Bacterial menageries inside insects

Comparative seasonal analysis of Eri silkworm (Samia ricini Donovan) gut composition: implications for lignocellulose degradation

Conversion of biomass such as lignocelluloses to an alternative energy source can contribute to sustainable development. Recently, biomass-degrading enzymes are reported to be common resources in insect-microbe interacting systems. Northeast India harbors ample sericigenous insect resources which are exploited for their silk products. Samia ricini Donovan is an economically important poly-phytophagous silkmoth capable of digesting foliage from different plant species, suggesting the versatility of a robust gut system. Here, a gut bacterial profile was determined by 16S rRNA gene characterization across the holometabolous life cycle during the summer and winter seasons, revealing 3 phyla, 13 families, and 22 genera. Comparative analysis among the seasonal gut isolates revealed a high diversity in summer, predominated by the genus Bacillus due to its high occurrence in all developmental stages. Shannon's diversity index demonstrated the second and fourth instars of summer as well as the fifth instar of winter to be relatively better developmental stages for gut bacteria assembly. Bacterial community shifts in concert to host developmental changes were found to be apparent between early instars and late instars in summer, which differed from those of winter. Forty-three and twenty-nine gut bacterial isolates were found to be cellulolytic and xylanolytic enzyme producers, respectively. The present results illustrate the gut microbiota of S. ricini over the seasons and support the holometabolous life cycle effect as the most likely factor shaping the gut bacterial microbiota. These findings may provide leads for the development of new cleaner and environmentally friendly lignocellulose-degrading enzymes.

Influence of Bacterial Secondary Symbionts in Sitobion avenae on Its Survival Fitness against Entomopathogenic Fungi, Beauveria bassiana and Metarhizium brunneum

The research was focused on the ability of wheat aphids Sitobion avenae, harboring bacterial secondary symbionts (BSS) Hamiltonella defensa or Regiella insecticola, to withstand exposure to fungal isolates of Beauveria bassiana and Metarhizium brunneum. In comparison to aphids lacking bacterial secondary symbionts, BSS considerably increased the lifespan of wheat aphids exposed to B. bassiana strains (Bb1022, EABb04/01-Tip) and M. brunneum strains (ART 2825 and BIPESCO 5) and also reduced the aphids' mortality. The wheat aphid clones lacking bacterial secondary symbionts were shown to be particularly vulnerable to M. brunneum strain BIPESCO 5. As opposed to wheat aphids carrying bacterial symbionts, fungal pathogens infected the wheat aphids lacking H. defensa and R. insecticola more quickly. When treated with fungal pathogens, bacterial endosymbionts had a favorable effect on the fecundity of their host aphids compared to the aphids lacking these symbionts, but there was no change in fungal sporulation on the deceased aphids. By defending their insect hosts against natural enemies, BSS increase the population of their host society and may have a significant impact on the development of their hosts.

Environments and Hosts Structure the Bacterial Microbiomes of Fungus-Gardening Ants and their Symbiotic Fungus Gardens

The fungus gardening-ant system is considered a complex, multi-tiered symbiosis, as it is composed of ants, their fungus, and microorganisms associated with either ants or fungus. We examine the bacterial microbiome of Trachymyrmex septentrionalis and Mycetomoellerius turrifex ants and their symbiotic fungus gardens, using 16S rRNA Illumina sequencing, over a region spanning approximately 350 km (east and central Texas). Typically, microorganisms can be acquired from a parent colony (vertical transmission) or from the environment (horizontal transmission). Because the symbiosis is characterized by co-dispersal of the ants and fungus, elements of both ant and fungus garden microbiome could be characterized by vertical transmission. The goals of this study were to explore how both the ant and fungus garden bacterial microbiome are acquired. The main findings were that different mechanisms appear to explain the structure the microbiomes of ants and their symbiotic fungus gardens. Ant associated microbiomes had a strong host ant signature, which could be indicative of vertical inheritance of the ant associated bacterial microbiome or an unknown mechanism of active uptake or screening. On the other hand, the bacterial microbiome of the fungus garden was more complex in that some bacterial taxa appear to be structured by the ant host species, whereas others by fungal lineage or the environment (geographic region). Thus bacteria in fungus gardens appear to be acquired both horizontally and vertically.

Not all animals need a microbiome

It is often taken for granted that all animals host and depend upon a microbiome, yet this has only been shown for a small proportion of species. We propose that animals span a continuum of reliance on microbial symbionts. At one end are the famously symbiont-dependent species such as aphids, humans, corals and cows, in which microbes are abundant and important to host fitness. In the middle are species that may tolerate some microbial colonization but are only minimally or facultatively dependent. At the other end are species that lack beneficial symbionts altogether. While their existence may seem improbable, animals are capable of limiting microbial growth in and on their bodies, and a microbially independent lifestyle may be favored by selection under some circumstances. There is already evidence for several ‘microbiome-free’ lineages that represent distantly related branches in the animal phylogeny. We discuss why these animals have received such little attention, highlighting the potential for contaminants, transients, and parasites to masquerade as beneficial symbionts. We also suggest ways to explore microbiomes that address the limitations of DNA sequencing. We call for further research on microbiome-free taxa to provide a more complete understanding of the ecology and evolution of macrobe-microbe interactions.

