Developing insect models for the study of current and emerging human pathogens

The study of human diseases requires the testing of microorganisms in model systems. Although mammals are typically used, we argue the validity of using insects as models in order to examine human diseases, particularly the growing number of opportunistic microorganisms. Insects can be used in large numbers, are easily manipulated, and are not subject to the same ethical concerns as mammalian systems. Insects and mammals have many parallels with respect to microbial pathogenesis, from proteinaceous integuments that require breaching before infection to similarities in their innate immune responses. Reactions of insects to Candida and Pseudomonas spp. infections show good correlation with mouse models, providing precedent-setting examples of the study of human pathogens using insects. Insects as pathogen hosts also warrant study because they may act as reservoirs for emerging human pathogens. Finally, insect models may be used to examine the evolutionary processes involved in the acquisition of virulence factors and host-jumping mechanisms indispensable to emerging pathogens. Insect models may be used in 'niche' investigations where large sample sizes can facilitate rapid, informative screening of opportunistic diseases and provide insights into pathogen evolution, while reducing the cost and ethical concerns associated with mammalian models.     

Antagonistic activities of lactobacilli and bifidobacteria against microbial pathogens

The gastrointestinal tract is a complex ecosystem that associates a resident microbiota and cells of various phenotypes lining the epithelial wall expressing complex metabolic activities. The resident microbiota in the digestive tract is a heterogeneous microbial ecosystem containing up to 1 x 10(14) colony-forming units (CFUs) of bacteria. The intestinal microbiota plays an important role in normal gut function and maintaining host health. The host is protected from attack by potentially harmful microbial microorganisms by the physical and chemical barriers created by the gastrointestinal epithelium. The cells lining the gastrointestinal epithelium and the resident microbiota are two partners that properly and/or synergistically function to promote an efficient host system of defence. The gastrointestinal cells that make up the epithelium, provide a physical barrier that protects the host against the unwanted intrusion of microorganisms into the gastrointestinal microbiota, and against the penetration of harmful microorganisms which usurp the cellular molecules and signalling pathways of the host to become pathogenic. One of the basic physiological functions of the resident microbiota is that it functions as a microbial barrier against microbial pathogens. The mechanisms by which the species of the microbiota exert this barrier effect remain largely to be determined. There is increasing evidence that lactobacilli and bifidobacteria, which inhabit the gastrointestinal microbiota, develop antimicrobial activities that participate in the host's gastrointestinal system of defence. The objective of this review is to analyze the in vitro and in vivo experimental and clinical studies in which the antimicrobial activities of selected lactobacilli and bifidobacteria strains have been documented.     

The survival of ingested Serratia marcescens in houseflies (Musca domestica L.) after electrocution with electric fly killers

Electric fly killers (EFKs) are commonly used to control flying insects that enter food establishments. For establishment of the incidence of pathogen-bearing insects in food establishments, insect samples obtained from EFK trays could be used. The principal difficulty with this approach is that the survival time of microorganisms on or within insect corpses after electrocution is unknown. This study determined the survival of Serratia marcescens (as a representative of the enteric bacteria) within houseflies following their electrocution by a commercial EFK. S. marcescens was successfully ingested by houseflies and survived on and within the corpses after electrocution for up to 5 weeks. Maximal levels of bacteria were recovered 24 h postelectrocution. The study also demonstrates the ability of ingested S. marcescens to out-compete resident microbial flora within houseflies. The findings are intended to pave the way for further research to determine the incidence of pathogen-laden flying insects in food establishments.     

