Subterranean termite prophylactic secretions and external antifungal defenses

Novel strains of Actinobacteria associated with neotropical social wasps (Vespidae; Polistinae, Epiponini) with antimicrobial potential for natural product discovery

Antimicrobial resistance has been considered a public health threat. The World Health Organization has warned about the urgency of detecting new antibiotics from novel sources. Social insects could be crucial in the search for new antibiotic metabolites, as some of them survive in places that favor parasite development. Recent studies have shown the potential of social insects to produce antimicrobial metabolites (e.g. ants, bees, and termites). However, most groups of social wasps remain unstudied. Here, we explored whether Actinobacteria are associated with workers in the Neotropical Social Wasps (Epiponini) of Costa Rica and evaluated their putative inhibitory activity against other bacteria. Most isolated strains (67%) have antagonistic effects, mainly against Bacillus thuringensis and Escherichia coli ATCC 25992. Based on genome analysis, some inhibitory Actinobacteria showed biosynthetic gene clusters (BGCs) related to the production of antimicrobial molecules such as Selvamycin, Piericidin A1, and Nystatin. The Actinobacteria could be associated with social wasps to produce antimicrobial compounds. For these reasons, we speculate that Actinobacteria associated with social wasps could be a novel source of antimicrobial compounds, mainly against Gram-negative bacteria.

Infection with a non-lethal fungal parasite is associated with increased immune investment in the ant Myrmica scabrinodis

Social insects, such as ants, are preferred host organisms of pathogens and parasites because colonies are densely populated, and the number of potential hosts is high in the same place and time. Within a colony, individuals are exposed differentially to risks according to their function and age. Thus, older individuals forage and are therefore the most exposed to infection, predation, or physical stress, while young workers mostly stay inside the sheltered nest being less exposed. Immune investment is considered to be dependent on an individual's age and pathogen pressure. Long-term exposure to a parasite could affect the immune activity of individuals in an intriguing way that interferes with the age-dependent decline in immunocompetence. However, there are only few cases in which such interferences can be studied. The myrmecopathogenic fungus Rickia wasmannii, which infects entire colonies without killing the workers, is a suitable candidate for such studies. We investigated the general immunocompetence of Myrmica scabrinodis ant workers associated with non-lethal fungal infection by measuring the levels of active phenoloxidase (PO) and total PO (PPO) (reflecting the amount of both active and inactive forms of the enzyme) in two age classes. The level of PO proved to be higher in infected workers than in uninfected ones, while the level of PPO increased with age but was not affected by infection. Overall, these results indicate that a long-term infection could go hand in hand with increased immune activity of ant workers, conferring them higher level of protection.

Subterranean termites raise the alarm when their anti-fungal weapon falters

Termicin is an anti-fungal defensin that is disseminated from termite salivary glands. The peptide appears to be critical for the elimination with mutual grooming (allogrooming) of pathogenic spores (conidia) that have attached to the insect cuticle. There has been a recent selective sweep for an advantageous variant of this peptide in the subterranean termite Reticulitermes flavipes. We tested the anti-mycotic activity of a recombinant termicin corresponding with this variant against the conidia of different Metarhizium fungal isolates from soil close to foraging R. flavipes workers. Termicin was most effective against isolates that had previously been shown to elicit a relatively weak alarm response, as indicated by brief bouts of rapid longitudinal oscillatory movement (LOM). These isolates that elicited weak alarm were also the deadliest apparently because the survival of termites exposed to the fungus depends on a strong social immune response (LOMs and allogrooming). The selective pressure for a single termicin variant may have been driven by the most dangerous isolates that elicit a weak behavioral response. The correlation between termicin anti-fungal activity and LOM suggests that pathogen-associated molecular patterns that affect termite recognition of conidial contamination and the onset of elevated allogrooming also affect the vulnerability of conidia to the disruption of their cell membranes by termicin.

Overcoming Immune Deficiency with Allogrooming

Allogrooming appears to be essential in many social animals for protection from routine exposure to parasites. In social insects, it appears to be critical for the removal of pathogenic propagules from the cuticle before they can start an infectious cycle. For subterranean termites, this includes fungal spores commonly encountered in the soil, such as Metarhizium conidia, that can quickly germinate and penetrate the cuticle. We investigated whether there is a difference in reliance on social and innate immunity in two closely related subterranean termites for protection from fatal infections by two locally encountered Metarhizium species. Our results indicate that relatively weak innate immunity in one termite species is compensated by more sustained allogrooming. This includes enhanced allogrooming in response to concentrations of conidia that reflect more routine contamination of the cuticle as well as to heavy cuticular contamination that elicits a networked emergency response.

OfGNBP silencing enhances the toxicity of Serratia marcescens Bizio (SM1) to Odontotermes formosanus (Shiraki)

The immunity of insects plays a vital role in their survival. Our experiments found that lipopolysaccharide (LPS) and glucono-δ-lactone (GDL) could influence the virulence of Serratia marcescens Bizio (SM1) to Odontotermes formosanus (Shiraki) by affecting the immunity. Gram-negative binding proteins (GNBPs) are an important pattern recognition proteins that play a crucial role in the innate immune system. Therefore, two OfGNBPs were cloned in O. formosanus. The expression of OfGNBPs was significantly changed by LPS,SM1 and GDL, not prick. In addition, the immune-related gene expression, the phenoloxidase activity and antibacterial activity of donor termites and recipient termites were significantly induced by SM1. Furthermore, the knockdown of OfGNBP by RNA interference reduced not only individual immunity but also social immunity in O. formosanus, which increased the virulence of SM1 to O. formosanus. Importantly, dsOfGNBP alone also had good control effect on O. formosanus. In summary, we concluded that dsOfGNBPs are important termite immunosuppressants.

