Presence of Culturable Bacteria in Cocoons of the Earthworm Eisenia fetida

Pontoscolex corethrurus: A homeless invasive tropical earthworm?

The presence of earthworm species in crop fields is as old as agriculture itself. The earthworms Pontoscolex corethrurus (invasive) and Balanteodrilus pearsei (native) are associated with the emergence of agriculture and sedentism in the region Amazon and Maya, respectively. Both species have shifted their preference from their natural habitat to the cropland niche. They contrast in terms of intensification of agricultural land use (anthropic impact to the symbiotic soil microbiome). P. corethrurus inhabits conventional agroecosystems, while B. pearsei thrives in traditional agroecosystems, i.e., P. corethrurus has not yet been recorded in soils where B. pearsei dwells. The demographic behavior of these two earthworm species was assessed in the laboratory over 100 days, according to their origin (OE; P. corethrurus and B. pearsei) food quality (FQ; soil only, maize stubble, Mucuna pruriens), and soil moisture (SM; 25, 33, 42%). The results showed that OE, FQ, SM, and the OE x FQ interaction were highly significant for the survival, growth, and reproduction of earthworms. P. corethrurus showed a lower survival rate (> mortality). P. corethrurus survivors fed a diet of low-to-intermediate nutritional quality (soil and stubble maize, respectively) showed a greater capacity to grow and reproduce; however, it was surpassed by the native earthworm when fed a high-quality diet (M. pruriens). Besides, P. corethrurus displayed a low cocoon hatching (emergence of juveniles). These results suggest that the presence of the invasive species was associated with a negative interaction with the soil microbiota where the native species dwells, and with the absence of natural mutualistic bacteria (gut, nephridia, and cocoons). These results are consistent with the absence of P. corethrurus in milpa and pasture-type agricultural niches managed by peasants (agroecologists) to grow food regularly through biological soil management. Results reported here suggest that P. corethrurus is an invasive species that is neither wild nor domesticated, that is, its eco-evolutionary phylogeny needs to be derived based on its symbionts.

Identification of novel lumbricin homologues in Eisenia andrei earthworms

Lumbricin and its orthologue antimicrobial peptides were typically isolated from annelids. In this report, mRNA for lumbricin and -serendipitously- a novel lumbricin-related mRNA sequence were identified in Eisenia andrei earthworms. The determined mRNA sequences of E. andrei lumbricin and lumbricin-related peptide consist of 477 and 575 nucleotides. The precursors of proline-rich E. andrei lumbricin and the related peptide contain 63 and 59 amino acids, respectively. Phylogenetic analysis indicated close relationship with other annelid lumbricins. Highest expression of both mRNAs appeared in the proximal part of the intestine (pharynx, gizzard), while other tested organs had moderate (body wall, midgut, ovary, metanephridium, seminal vesicles, ventral nerve cord) or low (coelomocytes) levels. During ontogenesis their expression revealed continuous increase in embryos. Following 48 h of in vivo Gram-positive bacteria challenge both mRNAs were significantly elevated in coelomocytes, while Gram-negative bacteria or zymosan stimulation had no detectable effects.

Diversity, structure and sources of bacterial communities in earthworm cocoons

Animals start interactions with the bacteria that will constitute their microbiomes at embryonic stage. After mating, earthworms produce cocoons externally which will be colonized with bacteria from their parents and the environment. Due to the key role bacterial symbionts play on earthworm fitness, it is important to study bacterial colonization during cocoon formation. Here we describe the cocoon microbiome of the earthworms Eisenia andrei and E. fetida, which included 275 and 176 bacterial species, respectively. They were dominated by three vertically-transmitted symbionts, Microbacteriaceae, Verminephrobacter and Ca. Nephrothrix, which accounted for 88% and 66% of the sequences respectively. Verminephrobacter and Ca. Nephrothrix showed a high rate of sequence variation, suggesting that they could be biparentally acquired during mating. The other bacterial species inhabiting the cocoons came from the bedding, where they accounted for a small fraction of the diversity (27% and 7% of bacterial species for E. andrei and E. fetida bedding). Hence, earthworm cocoon microbiome includes a large fraction of the vertically-transmitted symbionts and a minor fraction, but more diverse, horizontally and non-randomly acquired from the environment. These data suggest that horizontally-transmitted bacteria to cocoons may play an important role in the adaptation of earthworms to new environments or diets.

