Electrophysiological and Behavioral Responses of Virgin Female Bactrocera tryoni to Microbial Volatiles from Enterobacteriaceae

The Queensland fruit fly (Bactrocera tryoni) is a major polyphagous pest widespread in Australia and several Pacific Islands. Bacteria present on the host plant phyllosphere supply proteins, essential for egg development and female sexual maturity. We investigated the role of microbial volatile organic compounds (MVOCs) emitted by Enterobacteriaceae commonly found on the host plant and in the fly gut in attracting virgin females. Bacteria were cultured on artificial media and natural fruits, at various pH, and MVOCs were collected using different headspace volatile absorbent materials. The olfactory responses of virgin females to bacterial MVOCs were assessed via electrophysiology and behavioral assays. The production of MVOCs was strongly influenced qualitatively by the bacterial strain and the type of media, and it semi-quantitatively varied with pH and time. MVOCs emitted by Klebsiella oxytoca invoked the strongest antennal response and were the most attractive. Among the identified compounds triggering an olfactory response, D-limonene and 2-nonanone were both significantly behaviorally attractive, whereas phenol, nonanal, isoamyl alcohol, and some pyrazines appeared to be repulsive. This study deepens our understanding of the chemical ecology between fruit flies and their bacterial symbionts and paves the way for novel synthetic lures based on specifically MVOCs targeting virgin females.

Grain Inoculated with Different Growth Stages of the Fungus, Aspergillus flavus, Affect the Close-Range Foraging Behavior by a Primary Stored Product Pest, Sitophilus oryzae (Coleoptera: Curculionidae)

Although some research has investigated the interactions among stored product insects and microbes, little research has examined how specific fungal life stages affect volatile emissions in grain and linked it to the behavior of Sitophilus oryzae, the cosmopolitan rice weevil. Thus, our goals were to 1) isolate, culture, and identify two fungal life stages of Aspergillus flavus, 2) characterize the volatile emissions from grain inoculated by each fungal morphotype, and 3) understand how microbially-produced volatile organic compounds (MVOCs) from each fungal morphotype affect foraging, attraction, and preference by S. oryzae. We hypothesized that the headspace blends would be unique among our treatments and that this will lead to preferential mobility by S. oryzae among treatments. Using headspace collection coupled with GC-MS, we found the sexual life stage of A. flavus had the most unique emissions of MVOCs compared to the other semiochemical treatments. This translated to a higher interaction with kernels containing grain with the A. flavus sexual life stage, as well as a higher cumulative time spent in those zones by S. oryzae in a video-tracking assay in comparison to the asexual life stage. While fungal cues were important for foraging at close-range, the release-recapture assay indicated that grain volatiles were more important for attraction at longer distances. There was no significant preference between grain and MVOCs in a four-way olfactometer. Overall, this study enhances our understanding of how fungal cues affect the close and longer range foraging ecology of a primarily stored product insect.

An Investigation of Stability and Species and Strain-Level Specificity in Bacterial Volatilomes

Microbial volatilomics is a rapidly growing field of study and has shown great potential for applications in food, farming, and clinical sectors in the future. Due to the varying experimental methods and growth conditions employed in microbial volatilomic studies as well as strain-dependent volatilomic differences, there is limited knowledge regarding the stability of microbial volatilomes. Consequently, cross-study comparisons and validation of results and data can be challenging. In this study, we investigated the stability of the volatilomes of multiple strains of Staphylococcus aureus, Pseudomonas aeruginosa and Escherichia coli across three frequently used nutrient-rich growth media. Volatilomic stability was assessed based on media-, time- and strain-dependent variation across the examined bacterial volatilomes. Strain-level specificity of the observed volatilomes of E. coli and P. aeruginosa strains was further investigated by comparing the emission of selected compounds at varying stages of cell growth. Headspace solid phase microextraction (HS-SPME) sampling coupled with gas chromatography mass spectrometry (GC-MS) was used to analyze the volatilome of each strain. The whole volatilomes of the examined strains demonstrate a high degree of stability across the three examined growth media. At the compound-level, media dependent differences were observed particularly when comparing the volatilomes obtained in glucose-containing brain heart infusion (BHI) and tryptone soy broth (TSB) growth media with the volatilomes obtained in glucose-free Lysogeny broth (LB) media. These glucose-dependent volatilomic differences were primarily seen in the emission of primary metabolites such as alcohols, ketones, and acids. Strain-level differences in the emission of specific compounds in E. coli and P. aeruginosa samples were also observed across the media. These strain-level volatilomic differences were also observed across varying phases of growth of each strain, therefore confirming that these strains had varying core and accessory volatilomes. Our results demonstrate that, at the species-level, the examined bacteria have a core volatilome that exhibits a high-degree of stability across frequently-used growth media. Media-dependent differences in microbial volatilomes offer valuable insights into identifying the cellular origin of individual metabolites. The observed differences in the core and accessory volatilomes of the examined strains illustrate the complexity of microbial volatilomics as a study while also highlighting the need for more strain-level investigations to ultimately elucidate the whole volatilomic capabilities of microbial species in the future.