Rescue Strategy in a Termite: Workers Exposed to a Fungal Pathogen Are Reintegrated Into the Colony

Social insect colonies are characterized by an efficient division of labor, allowing high-value individuals (i.e., reproductives and brood) to be sheltered from tasks associated with increased risk of pathogen exposure, such as foraging or corpse disposal. This social organization helps limit the transmission of disease throughout the colony. Further, individuals can actively respond to imminent disease threats by altering their behaviors as a means of social immunity. In subterranean termites, although workers typically avoid detected pathogens, they can be attracted to pathogen cues when a nestmate is infected. Infected termites are usually groomed, but they may instead be cannibalized if the infection has already become lethal. The mechanisms governing these changes in behavior are unclear. We set out to examine immediate changes in individual behaviors, investigating the role that the infected individual plays in communicating its infection status to nestmates. We also assessed gradual changes in social organization after the re-introduction of an infected termite to the colony. Our results reveal that infected termites likely do not signal their infection status to nestmates through shaking behaviors and reduced movements, suggesting the occurrence of other mechanisms used in communicating infection. We also found that infected termites do not self-isolate and may travel to the densest part of the colony, where they can potentially benefit from grooming by large groups of nestmates. These results provide new insights into how individual changes in immune behaviors contribute to overall colony health, highlighting that, at early stages of infection, termites favor a rescuing strategy rather than isolation and/or cannibalization.

The chemical ecology of the fungus-farming termite symbiosis

Covering: September 1972 to December 2020Explorations of complex symbioses have often elucidated a plethora of previously undescribed chemical compounds that may serve ecological functions in signalling, communication or defence. A case in point is the subfamily of termites that cultivate a fungus as their primary food source and maintain complex bacterial communities, from which a series of novel compound discoveries have been made. Here, we summarise the origins and types of 375 compounds that have been discovered from the symbiosis over the past four decades and discuss the potential for synergistic actions between compounds within the complex chemical mixtures in which they exist. We go on to highlight how vastly underexplored the diversity and geographic distribution of the symbiosis is, which leaves ample potential for natural product discovery of compounds of both ecological and medical importance.

Reduced Environmental Microbial Diversity on the Cuticle and in the Galleries of a Subterranean Termite Compared to Surrounding Soil

Termites are intimately tied to the microbial world, as they utilize their gut microbiome for the conversion of plant cellulose into necessary nutrients. Subterranean termites must also protect themselves from the vast diversity of harmful microbes found in soil. However, not all soil microbes are harmful, such as Streptomyces and methanotrophic bacteria that some species of termites harbor in complex nest structures made of fecal material. The eastern subterranean termite, Reticulitermes flavipes, has a simple nest structure consisting of fecal lined galleries. We tested the hypothesis that R. flavipes maintains a select microbial community in its nests to limit the penetration of harmful soilborne pathogens and favor the growth of beneficial microbes. Using Illumina sequencing, we characterized the bacterial and fungal communities in the surrounding soil, in the nest galleries, and on the cuticle of workers. We found that the galleries provide a more beneficial microbial community than the surrounding soil. Bacterial and fungal diversity was highest in the soil, lower in the galleries, and least on the cuticle. Bacterial communities clustered together according to the substrate from which they were sampled, but this clustering was less clear in fungal communities. Most of the identified bacterial and fungal taxa were unique to one substrate, but the soil and gallery communities had very similar phylum-level taxonomic profiles. Notably, the galleries of R. flavipes also harbored both the potentially beneficial Streptomyces and the methanotrophic Methylacidiphilales, indicating that these microbial associations in fecal material pre-date the emergence of complex fecal nest structures. Surprisingly, several pathogenic groups were relatively abundant in the galleries and on the cuticle, suggesting that pathogens may accumulate within termite nests over time while putatively remaining at enzootic level during the lifetime of the colony.

Natural variation in colony inbreeding does not influence susceptibility to a fungal pathogen in a termite

Reduced genetic diversity through inbreeding can negatively affect pathogen resistance. This relationship becomes more complicated in social species, such as social insects, since the chance of disease transmission increases with the frequency of interactions among individuals. However, social insects may benefit from social immunity, whereby individual physiological defenses may be bolstered by collective-level immune responses, such as grooming or sharing of antimicrobial substance through trophallaxis. We set out to determine whether differences in genetic diversity between colonies of the subterranean termite, Reticulitermes flavipes, accounts for colony survival against pathogens. We sampled colonies throughout the United States (Texas, North Carolina, Maryland, and Massachusetts) and determined the level of inbreeding of each colony. To assess whether genetically diverse colonies were better able to survive exposure to diverse pathogens, we challenged groups of termite workers with two strains of a pathogenic fungus, one local strain present in the soil surrounding sampled colonies and another naïve strain, collected outside the range of this species. We found natural variation in the level of inbreeding between colonies, but this variation did not explain differences in susceptibility to either pathogen. Although the naïve strain was found to be more hazardous than the local strain, colony resistance was correlated between two strains, meaning that colonies had either relatively high or low susceptibility to both strains regardless of their inbreeding coefficient. Overall, our findings may reflect differential virulence between the strains, immune priming of the colonies via prior exposure to the local strain, or a coevolved resistance toward this strain. They also suggest that colony survival may rely more upon additional factors, such as different behavioral response thresholds or the influence of a specific genetic background, rather than the overall genetic diversity of the colony.

Insect architecture: structural diversity and behavioral principles

Insects build myriad structures out of diverse materials. These structures serve purposes that range from facilitating prey capture to housing their entire colony. Whereas some insects operate solitarily, others collectively build large and complex structures. The architecture of these structures vis-à-vis their ultimate function poses fascinating questions that require multidisciplinary investigations. Importantly, because the underlying design and architectural principles have evolved over millions of years to withstand multitude of environmental perturbations, they offer important lessons for human-made structures. The vast variety of insect-built structures reflects how diverse insects have adapted to resolve the ecological challenges in their specific habitats. Here, we survey insect-built structures from a comparative perspective to ask what design and architectural principles can be gleaned from them. We discuss the key hypotheses about building-behaviours, their neural underpinnings, the functional role of insect-built structures and their biomimetic potential.