The earthworm-Verminephrobacter symbiosis: an emerging experimental system to study extracellular symbiosis

Almost all Lumbricid earthworms (Oligochaeta: Lumbricidae) harbor extracellular species-specific bacterial symbionts of the genus Verminephrobacter (Betaproteobacteria) in their nephridia. The symbionts have a beneficial effect on host reproduction and likely live on their host's waste products. They are vertically transmitted and presumably associated with earthworms already at the origin of Lumbricidae 62–136 million years ago. The Verminephrobacter genomes carry signs of bottleneck-induced genetic drift, such as accelerated evolutionary rates, low codon usage bias, and extensive genome shuffling, which are characteristic of vertically transmitted intracellular symbionts. However, the Verminephrobacter genomes lack AT bias, size reduction, and pseudogenization, which are also common genomic hallmarks of vertically transmitted, intracellular symbionts. We propose that the opportunity for genetic mixing during part of the host—symbiont life cycle is the key to evade drift-induced genome erosion. Furthermore, we suggest the earthworm-Verminephrobacter association as a new experimental system for investigating host-microbe interactions, and especially for understanding genome evolution of vertically transmitted symbionts in the presence of genetic mixing.

Effect of earthworms on the biochemical characterization of biofilms in vermifiltration treatment of excess sludge

In this study, the biofilms formed in a vermifilter (VF) with earthworms and a conventional biofilter (BF) without earthworms were compared to investigate the effects of earthworms on the characteristics of biofilms during an excess sludge treatment period of 4months. Typical macrographs and micrographs of the biofilms showed that the feeding and casting actions of earthworms remarkably modified the VF morphology. Elemental analysis and fluorescence spectra indicated that earthworms enhanced the stabilization of organic matter by accelerating the mineralization and humification of organic materials during vermiconversion. In addition, bacterial communities inhabiting the VF biofilm showed that earthworms increased both bacterial diversity and metabolic activities in the film, as revealed by automatic testing bacteriology (ATB) expression and sequencing data. These results demonstrate that earthworms influence the structure and biochemical characteristics of biofilms and enhance their bacterial diversity and functions for improved sludge stabilization.

Effect of enzyme producing microorganisms on the biomass of epigeic earthworms (eisenia fetida) in vermicompost

We analyzed the bacterial community structure of the intestines of earthworms and determined the effect of enzyme producing microorganisms on the biomass of earthworms in vermicompost. Fifty-seven bacterial 16S rDNA clones were identified in the intestines of earthworms by using polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) analysis. Entomoplasma somnilux and Bacillus licheniformis were the dominant microorganisms; other strains included Aeromonas, Bacillus, Clostridium, Ferrimonas, and uncultured bacteria. Among these strains, Photobacterium ganghwense, Aeromonas hydrophila, and Paenibacillus motobuensis were enzyme-producing microorganisms. In the mixtures that were inoculated with pure cultures of A. hydrophila WA40 and P. motobuensis WN9, the highest survival rate was 100% and the average number of earthworms, young earthworms, and cocoons were 10, 4.00-4.33, and 3.00-3.33, respectively. In addition, P. motobuensis WN9 increased the growth of earthworms and production of casts in the vermicompost. These results show that earthworms and microorganisms have a symbiotic relationship.

Transmission of nephridial bacteria of the earthworm Eisenia fetida

The lumbricid earthworms (annelid family Lumbricidae) harbor gram-negative bacteria in their excretory organs, the nephridia. Comparative 16S rRNA gene sequencing of bacteria associated with the nephridia of several earthworm species has shown that each species of worm harbors a distinct bacterial species and that the bacteria from different species form a monophyletic cluster within the genus Acidovorax, suggesting that there is a specific association resulting from radiation from a common bacterial ancestor. Previous microscopy and culture studies revealed the presence of bacteria within the egg capsules and on the surface of embryos but did not demonstrate that the bacteria within the egg capsule were the same bacteria that colonized the nephridia. We present evidence, based on curing experiments, in situ hybridizations with Acidovorax-specific probes, and 16S rRNA gene sequence analysis, that the egg capsules contain high numbers of the bacterial symbiont and that juveniles are colonized during development within the egg capsule. Studies exposing aposymbiotic hatchlings to colonized adults and their bedding material suggested that juvenile earthworms do not readily acquire bacteria from the soil after hatching but must be colonized during development by bacteria deposited in the egg capsule. Whether this is due to the developmental stage of the host or the physiological state of the symbiont remains to be investigated.