Extending the Salinilactone Family

Five new members of the salinilactone family, salinilactones D-H, are reported. These bicyclic lactones are produced by Salinispora bacteria and display extended or shortened alkyl side chains relative to the recently reported salinilactones A-C. They were identified by GC/MS, gas chromatographic retention index, and comparison with synthetic samples. We further investigated the occurrence of salinilactones across six newly proposed Salinispora species to gain insight into how compound production varies among taxa. The growth-inhibiting effect of this compound family on multiple biological systems including non-Salinispora actinomycetes was analyzed. Additionally, we found strong evidence for significant cytotoxicity of the title compounds.

Volatilomes of Bacterial Infections in Humans

Sense of smell in humans has the capacity to detect certain volatiles from bacterial infections. Our olfactory senses were used in ancient medicine to diagnose diseases in patients. As humans are considered holobionts, each person's unique odor consists of volatile organic compounds (VOCs, volatilome) produced not only by the humans themselves but also by their beneficial and pathogenic micro-habitants. In the past decade it has been well documented that microorganisms (fungi and bacteria) are able to emit a broad range of olfactory active VOCs [summarized in the mVOC database (http://bioinformatics.charite.de/mvoc/)]. During microbial infection, the equilibrium between the human and its microbiome is altered, followed by a change in the volatilome. For several decades, physicians have been trying to utilize these changes in smell composition to develop fast and efficient diagnostic tools, particularly because volatiles detection is non-invasive and non-destructive, which would be a breakthrough in many therapies. Within this review, we discuss bacterial infections including gastrointestinal, respiratory or lung, and blood infections, focusing on the pathogens and their known corresponding volatile biomarkers. Furthermore, we cover the potential role of the human microbiota and their volatilome in certain diseases such as neurodegenerative diseases. We also report on discrete mVOCs that affect humans.

A Mass Spectrometry-Based Study Shows that Volatiles Emitted by Arthrobacter agilis UMCV2 Increase the Content of Brassinosteroids in Medicago truncatula in Response to Iron Deficiency Stress

Iron is an essential plant micronutrient. It is a component of numerous proteins and participates in cell redox reactions; iron deficiency results in a reduction in nutritional quality and crop yields. Volatiles from the rhizobacterium Arthrobacter agilis UMCV2 induce iron acquisition mechanisms in plants. However, it is not known whether microbial volatiles modulate other metabolic plant stress responses to reduce the negative effect of iron deficiency. Mass spectrometry has great potential to analyze metabolite alterations in plants exposed to biotic and abiotic factors. Direct liquid introduction-electrospray-mass spectrometry was used to study the metabolite profile in Medicago truncatula due to iron deficiency, and in response to microbial volatiles. The putatively identified compounds belonged to different classes, including pigments, terpenes, flavonoids, and brassinosteroids, which have been associated with defense responses against abiotic stress. Notably, the levels of these compounds increased in the presence of the rhizobacterium. In particular, the analysis of brassinolide by gas chromatography in tandem with mass spectrometry showed that the phytohormone increased ten times in plants grown under iron-deficient growth conditions and exposed to microbial volatiles. In this mass spectrometry-based study, we provide new evidence on the role of A. agilis UMCV2 in the modulation of certain compounds involved in stress tolerance in M. truncatula.

Advances in Attract-and-Kill for Agricultural Pests: Beyond Pheromones

Attract-and-kill has considerable potential as a tactic in integrated management of pests of agricultural crops, but the use of sex pheromones as attractants is limited by male multiple mating and immigration of mated females into treated areas. Attractants for both sexes, and particularly females, would minimize these difficulties. Volatile compounds derived from plants or fermentation of plant products can attract females and have been used in traps for monitoring and control, and in sprayable attract-and-kill formulations or bait stations. Recent advances in fundamental understanding of insect responses to plant volatiles should contribute to the development of products that can help manage a wide range of pests with few impacts on nontarget organisms, but theory must be tempered with pragmatism in the selection of volatiles and toxicants and in defining their roles in formulations. Market requirements and regulatory factors must be considered in parallel with scientific constraints if successful products are to be developed.

A suggested model for potato MIVOISAP involving functions of central carbohydrate and amino acid metabolism, as well as actin cytoskeleton and endocytosis

We have recently found that microbial species ranging from Gram-negative and Gram-positive bacteria to different fungi emit volatiles that strongly promote starch accumulation in leaves of both mono- and di-cotyledonous plants. Transcriptome and enzyme activity analyses of potato leaves exposed to volatiles emitted by Alternaria alternata revealed that starch over-accumulation was accompanied by enhanced 3-phosphoglycerate to Pi ratio, and changes in functions involved in both central carbohydrate and amino acid metabolism. Exposure to microbial volatiles also promoted changes in the expression of genes that code for enzymes involved in endocytic uptake and traffic of solutes. With the overall data we propose a metabolic model wherein important determinants of accumulation of exceptionally high levels of starch include (a) upregulation of ADPglucose-producing SuSy, starch synthase III and IV, proteins involved in the endocytic uptake and traffic of sucrose, (b) down-regulation of acid invertase, starch breakdown enzymes and proteins involved in internal amino acid provision, and (c) 3-phosphoglycerate-mediated allosteric activation of ADPglucose pyrophosphorylase.