Skin bacterial volatiles: propelling the future of vector control

Human microbiota in dengue infection: A narrative review

Dengue fever, a widespread mosquito-borne viral infection in tropical regions, typically manifests fever and gastrointestinal symptoms, including nausea, vomiting, and diarrhea. However, the human gut microbiota's role in dengue pathogenesis remains incompletely understood. Studies have demonstrated dysbiosis during dengue virus infection, characterized by increased abundance of potentially pathogenic bacteria like Bacteroidaceae and Proteobacteria, particularly during the critical phase. Furthermore, microbial translocation and leaky gut syndrome, characterized by the translocation of intestinal microbial products, have been observed in dengue patients and are associated with hypercytokinemia, plasma leakage, and disease severity. These findings underscore the necessity for an in-depth investigation into the role of human intestinal microbiota as a potential contributing factor in the pathogenesis and progression of dengue. Further research focusing on human intestinal microbiota, leaky gut syndrome, and the potential implications of treatment with oral bacteriotherapy, as previously observed in other viral diseases, is essential to clarify dengue pathology and evaluate new therapeutic strategies.

Sebaceous origins of human odor

The compounds that make up human body odor have been catalogued by researchers in many fields. Yet few are aware of exactly where these molecules come from. Volatile body-odor compounds are often cited as being produced primarily via microbial activity from precursors in sweat. While this is a source of many human volatiles, here we synthesize data showing that some of the most distinctive and abundant components of human odor instead originate from precursors in sebum, via reactions that do not involve the skin microbiome. We also review the unique biochemistry of human sebaceous glands and discuss evolutionary hypotheses that may partly explain why human sebum is so unique. Finally, we discuss how sebum-derived volatiles intersect with human health and disease, for example, via attraction of disease-vector mosquitoes or use in medical diagnostics. Our review draws insights from multiple fields, which together provide surprising clarity on some of the proximate and ultimate mechanisms underlying the distinctive composition of human odor.

Microbial interactions shaping host attractiveness: insights into dynamic behavioral relationships

Insects discern the presence of hosts (host plants) by integrating chemosensory, gustatory, and visual cues, with olfaction playing a pivotal role in this process. Among these factors, volatile signals produced by host-associated microbial communities significantly affect insect attraction. Microorganisms are widely and abundantly found on the surfaces of humans, plants, and insects. Notably, these microorganisms can metabolize compounds from the host surface and regulate the production of characteristic volatiles, which may guide the use of host microorganisms to modulate insect behavior. Essentially, the attraction of hosts to insects is intricately linked to the presence of their symbiotic microorganisms. This review underscores the critical role of microorganisms in shaping the dynamics of attractiveness between insects and their hosts.

Wearable solid-phase microextraction sampling for enhanced detection of volatile analytes in human ears

BACKGROUND

Investigating ear at molecule level is challenging task, since there is a lack of molecular detection by traditional diagnosis techniques such as otologic endoscopy, ear swab culture, and imaging diagnostic technique. Therefore, new development of noninvasive, highly sensitive, and convenient analytical method for investigating human ears is highly needed.

RESULTS

We developed a wearable sampling device for extracting trace analytes in ear by fixing solid-phase microextraction fibers into modified earmuffs (SPME-in-earmuffs). After sampling, SPME fiber was coupled with gas chromatography-mass spectrometry (GC-MS) for identification and quantification of extracted analytes. Enhanced detection of various analytes such as volatile metabolites, exposures, and therapeutic drugs of ears were demonstrated in this work. Particularly, sport-induced metabolic changes such as fatty acids, aldehyde compounds and oxidative produces were found from human ears using this method. Acceptable analytical performances were obtained by using this newly developed method for detecting ear medicines, e.g., low limit of detection (LOD, 0.005-0.021 ng/mL) and limit of quantification (LOQ, 0.018-0.071 ng/mL), excellent linear dynamic responses (R2 > 0.99, ranging from 0.050-8.00 ng/mL), good relative standard deviations (RSDs, 13.19 % ∼ 21.40 %, n = 6) and accuracy (84.43-150.18 %, n = 6) at different concentrations.

SIGNIFICANCE

For the first time, this work provides a simple, convenient, and wearable microextraction method for enhanced detection of trace volatiles in human ears. The enclosed space between ear and earmuff allows headspace SPME sampling of volatile analytes, and thus provides a new wearable method for monitoring ear metabolites and human exposures, showing potential applications in human health, disease diagnosis, and sport science.