Young but not defenceless: antifungal activity during embryonic development of a social insect

Termites live in environments heavily colonized by diverse microorganisms, including pathogens. Eggs laid within the nest are likely to experience similar pathogenic pressures as those experienced by older nest-mates. Consequently, eggs may be under selective pressures to be immune-competent. Through in vitro experiments using developing embryos of the dampwood termite, Zootermopsis angusticollis, we tested the ontogeny, location and strength of their antifungal activity against the fungus, Metarhizium brunneum. Exterior washes of the chorion (extra-chorionic) and components within the chorion (intra-chorionic) were incubated with fungal conidia, which were then scored for viability. The fungistatic activity was location and developmental stage dependent. Extra-chorionic washes had relatively weak antifungal activity. Intra-chorionic homogenates were highly antifungal, exhibiting increased potency through development. The positive correlation between intra-chorionic fungistasis and developmental stage is probably due to the expression of endogenous proteins during embryogenesis. Boiling of both the extra-chorionic washes and the intra-chorionic contents rescued conidia viability, indicating the antifungal agent(s) is (are) heat-sensitive and probably proteinaceous. This study is the first to address embryonic antifungal activity in a hemimetabolous, eusocial taxon. Our results support the hypothesis that microbes have been significant agents of selection in termites, fostering the evolution of antifungal properties even in the most immature stage of development.

A Novel Digestive GH16 β-1,3(4)-Glucanase from the Fungus-Growing Termite Macrotermes barneyi

β-1,3-glucanases are the main digestive enzymes of plant and fungal cell wall. Transcriptomic analysis of the fungus-growing termite Macrotermes barneyi revealed a high expression of a predicted β-1,3(4)-glucanase (Mbbgl) transcript in termite gut. Here, we described the cDNA cloning, heterologous expression, and enzyme characterization of Mbbgl. Sequence analysis and RT-PCR results showed that Mbbgl is a termite-origin GH16 β-1,3(4)-glucanase. The recombinant enzyme showed the highest activity towards laminarin and was active optimally at 50 °C, pH 5.5. The enzyme displayed endo/exo β-1,3(4)-glucanase activities. Moreover, Mbbgl had weak transglycosylation activity. The results indicate that Mbbgl is an endogenous digestive β-1,3(4)-glucanase, which contributes to the decomposition of plant biomass and fungal hyphae. Additionally, the multiple activities, pH, and ion stabilities make Mbbgl a potential candidate for application in the food industry.

Trichoderma Species Attract Coptotermes formosanus and Antagonize Termite Pathogen Metarhizium anisopliae

Although subterranean termites live within soil, little attention has been paid on the potential interaction among subterranean termites and soil microbes. Herein, we conducted different choice tests to investigate aggregation and tunneling behaviors of Coptotermes formosanus Shiraki responding to soil/sand treated with conidia of seven soil fungi, Trichoderma longibrachiatum Rifai, Trichoderma koningii Oud., Trichoderma hamatum (Bon.) Bain., Trichoderma atroviride Karsten, Trichoderma spirale Indira and Kamala, Trichoderma harzianum Rifai, and Trichoderma viride Pers. ex Fries. In aggregation-choice test, soil treated with nearly all Trichoderma species tested (except T. koningii) significantly increased termite aggregation compared with untreated soil. In tunneling-choice tests, termites produced significantly larger tunnels in sand treated with T. longibrachiatum or T. koningii than that in untreated sand. We hypothesized that Trichoderma species could benefit termites by protecting them from infection of the entomopathogenic fungus Metarhizium anisopliae (Metschn) Sorokin, and three Trichoderma species that attracted termites (T. longibrachiatum, T. atroviride, and T. harzianum) were tested. The antagonism tests showed that the three Trichoderma species suppressed growth of M. anisopliae. Also, the median lethal time (LT₅₀) of termites exposed to both Trichoderma species and M. anisopliae was significantly longer than termites exposed to M. anisopliae alone. Interestingly, though significantly fewer termites aggregated in soil treated with M. anisopliae conidia compared with untreated soil, M. anisopliae conidia mixed with T. longibrachiatum or T. harzianum were no longer repellent to termites. Our results showed that the fungi in the genus Trichoderma (1) exerted generally attractive effects on termites, (2) protected termites from the infection of entomopathogenic fungus, and (3) altered pathogen-avoiding behaviors of termites. Future studies will be required to understand the mechanisms underlying these newly discovered effects.

The Effects of Trichoderma Fungi on the Tunneling, Aggregation, and Colony-Initiation Preferences of Black-Winged Subterranean Termites, Odontotermes formosanus (Blattodea: Termitidae)

The black-winged subterranean termite, Odontotermes formosanus Shiraki, is a severe pest of plantations and forests in China. This termite cultures symbiotic Termitomyces in the fungal combs, which are challenged by antagonistic microbes such as Trichoderma fungi. In a previous study we showed that O. formosanus workers made significantly fewer tunnels in sand containing commercially formulated conidia of Trichoderma viride Pers. ex Fries compared with untreated sand. Herein, we hypothesize that fungi in the genus Trichoderma exert repellent effects on O. formosanus. Different choice tests were conducted to evaluate the tunneling and aggregation behaviors of O. formosanus workers reacting to sand/soil containing the unformulated conidia of seven Trichoderma fungi (Trichoderma longibrachiatum Rifai, Trichoderma koningii Oud., Trichoderma harzianum Rifai, Trichoderma hamatum (Bon.) Bain, Trichoderma atroviride Karsten, Trichoderma spirale Indira and Kamala, and T. viride). We also investigated the colony-initiation preference of paired O. formosanus adults to soil treated with Trichoderma conidia (T. koningii or T. longibrachiatum) versus untreated soil. Tunneling-choice tests showed that sand containing conidia of nearly all Trichoderma fungi tested (except T. harzianum) significantly decreased tunneling activity in O. formosanus workers compared with untreated sand. Aggregation-choice test showed that T. koningii, T. atroviride and T. spirale repelled O. formosanus workers, whereas T. longibrachiatum and T. hamatum attracted termites. There was no significant difference in proportions of paired adults that stayed and laid eggs in the soil blocks treated with conidia of Trichoderma fungi and untreated ones. Our study showed that Trichoderma fungi generally repelled tunneling in O. formosanus, but may exert varied effects on aggregation preference by workers.