Earthworm egg capsules as vectors for the environmental introduction of biodegradative bacteria

Earthworm egg capsules (cocoons) may acquire bacteria from the environment in which they are produced. We found that Ralstonia eutropha (pJP4) can be recovered from Eisenia fetida cocoons formed in soil inoculated with this bacterium. Plasmid pJP4 contains the genes necessary for 2,4-dichlorophenoxyacetic acid (2,4-D) and 2, 4-dichlorophenol (2,4-DCP) degradation. In this study we determined that the presence of R. eutropha (pJP4) within the developing earthworm cocoon can influence the degradation and toxicity of 2,4-D and 2,4-DCP, respectively. The addition of cocoons containing R. eutropha (pJP4) at either low or high densities (10(2) or 10(5) CFU per cocoon, respectively) initiated degradation of 2,4-D in nonsterile soil microcosms. Loss of 2,4-D was observed within the first week of incubation, and respiking the soil with 2,4-D showed depletion within 24 h. Microbial analysis of the soil revealed the presence of approximately 10(4) CFU R. eutropha (pJP4) g-1 of soil. The toxicity of 2,4-DCP to developing earthworms was tested by using cocoons with or without R. eutropha (pJP4). Results showed that cocoons containing R. eutropha (pJP4) were able to tolerate higher levels of 2,4-DCP. Our results indicate that the biodegradation of 2, 4-DCP by R. eutropha (pJP4) within the cocoons may be the mechanism contributing to toxicity reduction. These results suggest that the microbiota may influence the survival of developing earthworms exposed to toxic chemicals. In addition, cocoons can be used as inoculants for the introduction into the environment of beneficial bacteria, such as strains with biodegradative capabilities.

Plasmid Transfer between Spatially Separated Donor and Recipient Bacteria in Earthworm-Containing Soil Microcosms

Most gene transfer studies have been performed with relatively homogeneous soil systems in the absence of soil macrobiota, including invertebrates. In this study we examined the influence of earthworm activity (burrowing, casting, and feeding) on transfer of plasmid pJP4 between spatially separated donor (Alcaligenes eutrophus) and recipient (Pseudomonas fluorescens) bacteria in nonsterile soil columns. A model system was designed such that the activity of earthworms would act to mediate cell contact and gene transfer. Three different earthworm species (Aporrectodea trapezoides, Lumbricus rubellus, and Lumbricus terrestris), representing each of the major ecological categories (endogeic, epigeic, and anecic), were evaluated. Inoculated soil microcosms, with and without added earthworms, were analyzed for donor, recipient, and transconjugant bacteria at 5-cm-depth intervals by using selective plating techniques. Transconjugants were confirmed by colony hybridization with a mer gene probe. The presence of earthworms significantly increased dispersal of the donor and recipient strains. In situ gene transfer of plasmid pJP4 from A. eutrophus to P. fluorescens was detected only in earthworm-containing microcosms, at a frequency of (symbl)10(sup2) transconjugants per g of soil. The depth of recovery was dependent on the burrowing behavior of each earthworm species; however, there was no significant difference in the total number of transconjugants among the earthworm species. Donor and recipient bacteria were recovered from earthworm feces (casts) of all three earthworm species, with numbers up to 10(sup6) and 10(sup4) bacteria per g of cast, respectively. A. trapezoides egg capsules (cocoons) formed in the inoculated soil microcosms contained up to 10(sup7) donor and 10(sup6) recipient bacteria per g of cocoon. No transconjugant bacteria, however, were recovered from these microhabitats. To our knowledge, this is the first report of gene transfer between physically isolated bacteria in nonsterile soil, using burrowing earthworms as a biological factor to facilitate cell-to-cell contact.

Influence of earthworm activity on gene transfer from Pseudomonas fluorescens to indigenous soil bacteria

We have developed a model system to assess the influence of earthworm activity on the transfer of plasmid pJP4 from an inoculated donor bacterium, Pseudomonas fluorescens C5t (pJP4), to indigenous soil microorganisms. Three different earthworm species (Lumbricus terrestris, Lumbricus rubellus, and Aporrectodea trapezoides), each with unique burrowing, casting, and feeding behaviors, were evaluated. Soil columns were inoculated on the surface with 10(8) cells per g of soil of the donor bacterium, and after a 2-week incubation period, donor, transconjugant, and total bacteria were enumerated at 5-cm-depth intervals. Transconjugants were confirmed by use of colony hybridization with a mer gene probe. In situ gene transfer of plasmid pJP4 from P. fluorescens C5t to indigenous soil bacteria was detected in all inoculated microcosms. In the absence of earthworms, the depth of recovery was limited to the top 5 cm of the column, with approximately 10(3) transconjugants per g of soil. However, the total number of transconjugants recovered from soil was significantly greater in microcosms containing either L. rubellus or A. trapezoides, with levels reaching about 10(5) CFU/g of soil. In addition, earthworms distributed donor and transconjugant bacteria throughout the microcosm columns, with the depth of recovery dependent on the burrowing behavior of each earthworm species. Donor and transconjugant bacteria were also recovered from earthworm casts and inside developing cocoons. Transconjugant bacteria from the indigenous soil microflora were classified as belonging to Acidovorax spp., Acinetobacter spp., Agrobacterium spp., Pasteurella spp., Pseudomonas spp., and Xanthomonas spp.