Bacterial community structure and diversity in the gut of the muga silkworm, Antheraea assamensis (Lepidoptera: Saturniidae), from India

The muga silkworm, Antheraea assamensis, is exclusively present in the northeastern regions of India and rearing of this silkworm is a vocation unique to this region in the world. Through culture-dependent techniques, generic identification using 16S ribosomal RNA probes, diversity analysis and qualitative screening for enzyme activities, our studies have identified a number of bacterial isolates, viz. Bacillus spp., Serratia marcescens, Stenotrophomonas maltophilia, Pseudomonas stutzeri, Acinetobacter sp. and Alcaligens sp., inhabiting the gut of the muga silkworm. Analysis of the culturable bacterial community from the gut of An. assamensis revealed that Bacillus (54%) was the predominant bacterial genus followed by Serratia (24%), Pseudomonas (10%) and Alcaligens (6%). Significant differences in the Shannon-Wiener (H') and Simpson (D) diversity indices of gut bacteria were recorded for An. assamensis collected from different regions. H' and D values were found to be highest for An. assamensis from the Titabar region (H' = 4.73 ± 0.43; D = 10.00 ± 0.11) and lowest for individuals from the Mendipathar region (H' = 2.1 ± 0.05; D = 0.04 ± 0.00) of northeastern India. Qualitative screening for enzyme activities identified about 26 gut bacterial isolates having significantly higher cellulose, amylase and lipase activities. These isolates probably contribute to the digestion and nutrition of their host insect, An. assamensis.

Who lives in a fungus? The diversity, origins and functions of fungal endobacteria living in Mucoromycota

Bacterial interactions with plants and animals have been examined for many years; differently, only with the new millennium the study of bacterial-fungal interactions blossomed, becoming a new field of microbiology with relevance to microbial ecology, human health and biotechnology. Bacteria and fungi interact at different levels and bacterial endosymbionts, which dwell inside fungal cells, provide the most intimate example. Bacterial endosymbionts mostly occur in fungi of the phylum Mucoromycota and include Betaproteobacteria (Burkhoderia-related) and Mollicutes (Mycoplasma-related). Based on phylogenomics and estimations of divergence time, we hypothesized two different scenarios for the origin of these interactions (early vs late bacterial invasion). Sequencing of the genomes of fungal endobacteria revealed a significant reduction in genome size, particularly in endosymbionts of Glomeromycotina, as expected by their uncultivability and host dependency. Similar to endobacteria of insects, the endobacteria of fungi show a range of behaviours from mutualism to antagonism. Emerging results suggest that some benefits given by the endobacteria to their plant-associated fungal host may propagate to the interacting plant, giving rise to a three-level inter-domain interaction.

Transmission and retention of Salmonella enterica by phytophagous hemipteran insects

Several pest insects of human and livestock habitations are known as vectors of Salmonella enterica; however, the role of plant-feeding insects as vectors of S. enterica to agricultural crops remains unexamined. Using a hemipteran insect pest-lettuce system, we investigated the potential for transmission and retention of S. enterica. Specifically, Macrosteles quadrilineatus and Myzus persicae insects were fed S. enterica-inoculated lettuce leaf discs or artificial liquid diets confined in Parafilm sachets to allow physical contact or exclusively oral ingestion of the pathogen, respectively. After a 24-h acquisition access period, insects were moved onto two consecutive noninoculated leaf discs or liquid diets and allowed a 24-h inoculation access period on each of the two discs or sachets. Similar proportions of individuals from both species ingested S. enterica after a 24-h acquisition access period from inoculated leaf discs, but a significantly higher proportion of M. quadrilineatus retained the pathogen internally after a 48-h inoculation access period. S. enterica was also recovered from the honeydew of both species. After a 48-h inoculation access period, bacteria were recovered from a significantly higher proportion of honeydew samples from M. quadrilineatus than from M. persicae insects. The recovery of S. enterica from leaf discs and liquid diets postfeeding demonstrated that both species of insects were capable of transmitting the bacteria in ways that are not limited to mechanical transmission. Overall, these results suggest that phytophagous insects may serve as potential vectors of S. enterica in association with plants.

Analyzing arthropods for the presence of bacteria

Bacteria within arthropods can be identified using culture-independent methods. This unit describes protocols for surface sterilization of arthropods, DNA extraction of whole bodies and tissues, touchdown PCR amplification using 16S rDNA general bacteria primers, and profiling the bacterial community using denaturing gradient gel electrophoresis.

Systemic analysis of the symbiotic function of Buchnera aphidicola, the primary endosymbiont of the pea aphid Acyrthosiphon pisum

Buchnera aphidicola is the primary obligate intracellular symbiont of most aphid species. B. aphidicola and aphids have been evolving in parallel since their association started, about 150 Myr ago. Both partners have lost their autonomy, and aphid diversification has been confined to smaller ecological niches by this co-evolution. B. aphidicola has undergone major genomic and biochemical changes as a result of adapting to intracellular life. Several genomes of B. aphidicola from different aphid species have been sequenced in the last decade, making it possible to carry out analyses and comparative studies using system-level in silico methods. This review attempts to provide a systemic description of the symbiotic function of aphid endosymbionts, particularly of B. aphidicola from the pea aphid Acyrthosiphon pisum, by analyzing their structural genomic properties, as well as their genetic and metabolic networks.