Worker Defensive Behavior Associated with Toxins in the Neotropical Termite Neocapritermes braziliensis (Blattaria, Isoptera, Termitidae, Termitinae)

Termite societies are abundant in the tropics, and are therefore exposed to multiple enemies and predators, especially during foraging activity. Soldiers constitute a specialized defensive caste, although workers also participate in this process, and even display suicidal behavior, which is the case with the species Neocapritermes braziliensis. Here we describe the morphology, mechanisms of action, and proteomics of the salivary weapon in workers of this species, which due to the autothysis of the salivary glands causes their body rupture, in turn releasing a defensive secretion, observed during aggressiveness bioassays. Salivary glands are paired, composed of two translucent reservoirs, ducts and a set of multicellular acini. Histological and ultrastructural techniques showed that acini are composed of two types of central cells, and small parietal cells located in the acinar periphery. Type I central cells were abundant and filled with a large amount of secretion, while type II central cells were scarce and presented smaller secretion. Parietal cells were often paired and devoid of secretion. The gel-free proteomic approach (shotgun) followed by mass spectrometry revealed 235 proteins in the defensive secretion, which were classified into functional groups: (i) toxins and defensins, (ii) folding/conformation and post-translational modifications, (iii) salivary gland detoxification, (iv) housekeeping proteins and (v) uncharacterized and hypothetical proteins. We highlight the occurrence of neurotoxins previously identified in arachnid venoms, which are novelties for termite biology, and contribute to the knowledge regarding the defense strategies developed by termite species from the Neotropical region.

Subterranean Termite Social Alarm and Hygienic Responses to Fungal Pathogens

In social insects, alerting nestmates to the presence of a pathogen should be critical for limiting its spread and initiating social mechanisms of defense. Here we show that subterranean termites use elevated vibratory alarm behavior to help prevent fatal fungal infections. The elevated alarm leads to elevated social hygiene. This requires that termites coalesce so that they can groom each other’s cuticular surfaces of contaminating conidial spores. Groups of 12 Reticulitermes flavipes workers varied in their response when immersed in conidia solutions of nine different strains of Metarhizium. Pathogen alarm displays of short 2–7-second bursts of rapid longitudinal oscillatory movement (LOM), observed over 12 min following a fungal challenge, were positively correlated with the time that workers spent aggregated together grooming each other. The frequency of these LOMs was inversely correlated with fatal fungal infections. The variation in fatalities appeared to be largely attributable to a differential response to Metarhizium brunneum and Metarhizium robertsii in the time spent in aggregations and the frequency of allogrooming. Isolated workers challenged with conidia did not display LOMs, which suggests that the alarm is a conditional social response. LOMs appear to help signal the presence of fungal pathogens whose virulence depends on the level of this emergency alert.

Chemistry of the Secondary Metabolites of Termites

Isolation, structure determination, synthesis, and biochemistry of the low-molecular-weight compounds of the secretion of exocrine glands of termites are described, with an emphasis on pheromones and defensive compounds.

A Lab-Based Study of Temperate Forest Termite Impacts on Two Common Wood-Rot Fungi

Termites and fungi are the primary decomposers of dead wood. Interactions between wood-feeding termites and wood-rot fungi are inevitable given their shared food source. Termites have developed multiple defense strategies against infectious fungi, such as Metarhizium spp., that include antifungal proteins in their saliva and fungal inhibition properties in their gut. The antifungal properties of termite salivary secretions depend on β-1,3-glucanases that are likely to be effective against a broad spectrum of filamentous fungi. Given the overlap in niches, there is opportunity for interference competition between termites and wood-rot fungi to occur. Here we demonstrate that β-1,3-glucanases in the saliva and the antifungal properties of the gut of the eastern subterranean termite Reticulitermes flavipes (Kollar) (Blattodea: Rhinotermitidae) affects the growth of two common wood-rot fungi, Gloeophyllum trabeum Persoon (Murrill) (Gloeophyllales: Gloeophyllaceae) and Phanerochaete chrysosporium (Burdsall) (Polyporales: Phanerochaetaceae).

Highly potent host external immunity acts as a strong selective force enhancing rapid parasite virulence evolution

Virulence is often under selection during host-parasite coevolution. In order to increase fitness, parasites are predicted to circumvent and overcome host immunity. A particular challenge for pathogens are external immune systems, chemical defence systems comprised of potent antimicrobial compounds released by prospective hosts into the environment. We carried out an evolution experiment, allowing for coevolution to occur, with the entomopathogenic fungus, Beauveria bassiana, and the red flour beetle, Tribolium castaneum, which has a well-documented external immune system with strong inhibitory effects against B. bassiana. After just seven transfers of experimental evolution we saw a significant increase in parasite induced host mortality, a proxy for virulence, in all B. bassiana lines. This apparent virulence increase was mainly the result of the B. bassiana lines evolving resistance to the beetles' external immune defences, not due to increased production of toxins or other harmful substances. Transcriptomic analyses of evolved B. bassiana implicated the up-regulation of oxidative stress resistance genes in the observed resistance to external immunity. It was concluded that external immunity acts as a powerful selective force for virulence evolution, with an increase in virulence being achieved apparently entirely by overcoming these defences, most likely due to elevated oxidative stress resistance.

Isolation and assessment of gut bacteria from the Formosan subterranean termite, Coptotermes formosanus (Isoptera: Rhinotermitidae), for paratransgenesis research and application

Paratransgenesis targeting the gut protozoa is being developed as an alternative method for the control of the Formosan subterranean termite (FST). This method involves killing the cellulose-digesting gut protozoa using a previously developed antiprotozoal peptide consisting of a target specific ligand coupled to an antimicrobial peptide (Hecate). In the future, we intend to genetically engineer termite gut bacteria as "Trojan Horses" to express and spread ligand-Hecate in the termite colony. The aim of this study was to assess the usefulness of bacteria strains isolated from the gut of FST as "Trojan Horses." We isolated 135 bacteria from the guts of workers from 3 termite colonies. Sequencing of the 16S rRNA gene identified 20 species. We tested 5 bacteria species that were previously described as part of the termite gut community for their tolerance against Hecate and ligand-Hecate. Results showed that the minimum concentration required to inhibit bacteria growth was always higher than the concentration required to kill the gut protozoa. Out of the 5 bacteria tested, we engineered Trabulsiella odontotermitis, a termite specific bacterium, to express green fluorescent protein as a proof of concept that the bacteria can be engineered to express foreign proteins. Engineered T. odontotermitis was fed to FST to study if the bacteria are ingested. This feeding experiment confirmed that engineered T. odontotermitis is ingested by termites and can survive in the gut for at least 48 h. Here we report that T. odontotermitis is a suitable delivery and expression system for paratransgenesis in a termite species.