Engineered skin microbiome reduces mosquito attraction to mice

The skin microbiome plays a pivotal role in the production of attractive cues detected by mosquitoes. Here, we leveraged recent advances in genetic engineering to significantly reduce the production of L-(+)-lactic acid as a strategy to reduce mosquito attraction to the highly prominent skin commensals Staphylococcus epidermidis and Corynebacterium amycolatum. Engraftment of these engineered bacteria onto the skin of mice reduced mosquito attraction and feeding for up to 11 uninterrupted days, which is considerably longer than the several hours of protection conferred by the leading chemical repellent N,N-diethyl-meta-toluamide. Taken together, our findings demonstrate engineering the skin microbiome to reduce attractive volatiles represents an innovative untapped strategy to reduce vector attraction, preventing bites, and pathogen transmission. These findings set the stage for new classes of long-lasting microbiome-based repellent products.

Environment rather than breed or body site shapes the skin bacterial community of healthy sheep as revealed by metabarcoding

BACKGROUND

The skin is inhabited by a variety of micro-organisms, with bacteria representing the predominant taxon of the skin microbiome. In sheep, the skin bacterial community of healthy animals has been addressed in few studies, only with culture-based methods or sequencing of cloned amplicons.

OBJECTIVES

The objectives of this study were to determine the sheep skin bacterial community composition by using metabarcoding for a detailed characterisation and to determine the effect of body part, breed and environment.

MATERIALS AND METHODS

Overall, 267 samples were taken from 89 adult female sheep, belonging to three different breeds and kept on nine different farms in Switzerland. From every individual, one sample each was taken from belly, left ear and left leg and metabarcoding of the 16S rRNA V3-V4 hypervariable region was performed.

RESULTS

The main phyla identified were Actinobacteriota, Firmicutes, Proteobacteria and Bacteriodota. The alpha diversity as determined by Shannon's diversity index was significantly different between sheep from different farms. Beta diversity analysis by principal coordinate analysis (PCoA) showed clustering of the samples by farm and body site, while breed had only a marginal influence. A sparse partial least squares discriminant analysis (sPLS-DA) revealed seven main groups of operational taxonomic units (OTUs) of which groups of OTUs were specific for some farms.

CONCLUSIONS AND CLINICAL RELEVANCE

These findings indicate that environment has a larger influence on skin microbial variability than breed, although the sampled breeds, the most abundant ones in Switzerland, are phenotypically similar. Future studies on the sheep skin microbiome may lead to novel insights in skin diseases and prevention.

The behaviour of adult Anopheles gambiae, sub-Saharan Africa's principal malaria vector, and its relevance to malaria control: a review

BACKGROUND

Mosquitoes of the Anopheles gambiae complex are one of the major vectors of malaria in sub-Saharan Africa. Their ability to transmit this disease of major public health importance is dependent on their abundance, biting behaviour, susceptibility and their ability to survive long enough to transmit malaria parasites. A deeper understanding of this behaviour can be exploited for improving vector surveillance and malaria control.

FINDINGS

Adult mosquitoes emerge from aquatic habitats at dusk. After a 24 h teneral period, in which the cuticle hardens and the adult matures, they may disperse at random and search upwind for a mate or to feed. Mating generally takes place at dusk in swarms that form over species-specific 'markers'. Well-nourished females may mate before blood-feeding, but the reverse is true for poorly-nourished insects. Females are monogamous and only mate once whilst males, that only feed on nectar, swarm nightly and can potentially mate up to four times. Females are able to locate hosts by following their carbon dioxide and odour gradients. When in close proximity to the host, visual cues, temperature and relative humidity are also used. Most blood-feeding occurs at night, indoors, with mosquitoes entering houses mainly through gaps between the roof and the walls. With the exception of the first feed, females are gonotrophically concordant and a blood meal gives rise to a complete egg batch. Egg development takes two or three days depending on temperature. Gravid females leave their resting sites at dusk. They are attracted by water gradients and volatile chemicals that provide a suitable aquatic habitat in which to lay their eggs.

CONCLUSION

Whilst traditional interventions, using insecticides, target mosquitoes indoors, additional protection can be achieved using spatial repellents outdoors, attractant traps or house modifications to prevent mosquito entry. Future research on the variability of species-specific behaviour, movement of mosquitoes across the landscape, the importance of light and vision, reproductive barriers to gene flow, male mosquito behaviour and evolutionary changes in mosquito behaviour could lead to an improvement in malaria surveillance and better methods of control reducing the current over-reliance on the indoor application of insecticides.

