chironomus circumdatus was the most abundant. The presence of V. cholerae serogroup O1 and the cholera toxin genes were detected in samples from all chironomid species. However, serogroup O139 was detected in only two chironomid species. Besides PCR to detect specific genes, a metagenomic analysis that was performed in three selected C. ramosus larvae, identified a list of virulence genes associated with V. cholerae. The findings provide evidence that chironomids are natural reservoirs of toxigenic V. cholerae O1/O139. Chironomid populations and V. cholerae show biannual peak patterns. A similar pattern is found for cholera epidemics in the Bengal Delta region. Thus, we hypothesize that monitoring chironomids in endemic areas of the disease may provide a novel tool for predicting and preventing cholera epidemics. Moreover, serogroup O139 was detected only in two chironomid species that have a restricted distribution in the Indian subcontinent, possibly explaining why the distribution of the O139 serogroup is limited.

Quorum-sensing signaling by chironomid egg masses' microbiota, affects haemagglutinin/protease (HAP) production by Vibrio cholerae

Vibrio cholerae, the causative agent of cholera, is commonly isolated, along with other bacterial species, from chironomid insects (Diptera: Chironomidae). Nevertheless, its prevalence in the chironomid egg masses' microbiota is less than 0.5%. V. cholerae secretes haemagglutinin/protease (HAP) that degrades the gelatinous matrix of chironomid egg masses and prevents hatching. Quorum sensing (QS) activates HAP production in response to accumulation of bacterial autoinducers (AIs). Our aim was to define the impact of chironomid microbiota on HAP production by V. cholerae. To study QS signaling, we used V. cholerae bioluminescence reporter strains (QS-proficient O1 El-Tor wild-type and QS-deficient mutants) and different bacterial species that we isolated from chironomid egg masses. These egg mass isolates, as well as a synthetic AI-2, caused an enhancement in lux expression by a V. cholerae QS-deficient mutant. The addition of the egg mass bacterial isolate supernatant to the QS-deficient mutant also enhanced HAP production and egg mass degradation activities. Moreover, the V. cholerae wild-type strain was able to proliferate using egg masses as their sole carbon source, while the QS-deficient was not. The results demonstrate that members of the chironomid bacterial consortium produce external chemical cues that, like AI-2, induce expression of the hapA gene in V. cholerae. Understanding the interactions between V. cholerae and the insects' microbiota may help uncover the interactions between this pathogen and the human gut microbiota.

Seasonal dynamics of Chironomus transvaalensis populations and the microbial community composition of their egg masses

Chironomids (Diptera; Chironomidae) are the most abundant insects in freshwater environments and are considered natural reservoirs of Vibrio cholerae. We monitored the annual dynamics of chironomid populations along with their microbiota in order to better understand host–microbiota interactions. Chironomus transvaalensis populations peaked biannually in August and May–June. The composition of the endogenous bacterial communities of their egg masses clustered in two groups according to the sampling periods August–November and May–July. Nevertheless, a core bacterial community (43%) was present in all egg-mass samples. The most abundant phyla were: Proteobacteria, Firmicutes, Cyanobacteria and Bacteroidetes. The abundance of several genera (e.g. Rheinheimera and Pseudomonas) was positively correlated with C. transvaalensis population dynamics, while a predator–prey interaction was observed between the relative abundance of Vibrio OTUs and C. transvaalensis population size. Chironomids are known to tolerate toxic and stress conditions, and our results demonstrated that bacterial genera that may protect the insect under these conditions are present in the egg masses. After hatching, the first larval meal is the gelatinous matrix that surrounds the eggs. This meal contains a probiotic consortium that may protect the larva during its metamorphosis. The results provide important insights into the host–microbe interactions of chironomids.