Antimicrobial Activity of Actinobacteria Isolated From the Guts of Subterranean Termites

Subterranean termites need to minimize potentially pathogenic and competitive fungi in their environment in order to maintain colony health. We examined the ability of Actinobacteria isolated from termite guts in suppressing microorganisms commonly encountered in a subterranean environment. Guts from two subterranean termite species, Reticulitermes flavipes (Kollar) and Reticulitermes tibialis Banks, were extracted and plated on selective chitin media. A total of 38 Actinobacteria isolates were selected for in vitro growth inhibition assays. Target microbes included three strains of Serratia marcescens Bizio, two mold fungi (Trichoderma sp. and Metarhizium sp.), a yeast fungus (Candida albicans (C.P. Robin) Berkhout), and four basidiomycete fungi (Gloeophyllum trabeum (Persoon) Murrill, Tyromyces palustris (Berkeley & M.A. Curtis) Murrill, Irpex lacteus (Fries) Fries, and Trametes versicolor (L.) Lloyd). Results showed both broad and narrow ranges of antimicrobial activity against the mold fungi, yeast fungus, and S. marcescens isolates by the Actinobacteria selected. This suggests that termite gut-associated Actinobacteria produce secondary antimicrobial compounds that may be important for pathogen inhibition in termites. Basidiomycete fungi were strongly inhibited by the selected Actinobacteria isolates, with G. trabeum and T. versicolor being most inhibited, followed by I. lacteus and T. palustris The degree of inhibition was correlated with shifts in pH caused by the Actinobacteria. Nearly all Actinobacteria isolates raised pH of the growth medium to basic levels (i.e. pH ∼8.0-9.5). We summarize antimicrobial activity of these termite gut-associated Actinobacteria and examine the implications of these pH shifts.

Salecan Enhances the Activities of β-1,3-Glucanase and Decreases the Biomass of Soil-Borne Fungi

Salecan, a linear extracellular polysaccharide consisting of β-1,3-D-glucan, has potential applications in the food, pharmaceutical and cosmetic industries. The objective of this study was to evaluate the effects of salecan on soil microbial communities in a vegetable patch. Compositional shifts in the genetic structure of indigenous soil bacterial and fungal communities were monitored using culture-dependent dilution plating, culture-independent PCR-denaturing gradient gel electrophoresis (DGGE) and quantitative PCR. After 60 days, soil microorganism counts showed no significant variation in bacterial density and a marked decrease in the numbers of fungi. The DGGE profiles revealed that salecan changed the composition of the microbial community in soil by increasing the amount of Bacillus strains and decreasing the amount of Fusarium strains. Quantitative PCR confirmed that the populations of the soil-borne fungi Fusarium oxysporum and Trichoderma spp. were decreased approximately 6- and 2-fold, respectively, in soil containing salecan. This decrease in the amount of fungi can be explained by salecan inducing an increase in the activities of β-1,3-glucanase in the soil. These results suggest the promising application of salecan for biological control of pathogens of soil-borne fungi.

Termites as targets and models for biotechnology

Termites have many unique evolutionary adaptations associated with their eusocial lifestyles. Recent omics research has created a wealth of new information in numerous areas of termite biology (e.g., caste polyphenism, lignocellulose digestion, and microbial symbiosis) with wide-ranging applications in diverse biotechnological niches. Termite biotechnology falls into two categories: (a) termite-targeted biotechnology for pest management purposes, and (b) termite-modeled biotechnology for use in various industrial applications. The first category includes several candidate termiticidal modes of action such as RNA interference, digestive inhibition, pathogen enhancement, antimicrobials, endocrine disruption, and primer pheromone mimicry. In the second category, termite digestomes are deep resources for host and symbiont lignocellulases and other enzymes with applications in a variety of biomass, industrial, and processing applications. Moving forward, one of the most important approaches for accelerating advances in both termite-targeted and termite-modeled biotechnology will be to consider host and symbiont together as a single functional unit.

Wood decomposition as influenced by invertebrates

The diversity and habitat requirements of invertebrates associated with dead wood have been the subjects of hundreds of studies in recent years but we still know very little about the ecological or economic importance of these organisms. The purpose of this review is to examine whether, how and to what extent invertebrates affect wood decomposition in terrestrial ecosystems. Three broad conclusions can be reached from the available literature. First, wood decomposition is largely driven by microbial activity but invertebrates also play a significant role in both temperate and tropical environments. Primary mechanisms include enzymatic digestion (involving both endogenous enzymes and those produced by endo- and ectosymbionts), substrate alteration (tunnelling and fragmentation), biotic interactions and nitrogen fertilization (i.e. promoting nitrogen fixation by endosymbiotic and free-living bacteria). Second, the effects of individual invertebrate taxa or functional groups can be accelerative or inhibitory but the cumulative effect of the entire community is generally to accelerate wood decomposition, at least during the early stages of the process (most studies are limited to the first 2-3 years). Although methodological differences and design limitations preclude meta-analysis, studies aimed at quantifying the contributions of invertebrates to wood decomposition commonly attribute 10-20% of wood loss to these organisms. Finally, some taxa appear to be particularly influential with respect to promoting wood decomposition. These include large wood-boring beetles (Coleoptera) and termites (Termitoidae), especially fungus-farming macrotermitines. The presence or absence of these species may be more consequential than species richness and the influence of invertebrates is likely to vary biogeographically.