Symbiotic conversations are revealed under genetic interrogation

The recent development and application of molecular genetics to the symbionts of invertebrate animal species have advanced our knowledge of the biochemical communication that occurs between the host and its bacterial symbionts. In particular, the ability to manipulate these associations experimentally by introducing genetic variants of the symbionts into naive hosts has allowed the discovery of novel colonization mechanisms and factors. In addition, the role of the symbionts in inducing normal host development has been revealed, and its molecular basis described. In this Review, I discuss many of these developments, focusing on what has been discovered in five well-understood model systems.

The ubiquitous and varied role of infection in the lives of animals and plants

Parasitic and symbiotic infections are major forces governing the life histories of plant and animal hosts-a fact that is ever more evident because of recent findings emanating from diverse subdisciplines of biology. Yet, infectious organisms have been relatively little investigated by biologists who study natural populations. Now that new molecular and computational tools allow us to differentiate and track microscopic infectious agents in nature, we are beginning to establish a better appreciation of their effects on larger, more familiar organisms. This special issue on the ecological and evolutionary consequences of infection for plants and animals is based on the annual Vice Presidential Symposium at the meeting of the American Society of Naturalists held in Knoxville, Tennessee, in the summer of 2001.

Presymbiotic growth and sporal morphology are affected in the arbuscular mycorrhizal fungus Gigaspora margarita cured of its endobacteria

Some arbuscular mycorrhizal fungi contain endocellular bacteria. In Gigaspora margarita BEG 34, a homogenous population of beta-Proteobacteria is hosted inside the fungal spore. The bacteria, named Candidatus Glomeribacter gigasporarum, are vertically transmitted through fungal spore generations. Here we report how a protocol based on repeated passages through single-spore inocula caused dilution of the initial bacterial population eventually leading to cured spores. Spores of this line had a distinct phenotype regarding cytoplasm organization, vacuole morphology, cell wall organization, lipid bodies and pigment granules. The absence of bacteria severely affected presymbiotic fungal growth such as hyphal elongation and branching after root exudate treatment, suggesting that Ca. Glomeribacter gigasporarum is important for optimal development of its fungal host. Under laboratory conditions, the cured fungus could be propagated, i.e. could form mycorrhizae and sporulate, and can therefore be considered as a stable variant of the wild type. The results demonstrated that - at least for the G. margarita BEG 34 isolate - the absence of endobacteria affects the spore phenotype of the fungal host, and causes delays in the growth of germinating mycelium, possibly affecting its ecological fitness. This cured line is the first manipulated and stable isolate of an arbuscular mycorrhizal fungus.

Culture and manipulation of insect facultative symbionts

Insects from many different taxonomic groups harbor maternally transmitted bacterial symbionts. Some of these associations are ancient in origin and obligate in nature whereas others originated more recently and are facultative. Previous research focused on the biology of ancient obligate symbionts with essential nutritional roles in their insect hosts. However, recent important advances in understanding the biology of facultative associations have been driven by the development of techniques for the culture, genetic modification and manipulation of facultative symbionts. In this review, we examine these available experimental techniques and illustrate how they have provided fascinating new insight into the nature of associations involving facultative symbionts. We also propose a rationale for future research based on the integration of genomics and experimentation.

Metabolic interdependence of obligate intracellular bacteria and their insect hosts

Mutualistic associations of obligate intracellular bacteria and insects have attracted much interest in the past few years due to the evolutionary consequences for their genome structure. However, much less attention has been paid to the metabolic ramifications for these endosymbiotic microorganisms, which have to compete with but also to adapt to another metabolism--that of the host cell. This review attempts to provide insights into the complex physiological interactions and the evolution of metabolic pathways of several mutualistic bacteria of aphids, ants, and tsetse flies and their insect hosts.

Genome evolution in bacterial endosymbionts of insects

Many insect species rely on intracellular bacterial symbionts for their viability and fecundity. Large-scale DNA-sequence analyses are revealing the forces that shape the evolution of these bacterial associates and the genetic basis of their specialization to an intracellular lifestyle. The full genome sequences of two obligate mutualists, Buchnera aphidicola of aphids and Wigglesworthia glossinidia of tsetse flies, reveal substantial gene loss and an integration of host and symbiont metabolic functions. Further genomic comparisons should reveal the generality of these features among bacterial mutualists and the extent to which they are shared with other intracellular bacteria, including obligate pathogens.

Genome interdependence in insect-bacterium symbioses

Symbioses between unicellular and multicellular organisms have contributed significantly to the evolution of life on Earth. As exemplified by several studies of bacterium-insect symbioses, modern genomic techniques are providing exciting new information about the molecular basis and the biological roles of these complex relationships, revealing for instance that symbionts have lost many genes for functions that are provided by the host, but that they can provide amino acids that the host cannot synthesize.