Accumulating evidence suggests that some waterbird species are potential vectors of Vibrio cholerae

Vibrio cholerae is the causative agent of cholera, a life-threatening diarrheal disease. Cholera causes epidemics and pandemics, but the ways this disease spreads worldwide is still unclear. This review highlights a relatively new hypothesis regarding the way V. cholerae can be globally dispersed. Copepods and chironomids are natural reservoirs of V. cholerae and are part of different fish species’ diet. Furthermore, V. cholerae inhabits marine and freshwater fish species. Waterbird species feed on fish or on small invertebrates such as copepods and chironomids. Waterbirds have also been found to carry living copepods and/or chironomids internally or externally from one waterbody to another. All of the above points to the fact that some waterbird species might be vectors of V. cholerae. Indeed, we and others have found evidence for the presence of V. cholerae non-O1 as well as O1 in waterbird cloacal swabs, feces, and intestine samples. Moreover, hand-reared cormorants that were fed on tilapia, a fish that naturally carries V. cholerae, became infected with this bacterial species, demonstrating that V. cholerae can be transferred to cormorants from their fish prey. Great cormorants as well as other waterbird species can cover distances of up to 1,000 km/day and thus may potentially transfer V. cholerae in a short time across and between continents. We hope this review will inspire further studies regarding the understanding of the waterbirds' role in the global dissemination of V. cholerae.

Wild waterfowl as potential vectors of Vibrio cholerae and Aeromonas species

OBJECTIVE

To study the hypothesis that migratory waterfowl are possible disseminators of Vibrio cholerae and Aeromonas.

METHODS

We monitored the presence of V. cholerae and Aeromonas in three wild waterfowl species.

RESULTS

V. cholerae and Aeromonas species were isolated and identified from intestine samples of little egrets and black-crowned night herons. Only Aeromonas species were isolated from black-headed gulls. The majority of Aeromonas isolates were A. veronii. Twenty-three V. cholerae serogroups were identified. V. cholerae serogroup O1 was found in the intestine DNA extractions from four little egrets and black-crowned night herons; six birds carried cholera toxin subunit A gene.

CONCLUSION

Wild waterfowl species may carry pathogenic V. cholerae O1 and non-O1 serogroups and Aeromonas species in their intestine. The migration of waterfowl is a potential mechanism for global distribution of V. cholerae and Aeromonas.

Activation of Vibrio cholerae quorum sensing promotes survival of an arthropod host

Vibrio cholerae colonizes the human terminal ileum to cause cholera, and the arthropod intestine and exoskeleton to persist in the aquatic environment. Attachment to these surfaces is regulated by the bacterial quorum-sensing signal transduction cascade, which allows bacteria to assess the density of microbial neighbours. Intestinal colonization with V. cholerae results in expenditure of host lipid stores in the model arthropod Drosophila melanogaster. Here we report that activation of quorum sensing in the Drosophila intestine retards this process by repressing V. cholerae succinate uptake. Increased host access to intestinal succinate mitigates infection-induced lipid wasting to extend survival of V. cholerae-infected flies. Therefore, quorum sensing promotes a more favourable interaction between V. cholerae and an arthropod host by reducing the nutritional burden of intestinal colonization.

Otitis Media Caused by V. cholerae O100: A Case Report and Review of the Literature

Infections due to Vibrio cholerae are rarely documented in Israel. Here we report a case of recurrent otitis media in a young male, caused by V. cholerae non-O1/O139. This extra-intestinal infection was caused by V. cholerae O100 and has been associated with freshwater exposure and travel. Symptoms of chronic periodic earaches along with purulent exudate began about one week after the patient suffered a water skiing accident on a river in Australia. The condition lasted for three years, until his ear exudate was examined in a clinical laboratory, diagnosed and treated. Five bacterial isolates were identified as V. cholerae O100. The isolates were screened for genetic characteristics and were found positive for the presence of hapA, hlyA, and ompU virulence genes. All isolates were negative for the presence of ctxA. Based on antibiogram susceptibility testing, ciprofloxacin ear drops were used until the patient’s symptoms disappeared. This case demonstrates that exposure to freshwater can cause otitis media by V. cholerae non-O1/O139 in young and otherwise healthy humans.