Regulators and signalling in insect antimicrobial innate immunity: Functional molecules and cellular pathways

Insects possess an immune system that protects them from attacks by various pathogenic microorganisms that would otherwise threaten their survival. Immune mechanisms may deal directly with the pathogens by eliminating them from the host organism or disarm them by suppressing the synthesis of toxins and virulence factors that promote the invasion and destructive action of the intruder within the host. Insects have been established as outstanding models for studying immune system regulation because innate immunity can be explored as an integrated system at the level of the whole organism. Innate immunity in insects consists of basal immunity that controls the constitutive synthesis of effector molecules such as antimicrobial peptides, and inducible immunity that is activated after detection of a microbe or its product(s). Activation and coordination of innate immune defenses in insects involve evolutionary conserved immune factors. Previous research in insects has led to the identification and characterization of distinct immune signalling pathways that modulate the response to microbial infections. This work has not only advanced the field of insect immunology, but it has also rekindled interest in the innate immune system of mammals. Here we review the current knowledge on key molecular components of insect immunity and discuss the opportunities they present for confronting infectious diseases in humans.     

Honeybees and bumblebees share similar bacterial symbionts

Associations with symbiotic microorganisms are a major source for evolutionary innovation in eukaryotes. Arthropods have long served as model systems to study such associations, especially since Paul Buchner’s (1965) seminal work that beautifully illustrated the enormous diversity of microorganisms associated with insects. Particularly high taxonomic and functional diversities of microbial symbionts have been found in the guts and gut‐associated organs of insects. These microorganisms play important roles in the digestion, nutrition and defence of the host. However, most studies of gut microorganisms have focused on single host taxa, limiting the ability to draw general conclusions on composition and functional roles of the insect gut microbiota. This is especially true for the diverse and important insect order Hymenoptera that comprises the bees, wasps and ants. Recently, Russell et al. (2009) analysed the bacterial community associated with diverse ant species and found evidence for changes in the microbial gut community coinciding with the evolution of herbivory. In this issue of Molecular Ecology, Martinson et al. (2011) provide the first broad‐scale bacterial survey for bees. Their findings substantiate earlier evidence for a surprisingly simple gut microbiota in honeybees (Apis mellifera) that is composed of only six to ten major phylotypes. Importantly, Martinson et al. demonstrate for the first time that the same bacterial phylotypes are major constituents of other Apis as well as Bombus species, but not of any other bees and wasps outside of the corbiculate bees, a clade of four tribes within the subfamily Apinae. These results indicate that corbiculate bees harbour a specific and possibly co‐evolved bacterial community in their digestive tract. Furthermore, the comparison with other bees and wasps suggests that changes in social lifestyle may have had a stronger effect on the evolution of the gut microbiota than the dietary shift from predatory ancestors to pollen‐feeding (i.e. herbivorous) species. These findings have far‐reaching implications for research on the microbial symbionts of insects as well as on the nutritional physiology of the ecologically and economically important group of corbiculate bees.     

The Survival of Ingested <Emphasis Type="Italic">Serratia marcescens</Emphasis> in Houseflies (<Emphasis Type="Italic">Musca domestica</Emphasis> L.) After Electrocution with Electric Fly Killers

Electric fly killers (EFKs) are commonly used to control flying insects that enter food establishments. For establishment of the incidence of pathogen-bearing insects in food establishments, insect samples obtained from EFK trays could be used. The principal difficulty with this approach is that the survival time of microorganisms on or within insect corpses after electrocution is unknown. This study determined the survival of Serratia marcescens (as a representative of the enteric bacteria) within houseflies following their electrocution by a commercial EFK. S. marcescens was successfully ingested by houseflies and survived on and within the corpses after electrocution for up to 5 weeks. Maximal levels of bacteria were recovered 24 h postelectrocution. The study also demonstrates the ability of ingested S. marcescens to out-compete resident microbial flora within houseflies. The findings are intended to pave the way for further research to determine the incidence of pathogen-laden flying insects in food establishments. Received: 30 April 2002 / Accepted: 3 July 2002     

Complexity and variability of gut commensal microbiota in polyphagous lepidopteran larvae

Background

The gut of most insects harbours nonpathogenic microorganisms. Recent work suggests that gut microbiota not only provide nutrients, but also involve in the development and maintenance of the host immune system. However, the complexity, dynamics and types of interactions between the insect hosts and their gut microbiota are far from being well understood.