Symbiont-derived β-1,3-glucanases in a social insect: mutualism beyond nutrition

Termites have had a long co-evolutionary history with prokaryotic and eukaryotic gut microbes. Historically, the role of these anaerobic obligate symbionts has been attributed to the nutritional welfare of the host. We provide evidence that protozoa (and/or their associated bacteria) colonizing the hindgut of the dampwood termite Zootermopsis angusticollis, synthesize multiple functional β-1,3-glucanases, enzymes known for breaking down β-1,3-glucans, the main component of fungal cell walls. These enzymes, we propose, may help in both digestion of ingested fungal hyphae and protection against invasion by fungal pathogens. This research points to an additional novel role for the mutualistic hindgut microbial consortia of termites, an association that may extend beyond lignocellulolytic activity and nitrogen fixation to include a reduction in the risks of mycosis at both the individual- and colony-levels while nesting in and feeding on microbial-rich decayed wood.

Effects of caste on the expression of genes associated with septic injury and xenobiotic exposure in the Formosan subterranean termite

As social insects, termites live in densely populated colonies with specialized castes under conditions conducive to microbial growth and transmission. Furthermore, termites are exposed to xenobiotics in soil and their lignocellulose diet. Therefore, termites are valuable models for studying gene expression involved in response to septic injury, immunity and detoxification in relation to caste membership. In this study, workers and soldiers of the Formosan subterranean termite, Coptotermes formosanus, were challenged by bacterial injection or by no-choice feeding with a sublethal concentration (0.5%) of phenobarbital. Constitutive and induced expression of six putative immune response genes (two encoding for lectin-like proteins, one for a ficolin-precursor, one for the Down syndrome cell adhesion molecule, one for a chitin binding protein, and one for the gram-negative binding protein 2) and four putative detoxification genes (two encoding for cytochrome P450s, one for glutathione S-transferase, and one for the multi antimicrobial extrusion protein), were measured via quantitative real time polymerase chain reaction and compared within and among 1) colonies, 2) treatment types and 3) castes via ANOVA. Eight genes were inducible by septic injury, feeding with phenobarbital or both. Colony origin had no effect on inducibility or differential gene expression. However, treatment type showed significant effects on the expression of the eight inducible genes. Caste effects on expression levels were significant in five of the eight inducible genes with constitutive and induced expression of most target genes being higher in workers than in soldiers.

Multifaceted defense against antagonistic microbes in developing offspring of the parasitoid wasp Ampulex compressa (Hymenoptera, Ampulicidae)

Effective antimicrobial strategies are essential adaptations of insects to protect themselves, their offspring, and their foods from microbial pathogens and decomposers. Larvae of the emerald cockroach wasp, Ampulex compressa, sanitize their cockroach hosts, Periplaneta americana, with a cocktail of nine antimicrobials comprising mainly (R)-(-)-mellein and micromolide. The blend of these antimicrobials has broad-spectrum antimicrobial activity. Here we explore the spatio-temporal pattern of deployment of antimicrobials during the development from egg to adult as well as their physico-chemical properties to assess how these aspects may contribute to the success of the antimicrobial strategy. Using gas chromatography/mass spectrometry (GC/MS) we show that larvae start sanitizing their food as soon as they have entered their host to feed on its tissue. Subsequently, they impregnate the cockroach carcass with antimicrobials to create a hygienic substrate for cocoon spinning inside the host. Finally, the antimicrobials are incorporated into the cocoon. The antimicrobial profiles on cockroach and wasp cocoon differed markedly. While micromolide persisted on the cockroaches until emergence of the wasps, solid-phase microextraction sampling and GC/MS analysis revealed that (R)-(-)-mellein vaporized from the cockroaches and accumulated in the enclosed nest. In microbial challenge assays (R)-(-)-mellein in the headspace of parasitized cockroaches inhibited growth of entomopathogenic and opportunistic microbes (Serratia marcescens, Aspergillus sydowii, Metarhizium brunneum). We conclude that, in addition to food sanitation, A. compressa larvae enclose themselves in two defensive walls by impregnating the cocoon and the cockroach cuticle with antimicrobials. On top of that, they use vaporous (R)-(-)-mellein to sanitize the nest by fumigation. This multifaceted antimicrobial defense strategy involving the spatially and temporally coordinated deployment of several antimicrobials in solution and vapor form has apparently evolved to reliably protect the larvae themselves and their food against a broad range of antagonistic microbes.

Eco-taxonomic insights into actinomycete symbionts of termites for discovery of novel bioactive compounds

Termites play a major role in foraging and degradation of plant biomass as well as cultivating bioactive microorganisms for their defense. Current advances in "omics" sciences are revealing insights into function-related presence of these symbionts, and their related biosynthetic activities and genes identified in gut symbiotic bacteria might offer a significant potential for biotechnology and biodiscovery. Actinomycetes have been the major producers of bioactive compounds with an extraordinary range of biological activities. These metabolites have been in use as anticancer agents, immune suppressants, and most notably, as antibiotics. Insect-associated actinomycetes have also been reported to produce a range of antibiotics such as dentigerumycin and mycangimycin. Advances in genomics targeting a single species of the unculturable microbial members are currently aiding an improved understanding of the symbiotic interrelationships among the gut microorganisms as well as revealing the taxonomical identity and functions of the complex multilayered symbiotic actinofloral layers. If combined with target-directed approaches, these molecular advances can provide guidance towards the design of highly selective culturing methods to generate further information related to the physiology and growth requirements of these bioactive actinomycetes associated with the termite guts. This chapter provides an overview on the termite gut symbiotic actinoflora in the light of current advances in the "omics" science, with examples of their detection and selective isolation from the guts of the Sunshine Coast regional termite Coptotermes lacteus in Queensland, Australia.

Nest sanitation through defecation: antifungal properties of wood cockroach feces

The wood cockroach Cryptocercus punctulatus nests as family units inside decayed wood, a substrate known for its high microbial load. We tested the hypothesis that defecation within their nests, a common occurrence in this species, reduces the probability of fungal development. Conidia of the entomopathogenic fungus, Metarhizium anisopliae, were incubated with crushed feces and subsequently plated on potato dextrose agar. Relative to controls, the viability of fungal conidia was significantly reduced following incubation with feces and was negatively correlated with incubation time. Although the cockroach's hindgut contained abundant β-1,3-glucanase activity, its feces had no detectable enzymatic function. Hence, these enzymes are unlikely the source of the fungistasis. Instead, the antifungal compound(s) of the feces involved heat-sensitive factor(s) of potential microbial origin. When feces were boiled or when they were subjected to ultraviolet radiation and subsequently incubated with conidia, viability was "rescued" and germination rates were similar to those of controls. Filtration experiments indicate that the fungistatic activity of feces results from chemical interference. Because Cryptocercidae cockroaches have been considered appropriate models to make inferences about the factors fostering the evolution of termite sociality, we suggest that nesting in microbe-rich environments likely selected for the coupling of intranest defecation and feces fungistasis in the common ancestor of wood cockroaches and termites. This might in turn have served as a preadaptation that prevented mycosis as these phylogenetically related taxa diverged and evolved respectively into subsocial and eusocial organizations.