Great cormorants (Phalacrocorax carbo) as potential vectors for the dispersal of Vibrio cholerae

Vibrio cholerae is the cause of cholera, a devastating epidemic and pandemic disease. Despite its importance, the way of its global dissemination is unknown. V. cholerae is abundant in aquatic habitats and is known to be borne by copepods, chironomids and fishes. Our aim was to determine if fish-eating birds act as vectors in the spread of V. cholerae by consuming infected fish. We determined the existence of V. cholerae in the microbiome of 5/7 wild cormorants’ intestine. In three of these V. cholerae-positive wild cormorants, the presence of a gene for cholera toxin (ctxA) was detected. We subsequently tested eight captive, hand-reared cormorants, divided into two equal groups. Prior to the experiment, the feces of the cormorants were V. cholerae-negative. One group was fed exclusively on tilapias, which are naturally infected with V. cholerae, and the other was fed exclusively on goldfish or on koi that were V. cholerae-negative. We detected V. cholerae in the feces of the tilapia-fed, but not in the goldfish/koi-fed, cormorants. Hence, we demonstrate that fish-eating birds can be infected with V. cholerae from their fish prey. The large-scale movements of many fish-eating birds provide a potential mechanism for the global distribution of V. cholerae.

Fish as Hosts of Vibrio cholerae

Vibrio cholerae, the causative agent of pandemic cholera, is abundant in marine and freshwater environments. Copepods and chironomids are natural reservoirs of this species. However, the ways V. cholerae is globally disseminated are as yet unknown. Here we review the scientific literature that provides evidence for the possibility that some fish species may be reservoirs and vectors of V. cholerae. So far, V. cholerae has been isolated from 30 fish species (22 freshwater; 9 marine). V. cholerae O1 was reported in a few cases. In most cases V. cholerae was isolated from fish intestines, but it has also been detected in gills, skin, kidney, liver and brain tissue. In most cases the fish were healthy but in some, they were diseased. Nevertheless, Koch postulates were not applied to prove that V. cholerae and not another agent was the cause of the disease in the fish. Evidence from the literature correlates raw fish consumption or fish handling to a few cholera cases or cholera epidemics. Thus, we can conclude that V. cholerae inhabits some marine and freshwater fish species. It is possible that fish may protect the bacteria in unfavorable habitats while the bacteria may assist the fish to digest its food. Also, fish may disseminate the bacteria in the aquatic environment and may transfer it to waterbirds that consume them. Thus, fish are reservoirs of V. cholerae and may play a role in its global dissemination.

Aeromonas chitinase degrades chironomid egg masses

Chironomids are freshwater insects that undergo a complete metamorphosis of four life stages. Chironomid egg masses can be degraded by Vibrio cholerae and some Aeromonas species. Egg mass degradation by V. cholerae requires haemagglutinin protease activity. Our aim was to identify the egg mass degrading (EMD) factor secreted by Aeromonas dhkanesis 3K1C15. Following the hypothesis that the EMD factor of A. dhkanesis is also a protease, secreted proteases were screened, but none of them proved to have the same properties as the EMD factor. Using conventional protein purification methods, we found that the active fraction included chitinases. We further confirmed chitin as a building block of the egg masses. Interestingly, by supplementing bacterial growth media with chitin, we observed unexpected EMD factor activity in Aeromonas isolates that initially were not able to degrade egg masses. Accordingly, we concluded that although strain 3K1C15 secretes chitinases constitutively, most Aeromonas strains secrete chitinases inductively. Induction of chitinases in nature presumably occurs when bacteria are attached to the egg mass habitat, in which chitin is abundant. Considering that chitinases are highly conserved across bacteria phyla, we assume that the role of this enzyme in the bacteria-insect interplay could be wider than is currently thought.

High quality draft genome sequence of Brachymonas chironomi AIMA4(T) (DSM 19884(T)) isolated from a Chironomus sp. egg mass

Brachymonas chironomi strain AIMA4^T (Halpern et al., 2009) is a Gram-negative, non-motile, aerobic, chemoorganotroph bacterium. B. chironomi is a member of the Comamonadaceae, a family within the class Betaproteobacteria. This species was isolated from a chironomid (Diptera; Chironomidae) egg mass, sampled from a waste stabilization pond in northern Israel. Phylogenetic analysis based on the 16S rRNA gene sequences placed strain AIMA4^T in the genus Brachymonas. Here we describe the features of this organism, together with the complete genome sequence and annotation. The DNA GC content is 63.5%. The chromosome length is 2,509,395 bp. It encodes 2,382 proteins and 68 RNA genes. Brachymonas chironomi genome is part of the Genomic Encyclopedia of Type Strains, Phase I: the one thousand microbial genomes (KMG) project.

