Termite-Bacterial association

What is Termite-Bacterial association??

Lower termites are one of the best studied systems in insects. Their ability to feed on a nitrogen poor and wood-based diet with help from symbiotic microbes has been under investigation for almost a century. A microbial consortium living in the guts of lower termites is essential for wood feeding. Host and symbiont cellulolytic enzymes synergize each other in the gut of the insect to increase digestive efficiency. Because of their critical role in digestion the gut microbiota are driving forces in all aspects of termite microbiology. Social living comes with risks for termites. The combination of group living and a microbe rich habitat makes termites potentially vulnerable to infection from pathogens. However, the use of entomopathogens for termite control has been largely failed. One mechanism for the failure may be symbiotic collaboration; i.e., a very big reason termites have thrived in the first place. Symbiont contributions are thought to neutralize fungal spores as they pass through the gut of the insect. Also as and when the symbiont community is disrupted pathogen susceptibility increases. These discoveries have shed light on novel interactions for symbiotic microbes both within the termite as a host and with pathogenic invaders. Lower termite biology is therefore tightly linked to symbiosis and associations and their resulting physiological collaborations.

Introduction:

The association of lower termites with microbes is fundamental to their biology. For the last century, understanding the relationship between termites and their gut symbionts, i.e., the termite halobiont has been a major focus of termite research. The majority of this work emphasizes both the complexity and novelty of functions carried out to process lignocellulose degradation within the termite gut. For decades the termite wood digestion has been a excellent example of symbiotic collaboration; however, symbionts have also been associated with a myriad of similar functions in this system. For example in addition to synergistic digestive collaboration the symbionts of lower termites have also been shown to play protective roles against pathogens. This interaction between the termite symbiotic consortium and potential pathogens adds a layer of complexity within this already complex microbial community. We talk about the variety and roles the symbionts play in termites and highlight the broad implications of both topics for understanding termite microbiology and symbiotic evolution, and emphasize how a  approach to studying termite biology is necessary to encompass the impact of this symbiotic association.

Soil-Feeding Termites, Biology, Microbial Associations and Digestive Mechanisms:

Soil-feeding species are found in 3 subfamilies of termites and constitute 67% of all genera. The habit which may have evolved several times, is principally associated with lowland humid equatorial rainforests and there are some savannah forms. Soil feeders can generally be distinguished from wood feeders by the morphology of the intestines. The stable isotopic ratios of C and N, and by the higher activity of certain elements of the gut flora, notably methanogens and organisms able to ferment reduced and recalcitrant substrates including aromatics. Soil feeders emit more methane as free gas but do not appear to fix nitrogen in significant amounts. Organic material passing through the gut is further humified and is enriched with total C, N and fulvic acid compared with parent soil and humic acid is depleted. Materials made using faeces show enhanced cation exchange capacity, redistribution and stabilization of soil matter and an increase in  Phosphorus and Carbon availability. Bacterial activity is stimulated in fresh faeces and may contribute about the further processing of organic matter. The full range of substrates degraded by soil-feeders is not known: The possibilities discussed are:

1) That a range of compounds including polysaccharides are degraded to a limited extent by a generalist gut flora and that a specialized symbiont population degrades reduced substrates such as tannin-protein complex and polyaromatics.

Association of Actinomycete Like Bacteria with Soil-Feeding Termites:

Electron microscopy was carried out of the Gut which showed that there are some actinomycete like bacteria were the microbial associates of two African-species of soil feeding termites. Elongated circular spines are providing attachment:

Termites are well known for their associations with similar symbionts which play a significant and scientific  role in digestion. The writings in this literature has focused largely on the primary families (lower termites) in which the gut harbouring cellulose-degrading protozoa, whereas the Termitidae (higher termites, comprising four fifths of the described species of Isoptera) have been studied well.

Within the latter group, they have functionally significant gut microflora while the fungal gardens of the Macro-termitinae are also well-known phenomenon. The tropical group of soil feeders and utilizers however consists of a major proportion of the species in this family. In the wet land forests of West Africa and South America, these are present as  the most abundant insects species and evidently play a key and major role in nutrient cycling and consumption ,yet  but nothing is known of their symbiotic associations. Conclusion is that the actinomycete resembling bacteria are prominent among the intestinal microflora of two species of Termites. Attachment of  the filaments are provided by long cuticular spines in the posterior and anterior gut area.