Extended disease resistance emerging from the faecal nest of a subterranean termite

Social insects nesting in soil environments are in constant contact with entomopathogens but have evolved a range of defence mechanisms, resulting in both individual and social immunity that reduce the chance for epizootics in the colony, as in the case of subterranean termites. Coptotermes formosanus uses its faeces as building material for its nest structure that result into a ‘carton material’, and here, we report that the faecal nest supports the growth of Actinobacteria which provide another level of protection to the social group against entomopathogens. A Streptomyces species with in vivo antimicrobial activity against fungal entomopathogens was isolated from the nest material of multiple termite colonies. Termite groups were exposed to Metarhizium anisopliae, a fungal entomopathogen, during their foraging activity and the presence of Streptomyces within the nest structure provided a significant survival benefit to the termites. Therefore, this report describes a non-nutritional exosymbiosis in a termite, in the form of a defensive mutualism which has emerged from the use of faecal material in the nesting structure of Coptotermes. The association with an Actinobacteria community in the termite faecal material provides an extended disease resistance to the termite group as another level of defence, in addition to their individual and social immunity.

Immune-related transcriptome of Coptotermes formosanus Shiraki workers: the defense mechanism

Formosan subterranean termites, Coptotermes formosanus Shiraki, live socially in microbial-rich habitats. To understand the molecular mechanism by which termites combat pathogenic microbes, a full-length normalized cDNA library and four Suppression Subtractive Hybridization (SSH) libraries were constructed from termite workers infected with entomopathogenic fungi (Metarhizium anisopliae and Beauveria bassiana), Gram-positive Bacillus thuringiensis and Gram-negative Escherichia coli, and the libraries were analyzed. From the high quality normalized cDNA library, 439 immune-related sequences were identified. These sequences were categorized as pattern recognition receptors (47 sequences), signal modulators (52 sequences), signal transducers (137 sequences), effectors (39 sequences) and others (164 sequences). From the SSH libraries, 27, 17, 22 and 15 immune-related genes were identified from each SSH library treated with M. anisopliae, B. bassiana, B. thuringiensis and E. coli, respectively. When the normalized cDNA library was compared with the SSH libraries, 37 immune-related clusters were found in common; 56 clusters were identified in the SSH libraries, and 259 were identified in the normalized cDNA library. The immune-related gene expression pattern was further investigated using quantitative real time PCR (qPCR). Important immune-related genes were characterized, and their potential functions were discussed based on the integrated analysis of the results. We suggest that normalized cDNA and SSH libraries enable us to discover functional genes transcriptome. The results remarkably expand our knowledge about immune-inducible genes in C. formosanus Shiraki and enable the future development of novel control strategies for the management of Formosan subterranean termites.

Action on the Surface: Entomopathogenic Fungi versus the Insect Cuticle

Infections mediated by broad host range entomopathogenic fungi represent seminal observations that led to one of the first germ theories of disease and are a classic example of a co-evolutionary arms race between a pathogen and target hosts. These fungi are able to parasitize susceptible hosts via direct penetration of the cuticle with the initial and potentially determining interaction occurring between the fungal spore and the insect epicuticle. Entomogenous fungi have evolved mechanisms for adhesion and recognition of host surface cues that help direct an adaptive response that includes the production of: (a) hydrolytic, assimilatory, and/or detoxifying enzymes including lipase/esterases, catalases, cytochrome P450s, proteases, and chitinases; (b) specialized infectious structures, e.g., appressoria or penetrant tubes; and (c) secondary and other metabolites that facilitate infection. Aside from immune responses, insects have evolved a number of mechanisms to keep pathogens at bay that include: (a) the production of (epi) cuticular antimicrobial lipids, proteins, and metabolites; (b) shedding of the cuticle during development; and (c) behavioral-environmental adaptations such as induced fever, burrowing, and grooming, as well as potentially enlisting the help of other microbes, all intended to stop the pathogen before it can breach the cuticle. Virulence and host-defense can be considered to be under constant reciprocal selective pressure, and the action on the surface likely contributes to phenomena such as strain variation, host range, and the increased virulence often noted once a (low) virulent strain is "passaged" through an insect host. Since the cuticle represents the first point of contact and barrier between the fungus and the insect, the "action on the surface" may represent the defining interactions that ultimately can lead either to successful mycosis by the pathogen or successful defense by the host. Knowledge concerning the molecular mechanisms underlying this interaction can shed light on the ecology and evolution of virulence and can be used for rational design strategies at increasing the effectiveness of entomopathogenic fungi for pest control in field applications.

Comparative metatranscriptomic signatures of wood and paper feeding in the gut of the termite Reticulitermes flavipes (Isoptera: Rhinotermitidae)

Termites are highly eusocial insects that thrive on recalcitrant materials like wood and soil and thus play important roles in global carbon recycling and also in damaging wooden structures. Termites, such as Reticulitermes flavipes (Rhinotermitidae), owe their success to their ability to extract nutrients from lignocellulose (a major component of wood) with the help of gut-dwelling symbionts. With the aim to gain new insights into this enzymatic process we provided R. flavipes with a complex lignocellulose (wood) or pure cellulose (paper) diet and followed the resulting differential gene expression on a custom oligonucleotide-microarray platform. We identified a set of expressed sequence tags (ESTs) with differential abundance between the two diet treatments and demonstrated the source (host/symbiont) of these genes, providing novel information on termite nutritional symbiosis. Our results reveal: (1) the majority of responsive wood- and paper-abundant ESTs are from host and symbionts, respectively; (2) distinct pathways are associated with lignocellulose and cellulose feeding in both host and symbionts; and (3) sets of diet-responsive ESTs encode putative digestive and wood-related detoxification enzymes. Thus, this study illuminates the dynamics of termite nutritional symbiosis and reveals a pool of genes as potential targets for termite control and functional studies of termite-symbiont interactions.