High quality draft genome sequence of Leucobacter chironomi strain MM2LB(T) (DSM 19883(T)) isolated from a Chironomus sp. egg mass

Leucobacter chironomi strain MM2LB^T (Halpern et al., Int J Syst Evol Microbiol 59:665-70 2009) is a Gram-positive, rod shaped, non-motile, aerobic, chemoorganotroph bacterium. L. chironomi belongs to the family Microbacteriaceae, a family within the class Actinobacteria. Strain MM2LB^T was isolated from a chironomid (Diptera; Chironomidae) egg mass that was sampled from a waste stabilization pond in northern Israel. In a phylogenetic tree based on 16S rRNA gene sequences, strain MM2LB^T formed a distinct branch within the radiation encompassing the genus Leucobacter. Here we describe the features of this organism, together with the complete genome sequence and annotation. The DNA GC content is 69.90%. The chromosome length is 2,964,712 bp. It encodes 2,690 proteins and 61 RNA genes. L. chironomi genome is part of the Genomic Encyclopedia of Type Strains, Phase I: the one thousand microbial genomes (KMG) project.

Chironomid microbiome

Chironomids are abundant insects in freshwater habitats. They undergo a complete metamorphosis of four life stages: eggs, larvae, and pupae in water, and a terrestrial adult stage. Chironomids are known to be pollution-tolerant, but little is known about their resistance mechanisms to toxic substances. Here we review current knowledge regarding the chironomid microbiome. Chironomids were found as natural reservoirs of Vibrio cholerae and Aeromonas spp. A stable bacterial community was found in the egg masses and the larvae when both culture-dependent and -independent methods were used. A large portion of the endogenous bacterial species was closely related to species known as toxicant degraders. Bioassays based on Koch's postulates demonstrated that the chironomid microbiome plays a role in protecting its host from toxic hexavalent chromium and lead. V. cholerae, a stable resident in chironomids, is present at low prevalence. It degrades the egg masses by secreting haemagglutinin/protease, prevents eggs from hatching, and exhibits host pathogen interactions with chironomids. However, the nutrients from the degraded egg masses may support the growth of the other microbiome members and consequently control V. cholerae numbers in the egg mass. V. cholerae, other chironomid endogenous bacteria, and their chironomid host exhibit complex mutualistic relationships.

Molecular analysis of faecal samples from birds to identify potential crop pests and useful biocontrol agents in natural areas

Wild habitats adjoining farmland are potentially valuable sources of natural enemies, but also of pests. Here we tested the utility of birds as 'sampling devices', to identify the diversity of prey available to predators and particularly to screen for pests and natural enemies using natural ecosystems as refugia. Here we used PCR to amplify prey DNA from three sympatric songbirds foraging on small invertebrates in Phragmites reedbed ecosystems, namely the Reed Warbler (Acrocephalus scirpaceus), Sedge Warbler (Acrocephalus schoenobaenus) and Cetti's Warbler (Cettia cetti). A recently described general invertebrate primer pair was used for the first time to analyse diets. Amplicons were cloned and sequenced, then identified by reference to the Barcoding of Life Database and to our own sequences obtained from fresh invertebrates. Forty-five distinct prey DNA sequences were obtained from 11 faecal samples, of which 39 could be identified to species or genus. Targeting three warbler species ensured that species-specific differences in prey choice broadened the range of prey taken. Amongst the prey found in reedbeds were major pests (including the tomato moth Lacanobia oleracea) as well as many potentially valuable natural enemies including aphidophagous hoverflies and braconid wasps. Given the mobility of birds, this approach provides a practical way of sampling a whole habitat at once, providing growers with information on possible invasion by locally resident pests and the colonization potential of natural enemies from local natural habitats.