Methods/Principal Findings

To determine the composition of the gut microbiota of two lepidopteran pests, Spodoptera littoralis and Helicoverpa armigera, we applied cultivation-independent techniques based on 16S rRNA gene sequencing and microarray. The two insect species were very similar regarding high abundant bacterial families. Different bacteria colonize different niches within the gut. A core community, consisting of Enterococci, Lactobacilli, Clostridia, etc. was revealed in the insect larvae. These bacteria are constantly present in the digestion tract at relatively high frequency despite that developmental stage and diet had a great impact on shaping the bacterial communities. Some low-abundant species might become dominant upon loading external disturbances; the core community, however, did not change significantly. Clearly the insect gut selects for particular bacterial phylotypes.

Conclusions

Because of their importance as agricultural pests, phytophagous Lepidopterans are widely used as experimental models in ecological and physiological studies. Our results demonstrated that a core microbial community exists in the insect gut, which may contribute to the host physiology. Host physiology and food, nevertheless, significantly influence some fringe bacterial species in the gut. The gut microbiota might also serve as a reservoir of microorganisms for ever-changing environments. Understanding these interactions might pave the way for developing novel pest control strategies.     

肠道微生物与昆虫的共生关系

昆虫肠道栖息着大量的微生物。随着近年来研究肠道微生物的方法不断进步,尤其是基于16S rDNA的分子生物学方法的应用,人们对肠道微生物的了解逐渐加深。昆虫肠道对于微生物的拓殖存在一定的选择作用。肠道微生物对昆虫寄主的作用包括提供营养、利用拓殖抗性抵抗外来微生物侵袭、参与多重营养关系、引起昆虫免疫反应。长期进化过程中肠道微生物与昆虫发展出紧密的共生关系,微生物发展出一系列手段适应昆虫肠道环境。文章从以上几个方面对近年来的研究进展进行总结,并对昆虫肠道微生态学的实践意义和将来可能的研究热点进行展望。     

微生物对昆虫行为的影响研究进展

在漫长的进化过程中,微生物与昆虫形成了多种形式的互作关系.微生物的广泛分布为与昆虫接触并影响昆虫的行为提供了背景条件.为了深入探究微生物影响昆虫行为的现象和机制,本文综述了微生物影响昆虫行为方面的研究进展.微生物通过产生可被昆虫识别的化学信号物质、参与昆虫或寄主植物信息化合物的合成等方式可影响昆虫对其寄主的定位和选择....     

Microbial Community Profiling to Investigate Transmission of Bacteria Between Life Stages of the Wood-Boring Beetle, Anoplophora glabripennis

Many insects harbor specific bacteria in their digestive tract, and these gut microbiota often play important roles in digestion and nutrient provisioning. While it is common for a given insect species to harbor a representative gut microbial community as a population, how this community is acquired and maintained from generation to generation is not known for most xylophagous insects, except termites. In this study, we examined acquisition of gut microbiota by the wood-feeding beetle, Anoplophora glabripennis, by identifying and comparing microbial community members among different life stages of the insect and with microbes it encounters in the environment. Automated ribosomal intergenic spacer analysis was employed to compare bacterial communities present in the egg and larval stages of A. glabripennis as well as with microbes found in the oviposition site and the surrounding woody tissue. Multivariate analyses were used to identify relationships between sample type and specific bacterial types (operational taxonomic units). From this analysis, bacteria that were derived from the environment, the oviposition site, and/or the egg were identified and compared with taxa found in larvae. Results showed that while some larval microbes were derived from environmental sources, other members of the larval microbial community appear to be vertically transmitted. These findings could lead to a better understanding of which microbial species are critical for the survival of this insect and to development of techniques that could be used to alter this community to disrupt the digestive physiology of the host insect as a biological control measure.     