Antifungal proteins: More than antimicrobials?

Antimicrobial proteins (AMPs) are widely distributed in nature. In higher eukaryotes, AMPs provide the host with an important defence mechanism against invading pathogens. AMPs of lower eukaryotes and prokaryotes may support successful competition for nutrients with other microorganisms of the same ecological niche. AMPs show a vast variety in structure, function, antimicrobial spectrum and mechanism of action. Most interestingly, there is growing evidence that AMPs also fulfil important biological functions other than antimicrobial activity. The present review focuses on the mechanistic function of small, cationic, cysteine-rich AMPs of mammals, insects, plants and fungi with antifungal activity and specifically aims at summarizing current knowledge concerning additional biological properties which opens novel aspects for their future use in medicine, agriculture and biotechnology.

Lignin-associated metagene expression in a lignocellulose-digesting termite

Lignin is a component of plant biomass that presents a significant obstacle to biofuel production. It is composed of a highly stable phenylpropanoid matrix that upon degradation, releases toxic metabolites. Termites have specialized digestive systems that overcome the lignin barrier in wood lignocellulose to efficiently release fermentable simple sugars; however, how termites specifically degrade lignin and tolerate its toxic byproducts remains unknown. Here, using the termite Reticulitermes flavipes and its symbiotic (protozoan) gut fauna as a model system, we used high throughput Roche 454-titanium pyrosequencing and proteomics approaches to (i) experimentally compare the effects of diets containing varying degrees of lignin complexity on host-symbiont digestome composition, (ii) deeply sample host and symbiont lignocellulase diversity, and (iii) identify promising lignocellulase candidates for functional characterization. In addition to revealing over 9500 differentially expressed transcripts related to a wide range of physiological processes, our findings reveal two detoxification enzyme families not generally considered in connection with lignocellulose digestion: aldo-keto reductases and catalases. Recombinant versions of two host enzymes from these enzyme families, which apparently play no roles in cellulose or hemicellulose digestion, significantly enhance lignocellulose saccharification by cocktails of host and symbiont cellulases. These hypothesis-driven results provide important new insights into (i) dietary lignin as a xenobiotic challenge, (ii) the complex mechanisms used by termites to cope with their lignin-rich diets, and (iii) novel lignin-targeted enzymatic approaches to enhance biofuel and biomaterial production.

Differential undertaking response of a lower termite to congeneric and conspecific corpses

Undertaking behaviour is an essential activity in social insects. Corpses are often recognized by a postmortem change in a chemical signature. Reticulitermes flavipes responded to corpses within minutes of death. This undertaking behaviour did not change with longer postmortem time (24 h); however, R. flavipes exhibited distinctively different behaviours toward dead termites from various origins. Corpses of the congeneric species, Reticulitermes virginicus, were buried onsite by workers with a large group of soldiers guarding the burial site due to the risk of interspecific competition; while dead conspecifics, regardless of colony origin, were pulled back into the holding chamber for nutrient recycling and hygienic purposes. The burial task associated with congeneric corpses was coupled with colony defence and involved ten times more termites than retrieval of conspecific corpses. Our findings suggest elicitation of undertaking behaviour depends on the origin of corpses which is associated with different types of risk.

Resource competition between two fungal parasites in subterranean termites

Subterranean termites live in large groups in underground nests where the pathogenic pressure of the soil environment has led to the evolution of a complex interaction among individual and social immune mechanisms in the colonies. However, groups of termites under stress can show increased susceptibility to opportunistic parasites. In this study, an isolate of Aspergillus nomius Kurtzman, Horn & Hessltine was obtained from a collapsed termite laboratory colony. We determined that it was primarily a saprophyte and, secondarily, a facultative parasite if the termite immunity is undergoing a form of stress. This was determined by stressing individuals of the Formosan subterranean termite Coptotermes formosanus Shiraki via a co-exposure to the virulent fungal parasite Metarhizium anisopliae (Metch.) Sorokin. We also examined the dynamics of a mixed infection of A. nomius and M. anisopliae in a single termite host. The virulent parasite M. anisopliae debilitated the termite immune system, but the facultative, fast growing parasite A. nomius dominated the mixed infection process. The resource utilization strategy of A. nomius during the infection resulted in successful conidia production, while the chance for M. anisopliae to complete its life cycle was reduced. Our results also suggest that the occurrence of opportunistic parasites such as A. nomius in collapsing termite laboratory colonies is the consequence of a previous stress, not the cause of the stress.

When subterranean termites challenge the rules of fungal epizootics

Over the past 50 years, repeated attempts have been made to develop biological control technologies for use against economically important species of subterranean termites, focusing primarily on the use of the entomopathogenic fungus Metarhizium anisopliae. However, no successful field implementation of biological control has been reported. Most previous work has been conducted under the assumption that environmental conditions within termite nests would favor the growth and dispersion of entomopathogenic agents, resulting in an epizootic. Epizootics rely on the ability of the pathogenic microorganism to self-replicate and disperse among the host population. However, our study shows that due to multilevel disease resistance mechanisms, the incidence of an epizootic within a group of termites is unlikely. By exposing groups of 50 termites in planar arenas containing sand particles treated with a range of densities of an entomopathogenic fungus, we were able to quantify behavioral patterns as a function of the death ratios resulting from the fungal exposure. The inability of the fungal pathogen M. anisopliae to complete its life cycle within a Coptotermes formosanus (Isoptera: Rhinotermitidae) group was mainly the result of cannibalism and the burial behavior of the nest mates, even when termite mortality reached up to 75%. Because a subterranean termite colony, as a superorganism, can prevent epizootics of M. anisopliae, the traditional concepts of epizootiology may not apply to this social insect when exposed to fungal pathogens, or other pathogen for which termites have evolved behavioral and physiological means of disrupting their life cycle.