Associations and dynamics of Vibrionaceae in the environment, from the genus to the population level

The Vibrionaceae, which encompasses several potential pathogens, including V. cholerae, the causative agent of cholera, and V. vulnificus, the deadliest seafood-borne pathogen, are a well-studied family of marine bacteria that thrive in diverse habitats. To elucidate the environmental conditions under which vibrios proliferate, numerous studies have examined correlations with bulk environmental variables—e.g., temperature, salinity, nitrogen, and phosphate—and association with potential host organisms. However, how meaningful these environmental associations are remains unclear because data are fragmented across studies with variable sampling and analysis methods. Here, we synthesize findings about Vibrio correlations and physical associations using a framework of increasingly fine environmental and taxonomic scales, to better understand their dynamics in the wild. We first conduct a meta-analysis to determine trends with respect to bulk water environmental variables, and find that while temperature and salinity are generally strongly predictive correlates, other parameters are inconsistent and overall patterns depend on taxonomic resolution. Based on the hypothesis that dynamics may better correlate with more narrowly defined niches, we review evidence for specific association with plants, algae, zooplankton, and animals. We find that Vibrio are attached to many organisms, though evidence for enrichment compared to the water column is often lacking. Additionally, contrary to the notion that they flourish predominantly while attached, Vibrio can have, at least temporarily, a free-living lifestyle and even engage in massive blooms. Fine-scale sampling from the water column has enabled identification of such lifestyle preferences for ecologically cohesive populations, and future efforts will benefit from similar analysis at fine genetic and environmental sampling scales to describe the conditions, habitats, and resources shaping Vibrio dynamics.

Environmental reservoirs and mechanisms of persistence of Vibrio cholerae

It is now well accepted that Vibrio cholerae, the causative agent of the water-borne disease cholera, is acquired from environmental sources where it persists between outbreaks of the disease. Recent advances in molecular technology have demonstrated that this bacterium can be detected in areas where it has not previously been isolated, indicating a much broader, global distribution of this bacterium outside of endemic regions. The environmental persistence of V. cholerae in the aquatic environment can be attributed to multiple intra- and interspecific strategies such as responsive gene regulation and biofilm formation on biotic and abiotic surfaces, as well as interactions with a multitude of other organisms. This review will discuss some of the mechanisms that enable the persistence of this bacterium in the environment. In particular, we will discuss how V. cholerae can survive stressors such as starvation, temperature, and salinity fluctuations as well as how the organism persists under constant predation by heterotrophic protists.

The protective role of endogenous bacterial communities in chironomid egg masses and larvae

Insects of the family Chironomidae, also known as chironomids, are distributed worldwide in a variety of water habitats. These insects display a wide range of tolerance toward metals and organic pollutions. Bacterial species known for their ability to degrade toxicants were identified from chironomid egg masses, leading to the hypothesis that bacteria may contribute to the survival of chironomids in polluted environments. To gain a better understanding of the bacterial communities that inhabit chironomids, the endogenous bacteria of egg masses and larvae were studied by 454-pyrosequencing. The microbial community of the egg masses was distinct from that of the larval stage, most likely due to the presence of one dominant bacterial Firmicutes taxon, which consisted of 28% of the total sequence reads from the larvae. This taxon may be an insect symbiont. The bacterial communities of both the egg masses and the larvae were found to include operational taxonomic units, which were closely related to species known as toxicant degraders. Furthermore, various bacterial species with the ability to detoxify metals were isolated from egg masses and larvae. Koch-like postulates were applied to demonstrate that chironomid endogenous bacterial species protect the insect from toxic heavy metals. We conclude that chironomids, which are considered pollution tolerant, are inhabited by stable endogenous bacterial communities that have a role in protecting their hosts from toxicants. This phenomenon, in which bacteria enable the continued existence of their host in hostile environments, may not be restricted only to chironomids.

Bacterial community composition associated with chironomid egg masses

Chironomids (Diptera: Chironomidae) are the most widely distributed and often the most abundant insect in freshwater. They undergo a complete metamorphosis of four life stages, of which the egg, larva, and pupae are aquatic and the adult is terrestrial. Chironomid egg masses were found to be natural reservoirs of Vibrio cholerae and Aeromonas species. To expand the knowledge of the endogenous bacterial community associated with chironomid egg masses, denaturing gradient gel electrophoresis and clone analysis of 16S rRNA gene libraries were used in this study. Bacterial community composition associated with chironomid egg masses was found to be stable among different sampling periods. Cloned libraries of egg masses revealed that about 40% of the clones were related to bacteria known to degrade various toxicants. These findings were further supported when bacterial species that showed resistance to different toxic metals were isolated from egg masses and larval samples. Chironomids are found under a wide range of water conditions and are able to survive pollutants. However, little is known about their protective mechanisms under these conditions. Chironomid egg masses are inhabited by a stable endogenous bacterial community, which may potentially play a role in protecting chironomids from toxicants in polluted environments. Further study is needed to support this hypothesis.