Gut microflora was studied by scanning electron microscopy and transmission electron microscopy in Procubitermes aburiensis, Sjostedt and Cubitermes severus Silvestri (Termitidae, Termitinae). Mounds were flown to the United Kingdom within a few days of collection in an alluvial secondary forest at Rabba, Nigeria, and maintained in sterilized soil or mixtures of sterilized potting compost and millipede frass at 25’C. For scanning electron microscopy gut sections were taken by longitudinal incisions and shaken vigorously for 20 s in bacteriological Ringer solution to remove loose mineral debris and particles. Fixation was for 1N in 2.5% glutaraldehyde in 0.1 M phosphate buffer, pH 7.2. For the purpose of electron microscopy dissected guts were fixed for 3 hours in glutaraldehyde without agitation, post fixed for 1 h in 1% oxo sulphate, and put in resin.

A previous investigation by light microscopy of gut organisms in C. severus showed that the P3 and P4 regions (Fig. 1) contained large concentrations of non-filamentous bacteria. This was confirmed by the research, but by scanning of electron microscopy showed that fine septa showing filaments were also abundant in the hindgut in the posterior part to the enteric valve. The filaments formed a dense network extending through the P3 and P4, free of attachment to the gut wall except in the posterior dilation of the colon. (designated P4b) where attachment was provided by about 200 spines having cuticles(Fig. 2). The spines were uniformly distributed over the surface of the gut, about 70 um apart from the base, and were 180 to 210 um long and 2 um in diameter at the base narrowing to the tips. Each spine was supported and was almost entirely enclosed by a complex aggregation of filaments and minerals materials. The filaments, of various sizes and diameters.

FIG. 1. General morphology of the gut. The cecum is vestigial in C. severus

range was  from 0.2 to 2.0um. lt could be seen at higher magnification microscopy that the attachment to the surface of the spines (Fig. 3). Sporing structures were not present apparently but transmission electron microscopy of the microsporic complex showed that all cells present were typically prokaryotic in structure and  lack of nuclear membranes and mitochondria (Fig. 4). Inoculation on starch casein or chitin nutrient agar plates with small numbers of the cuticular spines showed that a variety of actinomycetes could be easily isolated. Many of the filaments surrounding the spines were supposed be actinomycetes. Some of the cells showed a large electron-dense core with a groove (Fig. 4, arrows), which is not a characteristic of described a actinomycetes intracellular secretion were also present.

FIG. 3. Scanning electron micrograph of part of a single cuticular spine of C. severus, showing the attachment of the filaments (arrows).
FIG. 4. Transmission electron micrograph of microbial complex adjacent to cuticular spine in P. aburiensis.

insect hindguts, both and also in relation to the gut diameter, which is shown in the P4b does not exceed 400 ltm. In situ the spines are sloped obliquely in the position of the rectum (Fig. 1,P5), spines from angular sides of the gut wall intersecting in some cases with one another at the tip. The spines and associated microflora that therefore formed a complex across the lumen of the gut through which the  soil eaten was filtered during peristalsis. The association was observed in both species of termites and did not differ between freshly freighted insects and those kept for up to 4 months in the laboratory. Actinomycetes have been isolated  from the guts of wood-feeder termites but no information has been available on their variety or orientation within the host. The attachment of bacteria to cuticles and spines has been reported in the hindgut of the cockroach Blaberus posticus but filamentous forms appeared less numerous than in the present study with termites. The prominence of actinomycetes like bacteria in soil and manure feeders and attachment to the  structures within the hindgut suggests a possible role in digestion and excretion. Despite assumptions to the contrary, it was found that no evidence that soil feeders select or degrade the cellular remains of plants. Gut content analysis, supported by particle size, organic carbon, and mineral analyses of P. aburiensis and C. severus mound material, showed that mineral-humus complexes were the principal organic substances ingested. There is no  evidence that actinomycetes can degrade humus and manure, but the ability of many soil-dwelling species to secrete phenol oxidases has led to the proposition that they assist in humus formation from phenolic complexes derived from plant tissues. Thus, the presence of actinomycetes like bacteria in the guts of soil feeder termites raises the possibility that their nourishment and enrichment include a significant component from soil organic matter fractions.

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