The impact of volatiles on tick-host interaction and vector competence

Ticks are obligatory hematophagous arachnids, serving as vectors for a wide array of pathogens that can be transmitted to humans or animals. The ability of tick-borne pathogens to maintain within natural reservoirs is intricately influenced by the attractiveness of ticks to their animal hosts, including humans. However, the complex dynamics of tick behavior and host-seeking strategies remain understudied. This review aims to summarize the impact of volatiles or odors on tick behavior and vector competence. Our literature review has identified a selection of compounds, such as 1-octen-3-ol, hexanal, heptanal, nonanal, 6-methyl-5-hepten-2-one, acetone, and octanal, as having the potential to impact both ticks' and mosquitos' behaviors. In addition, carbon dioxide (CO2) is a universal attractant for hematophagous arthropods. Moreover, we have gathered some clues indicating that volatiles emitted by infected animal hosts might play a role in the transmission of tick-borne pathogens. Nonetheless, our understanding of this phenomenon remains largely inadequate, particularly with regarding to whether the tick microbiome or the skin microbiota of the feeding mammals, including humans, can actively modulate tick-host-seeking behavior. Further investigations in this emerging field hold immense promise for the development of innovative strategies aimed at controlling vectors and curtailing the spread of tick-borne diseases.

Identification of human skin microbiome odorants that manipulate mosquito landing behavior

The resident human skin microbiome is responsible for the production of most of the human scents that are attractive to mosquitoes. Hence, engineering the human skin microbiome to synthesize less of mosquito attractants or produce repellents could potentially reduce bites and prevent the transmission of deadly mosquito-borne pathogens. In order to further characterize the human skin volatilome, we quantified the major volatiles of 39 strains of skin commensals (Staphylococci and Corynebacterium). Importantly, to validate the behavioral activity of these volatiles, we first assessed landing behavior triggered by human skin volatiles. We demonstrated that landing behavior is gated by the presence of carbon dioxide and L-(+)-lactic acid. This is similar to the combinatorial coding triggering mosquito short range attraction. Repellency behavior to selected skin volatiles and terpenes was tested in the presence of carbon dioxide and L-(+)-lactic acid. In a 2-choice landing behavior context, the skin volatiles 2- and 3-methyl butyric acids reduced mosquito landing by 62.0-81.6% and 87.1-99.6%, respectively. Similarly, the terpene geraniol was capable of reducing mosquito landing behavior by 74.9%. We also tested the potential repellency effects of terpenes in mosquitoes at short-range using a 4-port olfactometer. In these assays, geraniol reduced mosquito attraction (69-78%) to a mixture of key human kairomones carbon dioxide, L-(+)-lactic acid, and ammonia. These findings demonstrate that carbon dioxide and L-(+)-lactic acid change the valence of other skin volatiles towards mosquito landing behavior. Moreover, this study offers candidate odorants to be targeted in a novel strategy to reduce attractants or produce repellents by the human skin microbiota that may curtail mosquito bites, and subsequent mosquito-borne disease.

Engineered Skin Microbiome Reduces Mosquito Attraction to Mice

The skin microbiome plays a pivotal role in the production of attractive cues detected by mosquitoes. Here we leveraged recent advances in genetic engineering to significantly reduce the production of L-(+)-lactic acid as a strategy to reduce mosquito attraction to the highly prominent skin commensals Staphylococcus epidermidis and Corynebacterium amycolatum . Engraftment of these engineered bacteria onto the skin of mice reduced mosquito attraction and feeding for up to 11 uninterrupted days, which is considerably longer than the several hours of protection conferred by the leading chemical repellent DEET. Taken together, our findings demonstrate engineering the skin microbiome to reduce attractive volatiles represents an innovative untapped strategy to reduce vector attraction, preventing bites, and pathogen transmission setting the stage for new classes of long-lasting microbiome-based repellent products.

One-Sentence Summary

Modified microbes make skin less attractive to mosquitoes.