Re-identification of Aeromonas isolates from chironomid egg masses as the potential pathogenic bacteria Aeromonas aquariorum

Egg masses of the non-biting midge Chironomous sp. have recently been found to serve as a reservoir for Vibrio cholerae and Aeromonas species. These insects are widely distributed in freshwater and evidence suggests that they may disseminate pathogenic bacteria species into drinking water systems. In the current study the taxonomy of 26 Aeromonas isolates, previously recovered from chironomid egg masses, was re-evaluated. It was found that 23 isolates, which had previously been identified as Aeromonas caviae, could belong to the recently described species Aeromonas aquariorum by their biochemical traits. To date, A. aquariorum has been found in ornamental fish and also in human extra-intestinal infections. ERIC-PCR genotyping differentiated 11 strains within the 23 A. aquariorum isolates, whose identity was confirmed by their rpoD gene sequences. Strains were found to possess the following virulence-associated genes: alt (90.9%), ahpB (81.8%), pla/lip/lipH3/apl-1/lip (54.5%), fla (27.3%), act/hylA/aerA (27.3%), ascF-ascG (81.8%) and aexT (9%) encoding for the cytotonic heat-labile enterotoxin, elastase, lipase, flagella, cytotoxic enterotoxins, the Type III Secretion System and the AexT toxin delivered by this system respectively. These findings indicate that chironomid egg masses harbour strains of A. aquariorum, which bear an important number of virulence genes, and that this species was misidentified originally as A. caviae.

Fish as reservoirs and vectors of Vibrio cholerae

Vibrio cholerae, the etiologic agent of cholera, is autochthonous to various aquatic environments, but despite intensive efforts its ecology remains an enigma. Recently, it was suggested that copepods and chironomids, both considered as natural reservoirs of V. cholerae, are dispersed by migratory waterbirds, thus possibly distributing the bacteria between water bodies within and between continents. Although fish have been implicated in the scientific literature with cholera cases, as far as we know, no study actually surveyed the presence of the bacteria in the fish. Here we show for the first time that fish of various species and habitats contain V. cholerae in their digestive tract. Fish (n = 110) were randomly sampled from freshwater and marine habitats in Israel. Ten different fish species sampled from freshwater habitats (lake, rivers and fish ponds), and one marine species, were found to carry V. cholerae. The fish intestine of Sarotherodon galilaeus harboured ca. 5×10³V. cholerae cfu per 1 gr intestine content—high rates compared with known V. cholerae cfu numbers in the bacteria's natural reservoirs. Our results, combined with evidence from the literature, suggest that fish are reservoirs of V. cholerae. As fish carrying the bacteria swim from one location to another (some fish species move from rivers to lakes or sea and vice versa), they serve as vectors on a small scale. Nevertheless, fish are consumed by waterbirds, which disseminate the bacteria on a global scale. Moreover, V. cholerae isolates had the ability to degrade chitin, indicating a commensal relationship between V. cholerae and fish. Better understanding of V. cholerae ecology can help reduce the times that human beings come into contact with this pathogen and thus minimize the health risk this poses.