Gut microbiota and parasite transmission by insect vectors

In the gut of some insect vectors, parasites ingested with the bloodmeal decrease in number before coming into contact with host tissues. Many factors could be responsible for this reduction in parasite number but the potentially important role of the large communities of naturally occurring microorganisms that exist alongside the newly ingested parasites in the vector midgut has been largely overlooked. Some previous reports exist of the inhibition of parasite development by vector gut microbiota and of the killing of Trypanosoma cruzi and Plasmodium spp. by prodigiosin produced by bacteria. Based on this evidence, we believe that the microbiota present in the midgut of vector insects could have important roles as determinants of parasite survival and development in insect vector hosts and, therefore, contribute to the modulation of vector competence for many important diseases.     

Exploiting the potential of insects for in vivo pathogenicity testing of microbial pathogens

Conventional assays for quantifying the virulence of microbial pathogens and mutants have traditionally relied upon the use of a range of mammalian species. A number of workers have demonstrated that insects can be used for evaluating microbial pathogenicity and provide results comparable to those that can be obtained with mammals since one component of the vertebrate immune system, the innate immune response, remains similar to that found in insects. Larvae of the Greater Wax Moth Galleria mellonella have been used to evaluate the virulence of a range of bacterial and fungal pathogens and a correlation with the virulence of these microbes in mice has been established. This review highlights the similarities of the vertebrate and insect innate immune responses to infection and identifies the potential use of insects for the in vivo evaluation of the microbial pathogenicity.     

培菌昆虫相关放线菌的次级代谢产物研究进展

昆虫共生微生物是一种特殊的微生物资源,其中放线菌在昆虫肠道、体表和巢穴中广泛分布。近年来,人们从培菌昆虫来源的放线菌中分离得到多种新型化合物,可以选择性抑制菌圃的致病真菌,部分还对植物致病真菌、昆虫致病真菌、人类病原菌和癌细胞有抑制活性。因此,研究培菌昆虫相关微生物不仅有助于了解宿主与微生物的共生机制,还能发掘新的活性物质,用于生物农药、生物医药的开发。本文对培菌昆虫来源放线菌次级代谢产物的研究进展进行了综述。     

Detoxifying symbionts in agriculturally important pest insects

Pest insects lead to excessive agricultural and therefore economical losses on crops worldwide. These insects have to withstand toxic molecules that are inherent to plant defences, as well as those that are produced and introduced by humans in the form of insecticides. In recent years, research on insect–microbe symbioses has recognized that microbial symbionts may play a role protecting against these toxins, leading to a form of defensive symbiosis between the pest insect and different types of microorganisms that we term detoxifying symbioses. In this minireview, we will highlight well‐characterized and emerging insect model systems of detoxifying symbioses and assess how the microorganisms influence the host's success.     

The skin microbiome

The skin is the human body's largest organ, colonized by a diverse milieu of microorganisms, most of which are harmless or even beneficial to their host. Colonization is driven by the ecology of the skin surface, which is highly variable depending on topographical location, endogenous host factors and exogenous environmental factors. The cutaneous innate and adaptive immune responses can modulate the skin microbiota, but the microbiota also functions in educating the immune system. The development of molecular methods to identify microorganisms has led to an emerging view of the resident skin bacteria as highly diverse and variable. An enhanced understanding of the skin microbiome is necessary to gain insight into microbial involvement in human skin disorders and to enable novel promicrobial and antimicrobial therapeutic approaches for their treatment.     

昆虫体内微生物多样性的影响因素研究进展

昆虫体内的微生物种类繁多,在长期的协同进化中与其宿主昆虫形成了密切的共生关系。近年来,随着分子生物学技术的快速发展与应用,昆虫体内微生物多样性得到广泛的研究。本文综述了影响昆虫体内微生物多样性的各种因素。体内微生物多样性除与昆虫种类相关外,还与昆虫自身因素(性别、发育龄期和种群)、生态因子(食物、温度、CO_2浓度和辐照)以及技术本身的局限性等方面密切相关。     

Host-mediated inflammation disrupts the intestinal microbiota and promotes the overgrowth of Enterobacteriaceae

