Relationship of symbiotic microorganisms to metanephridium: Phagoctic activity in the metanephridial epithelium of two species of oligochaeta

Earthworm immunity

Earthworms belonging to oligochaete annelids became a model for comparative immunologists in the early sixties with the publication of results from transplantation experiments that proved the existence of self/nonself recognition in earthworms. This initiated extensive studies on the earthworm immune mechanisms that evolved to prevent the invasion of pathogens. In the last four decades important cellular and humoral pathways were described and numerous biologically active compounds were characterized and often cloned.

Up-regulation of Neurohemerythrin Expression in the Central Nervous System of the Medicinal Leech, Hirudo medicinalis, following Septic Injury

We report here some results of a proteomic analysis of changes in protein expression in the leech Hirudo medicinalis in response to septic injury. Comparison of two-dimensional protein gels revealed several significant differences between normal and experimental tissues. One protein found to be up-regulated after septic shock was identified, through a combination of Edman degradation, mass spectrometry, and molecular cloning, as a novel member of the hemerythrin family, a group of non-heme-iron oxygen transport proteins found in four invertebrate phyla: sipunculids, priapulids, brachiopods, and annelids. We found by in situ hybridization and immunocytochemistry that the new leech protein, which we have called neurohemerythrin, is indeed expressed in the leech central nervous system. Both message and protein were detected in the pair of large glia within the ganglionic neuropile, in the six packet glia that surround neuronal somata in each central ganglion, and in the bilateral pair of glia that separate axonal fascicles in the interganglionic connective nerves. No expression was detected in central neurons or in central nervous system microglia. Expression was also observed in many other, non-neuronal tissues in the body wall. Real-time PCR experiments suggest that neurohemerythrin is up-regulated posttranscriptionaly. We consider potential roles of neurohemerythrin, associated with its ability to bind oxygen and iron, in the innate immune response of the leech nervous system to bacterial invasion.

Acidovorax-like symbionts in the nephridia of earthworms

Dense accumulations of bacteria in the excretory organs, nephridia, were first described more than 75 years ago in members of the annelid family Lumbricidae (earthworms). These nephridial symbionts were assumed to play a role in the degradation of proteins in the excretory fluid for nitrogen recycling. In the present study, the phylogenetic affiliation of the nephridial bacteria of the earthworms Lumbricus terrestris, Aporrectodea tuberculata, Octolasion lacteum and Eisenia foetida was resolved. The 16S rRNA gene sequences of the symbionts formed a monophyletic cluster within the genus Acidovorax. Similarity between symbiont sequences from different host species was 95.5-97.6%, whereas similarity was> 99% between symbiont sequences from individuals of the same species. Densely packed bacteria were detected in the ampulla of the nephridia by fluorescence in situ hybridization (FISH) using Acidovorax-specific oligonucleotide probes. No other bacterial cells could be found by FISH, although a few sequences other than Acidovorax had been found by PCR and cloning. These results suggest that the Acidovorax-earthworm symbiosis is a stable, host-specific association that has evolved from a common bacterial ancestor. Given the close phylogenetic relationship of the symbionts to proteolytic, free-living Acidovorax species, they may indeed play a role in protein degradation during nitrogen excretion by earthworms.

Antimicrobial defense of the earthworm

Discrimination of self and nonself is one of the features of all animal species but the ways of elimination of nonself are different. Defense strategies of invertebrates, which lack antibodies and lymphocytes, are based on innate defense mechanisms. The study of such, undoubtedly less complex, defense mechanisms in invertebrates may shed a new light on the more sophisticated immunity of vertebrates. The main aim of this review is to show on one experimental model--an oligochaete annelid--cellular and humoral defense pathways protecting against microbial infection.

Presence of Culturable Bacteria in Cocoons of the Earthworm Eisenia fetida

Viable bacteria were found to coexist with developing embryos in egg capsules (cocoons) of the earthworm Eisenia fetida . Earthworms were reared under standardized conditions, and bacterial densities were measured in distinct batches of cocoons collected weekly for 10 weeks. Cocoons weighing 12 mg contained a mean viable bacterial population of approximately 10 8 CFU/g of cocoons. No difference was found in viable counts obtained from cocoons incubated at 15°C and cocoons incubated at 24°C. Viable bacterial numbers increased with cocoon age, while acridine orange direct counts of microbial cells were stable at approximately 10 9 cells per g of cocoons. Bacteria isolated from cocoons were used to develop antisera in rabbits for the production of strain-specific fluorescent antibodies. Fluorescent antibody and selective plating techniques were used to monitor populations of these bacteria in earthworm bedding and to determine whether cocoons acquire bacteria from the environment in which they are formed. Cocoon isolates were readily recovered from cocoons formed in inoculated bedding at densities of 10 8 CFU/g of cocoons. Bradyrhizobium japonicum USDA 110 and UMR 161 added to bedding were also recovered from cocoons, but at lower densities than cocoon isolates. Escherichia coli K-12(pJP4) inoculum was recovered from bedding but not from cocoons. The bacterial complement of Eisenia fetida cocoons is affected by inoculation of selected bacterial isolates in the worm growth environment.