Reflected polarization guides chironomid females to oviposition sites

Chironomids (Diptera: Chironomidae; non-biting midges) are known to be carriers of the Vibrio cholerae bacterium, responsible for the fatal cholera disease in humans. It was recently discovered that chironomid females choose their oviposition site by a visual cue. In this study, we test the hypothesis that this visual cue is the linear polarization of light reflected from the water surface. We conducted two multiple choice field experiments using egg traps with different light intensities and polarizations. With controlled illumination, a higher number of eggs was found under both high intensity and high polarization. Under natural illumination, no eggs were found in the unpolarized traps, and the egg number increased with the percentage polarization regardless of the light intensity. Field measurements showed that at sunset, when chironomids are active, the intensity of light reflected from their natural ponds decreases by 96%, while the percentage polarization remains stable and high at 60%. Furthermore, the percentage polarization is positively correlated with the total organic carbon (TOC) concentration in the water. Orthogonal alignment of the microvilli found in ommatidia from the ventral part of the female eye may provide the anatomical basis for polarization sensitivity. We conclude that the percentage polarization of reflected light is the cue by which chironomid females choose their oviposition site. It is a stable cue and can provide information on the amount of food available to the larvae in the water. Based on our results, we suggest that manipulating the polarization of reflected light is a viable way to control chironomid populations and mitigate cholera dispersion.

Oceanobacillus chironomi sp. nov., a halotolerant and facultatively alkaliphilic species isolated from a chironomid egg mass

Chironomids (Diptera; Chironomidae) are the most abundant insects in freshwater aquatic habitats. Females of the genus Chironomus lay egg masses containing hundreds of eggs embedded in a gelatinous matrix. A bacterial strain, designated T3944D(T), was isolated from a chironomid egg mass sampled from a waste-stabilization pond in northern Israel and was found to be Gram-positive, motile by peritrichous flagella, endospore-forming, halotolerant and facultatively alkaliphilic. Comparative 16S rRNA gene sequence analysis showed that strain T3944D(T) belonged to the genus Oceanobacillus, exhibiting the highest levels of similarity with the sequences of Oceanobacillus oncorhynchi subsp. incaldanensis DSM 16557(T) (94.9 %), Oceanobacillus oncorhynchi subsp. oncorhynchi JCM 12661(T) (94.8 %), Oceanobacillus iheyensis JCM 11309(T) (94.7 %) and Oceanobacillus picturae LMG 19416 (94.5 %). Strain T3944D(T) grew optimally at 1-3 % NaCl, pH 8.5 and 37 degrees C. The major cellular fatty acids were anteiso-C(15 : 0) (60.0 %) and anteiso-C(17 : 0) (12.9 %) and the DNA G+C content was 38.1 mol%. On the basis of its phenotypic properties and phylogenetic distinctiveness, strain T3944D(T) represents a novel species in the genus Oceanobacillus, for which the name Oceanobacillus chironomi sp. nov. is proposed. The type strain is T3944D(T) (=LMG 23627(T)=DSM 18262(T)).

Culturable and VBNC Vibrio cholerae: interactions with chironomid egg masses and their bacterial population

Vibrio cholerae, the etiologic agent of cholera, is autochthonous to various aquatic environments. Recently, it was found that chironomid (nonbiting midges) egg masses serve as a reservoir for the cholera bacterium and that flying chironomid adults are possible windborne carriers of V. cholerae non-O1 non-O139. Chironomids are the most widely distributed insect in freshwater. Females deposit egg masses at the water's edge, and each egg mass contains eggs embedded in a gelatinous matrix. Hemagglutinin/protease, an extracellular enzyme of V. cholerae, was found to degrade chironomid egg masses and to prevent them from hatching. In a yearly survey, chironomid populations and the V. cholerae in their egg masses followed phenological succession and interaction of host-pathogen population dynamics. In this report, it is shown via FISH technique that most of the V. cholerae inhabiting the egg mass are in the viable but nonculturable (VBNC) state. The diversity of culturable bacteria from chironomid egg masses collected from two freshwater habitats was determined. In addition to V. cholerae, representatives of the following genera were isolated: Acinetobacter, Aeromonas, Klebsiella, Shewanella, Pseudomonas, Paracoccus, Exiguobacterium, and unidentified bacteria. Three important human pathogens, Aeromonas veronii, A. caviae, and A. hydrophila, were isolated from chironomid egg masses, indicating that chironomid egg masses may be a natural reservoir for pathogenic Aeromonas species in addition to V. cholerae. All isolates of V. cholerae were capable of degrading chironomid egg masses. This may help explain their host-pathogen relationship with chironomids. In contrast, almost none of the other bacteria that were isolated from the egg masses possessed this ability. Studying the interaction between chironomid egg masses, the bacteria inhabiting them, and V. cholerae could contribute to our understanding of the nature of the V. cholerae-egg mass interactions.