Identification of human skin microbiome odorants that manipulate mosquito landing behavior

The resident human skin microbiome is responsible for the production of most of the human scents that are attractive to mosquitoes. Hence, engineering the human skin microbiome to synthesize less of mosquito attractants or produce repellents could potentially reduce bites and prevent the transmission of deadly mosquito-borne pathogens. In order to further characterize the human skin volatilome, we quantified the major volatiles of 39 strains of skin commensals (Staphylococci and Corynebacterium). Importantly, to validate the behavioral activity of these volatiles, we first assessed landing behavior triggered by human skin bacteria volatiles. We demonstrated that this behavioral step is gated by the presence of carbon dioxide and L-(+)-lactic acid, similar to the combinatorial coding triggering short range attraction. Repellency behavior to selected skin volatiles and the geraniol terpene was tested in the presence of carbon dioxide and L-(+)-lactic acid. In a 2-choice landing behavior context, the skin volatiles 2- and 3-methyl butyric acids reduced mosquito landing by 62.0-81.6% and 87.1-99.6%, respectively. Similarly, geraniol was capable of reducing mosquito landing behavior by 74.9%. We also tested the potential repellency effects of geraniol on mosquitoes at short-range using a 4-port olfactometer. In these assays, geraniol reduced mosquito attraction (69-78%) to a mixture of key human kairomones carbon dioxide, L-(+)-lactic acid, and ammonia. These findings demonstrate that carbon dioxide and L-(+)-lactic acid changes the valence of other skin volatiles towards mosquito landing behavior. Moreover, this study offers candidate odorants to be targeted in a novel strategy to reduce attractants or produce repellents by the human skin microbiota that may curtail mosquito bites, and subsequent mosquito-borne disease.

Carboxylic acids that drive mosquito attraction to humans activate ionotropic receptors

The mosquito, Aedes aegypti, is highly anthropophilic and transmits debilitating arboviruses within human populations and between humans and non-human primates. Female mosquitoes are attracted to sources of blood by responding to odor plumes that are emitted by their preferred hosts. Acidic volatile compounds, including carboxylic acids, represent particularly salient odors driving this attraction. Importantly, carboxylic acids are major constituents of human sweat and volatiles generated by skin microbes. As such, they are likely to impact human host preference, a dominant factor in disease transmission cycles. A more complete understanding of mosquito host attraction will necessitate the elucidation of molecular mechanisms of volatile odor detection that function in peripheral sensory neurons. Recent studies have shown that members of the variant ionotropic glutamate receptor gene family are necessary for physiological and behavioral responses to acidic volatiles in Aedes. In this study, we have identified a subfamily of variant ionotropic receptors that share sequence homology across several important vector species and are likely to be activated by carboxylic acids. Moreover, we demonstrate that selected members of this subfamily are activated by short-chain carboxylic acids in a heterologous cell expression system. Our results are consistent with the hypothesis that members of this receptor class underlie acidic volatile sensitivity in vector mosquitoes and provide a frame of reference for future development of novel mosquito attractant and repellent technologies.

A novel volatile deterrent from symbiotic bacteria of entomopathogenic nematodes fortifies field performances of nematodes against fall armyworm larvae

The core elements of entomopathogenic nematode toxicity towards the fall armyworm Spodoptera frugiperda are associated with symbiotic bacteria. These microbes provide independent control effects and are reported to have repellency to insect pests. However, the ecological background of this nematode-bacteria-insect communication module is elusive. This work aims to identify key chemical cues which drive the trophic interactions through olfactory reception of S. frugiperda, and to inspire implementations with these isolated behavioral regulators in the corn field. A total of 657 volatiles were found within 13 symbiotic bacterial strains, and five of them induced significant electrophysiological responses of S. frugiperda larvae. 2-Hexynoic acid was demonstrated to exhibit a dominant role in deterring S. frugiperda larvae from feeding and localization. Field implementations with this novel volatile deterrent have resulted in fortified nematode applications. 2-Hexynoic acid acts as an excellent novel deterrent and presents remarkable application potential against fall armyworm larvae. Emissions from symbiotic bacteria of entomopathogenic nematodes are key players in chemical communication among insects, nematodes, and microbes. The olfactory perceptions and molecular targets for this volatile are worthy of future research.

Update on mosquito bite reaction: Itch and hypersensitivity, pathophysiology, prevention, and treatment

Mosquito bites are endured by most populations worldwide. Reactions to mosquito bites range from localized wheals and papules with associated pruritus to rare systemic reactions and anaphylaxis in certain populations. The mechanism of itch is due to introduction of mosquito saliva components into the cutaneous tissue, although the exact pathophysiology is unclear. Histamine is thought to be a key player through mosquito saliva itself or through activation of mast cells by IgE or through an IgE-independent pathway. However, other salivary proteins such as tryptase and leukotrienes may induce non-histaminergic itch. Some individuals have a genetic predisposition for mosquito bites, and people with hematologic cancers, HIV, and other conditions are susceptible to robust reactions. Prevention of mosquito bites is key with physical barriers or chemical repellents. Treatment consists of second-generation antihistamines and topical corticosteroids. Further research on topical treatments that target neural-mediated itch is needed.