While the normal microbiota has been implicated as a critical defense against invading pathogens, the impact of enteropathogenic infection and host inflammation on intestinal microbial communities has not been elucidated. Using mouse models of Citrobacter rodentium, which closely mimics human diarrheal pathogens inducing host intestinal inflammation, and Campylobacter jejuni infection, as well as chemically and genetically induced models of intestinal inflammation, we demonstrate that host-mediated inflammation in response to an infecting agent, a chemical trigger, or genetic predisposition markedly alters the colonic microbial community. While eliminating a subset of indigenous microbiota, host-mediated inflammation supported the growth of either the resident or introduced aerobic bacteria, particularly of the Enterobacteriaceae family. Further, assault by an enteropathogen and host-mediated inflammation combined to significantly reduce the total numbers of resident colonic bacteria. These findings underscore the importance of intestinal microbial ecosystems in infectious colitis and noninfectious intestinal inflammatory conditions, such as inflammatory bowel disease.     

The Molecular Basis of Bacterial–Insect Symbiosis

Insects provide experimentally tractable and cost-effective model systems to investigate the molecular basis of animal–bacterial interactions. Recent research is revealing the central role of the insect innate immune system, especially anti-microbial peptides and reactive oxygen species, in regulating the abundance and composition of the microbiota in various insects, including Drosophila and the mosquitoes Aedes and Anopheles. Interactions between the immune system and microbiota are, however, bidirectional with evidence that members of the resident microbiota can promote immune function, conferring resistance to pathogens and parasites by both activation of immune effectors and production of toxins. Antagonistic and mutualistic interactions among bacteria have also been implicated as determinants of the microbiota composition, including exclusion of pathogens, but the molecular mechanisms are largely unknown. Some bacteria are crucial for insect nutrition, through provisioning of specific nutrients (e.g., B vitamins, essential amino acids) and modulation of the insect nutritional sensing and signaling pathways (e.g., insulin signaling) that regulate nutrient allocation, especially to lipid and other energy reserves. A key challenge for future research is to identify the molecular interaction between specific bacterial effectors and animal receptors, as well as to determine how these interactions translate into microbiota-dependent signaling, metabolism, and immune function in the host.     

Host-mediated inflammation disrupts the intestinal microbiota and promotes the overgrowth of Enterobacteriaceae

While the normal microbiota has been implicated as a critical defense against invading pathogens, the impact of enteropathogenic infection and host inflammation on intestinal microbial communities has not been elucidated. Using mouse models of Citrobacter rodentium, which closely mimics human diarrheal pathogens inducing host intestinal inflammation, and Campylobacter jejuni infection, as well as chemically and genetically induced models of intestinal inflammation, we demonstrate that host-mediated inflammation in response to an infecting agent, a chemical trigger, or genetic predisposition markedly alters the colonic microbial community. While eliminating a subset of indigenous microbiota, host-mediated inflammation supported the growth of either the resident or introduced aerobic bacteria, particularly of the Enterobacteriaceae family. Further, assault by an enteropathogen and host-mediated inflammation combined to significantly reduce the total numbers of resident colonic bacteria. These findings underscore the importance of intestinal microbial ecosystems in infectious colitis and noninfectious intestinal inflammatory conditions,such as inflammatory bowel disease.     

昆虫肠道防御及微生物稳态维持机制

在长期的进化过程中,昆虫形成了独特的肠道防御系统,主要由物理屏障和免疫系统共同作用来抵御外来微生物的入侵。如大部分后生动物一样,昆虫肠道上皮细胞无时无刻不与微生物接触,其种类从有益的共生菌、随食物进入的微生物到影响宿主生命的病原菌。在这样一种复杂的环境中,为了实现防御肠道病原微生物的同时又能维持共生微生物稳定的目的,宿主肠道上皮细胞必须在免疫应激和免疫耐受之间保持一种稳态平衡。Duox-ROS免疫系统和免疫缺陷(immune deficiency,Imd)信号通路作为肠道免疫反应的基本途径,必然参与调节此过程。本文从昆虫肠道防御组成、肠道免疫信号通路作用分子机制以及肠道免疫系统在肠道微生物群落稳态维持中的作用的最新研究进展进行综述。