Over 5000 species of earthworms distributed worldwide alter the soils they inhabit through mixing and processing of organic matter. Soil processing involves activities mediated by both the earthworm and associated microbiota.

Most studies of the microbial contribution to soil processing have focused on the gut and casts (fecal pellets) employing both culture based and more recently, molecular methods (Furlong, et al. 2002; Toyota and Makoto, 2000; Schönholzer, et al. 2002). We are investigating a less familiar association of a dense culture of bacteria in the nephridia, excretory organs present in each segment of the worm (Fig. 1). Although discovered around 1926, the identity and the activity of these bacteria within the worm had not been extensively investigated.

The nephridia pass fluid from the coelom of the worm to the outside through a continuous winding tube that forms three major loops for osmoregulation and excretion. The second loop contains a narrow part that widens into the ampulla, which is packed with bacterial cells (Fig. 1B). Our recently published comparative 16S rRNA gene sequence analyses revealed that the bacterial symbionts form a monophyletic group within the Acidovorax genus of the Betaproteobacteria (Schramm, et al. 2003). Different species of worms seem to harbor distinct strains of these associated, Acidovorax-like bacteria, suggesting a certain degree of co-evolution.

Acidovorax-like bacteria have been isolated from the nephridia of the compost earthworm Eisenia foetida, and a systematic description of the symbiotic organisms is being conducted (Pinel, et al., in preparation). In collaboration with the Joint Genome Institute, the genome of this symbiotic bacterium, and that of two other closely related Acidovorax species are being sequenced. Nephridial symbionts have been isolated from other species of earthworms, and they are being characterized in order to explore the phenotypic plasticity of nephridial bacteria accross species of hosts.

Figure 1. Verminephrobacter eiseniae symbionts in the nephridia of the earthworm Eisenia foetida. A., E. foetida and location of nephridia. B., diagram of a nephridium illustrating three main loops and the ampulla of the second loop that harbors the bacterial symbionts. C., a laser scanning confocal micrograph of the ampulla showing the bacterial cells in yellow and earthworm tissue in green. Bacterial cells the nephridium were labeled by fluorescent in situ hybridization (FISH) and imaged using a Zeiss Pascal Laser Scanning Confocal microscope. For more photos click here.

This association offers opportunities to study interactions between a beneficial bacterium and its host. Furthermore, this interaction may have influence on the ability of earthworms to function in the soil and thus on soil ecology. Currently we have NSF support to investigate the physiology of the symbiosis and interactions between bacterial symbiont and the host worm.

1. function of bacterial symbiont in the nephridium
• alteration of host chemistry
• influence of bacteria on overall health/fitness of worm
2. bacterial transmission from parent to juvenile
• mechanisms of transport of bacteria into egg capsules
• microbial ecology of the egg capsule community
• timing of infection and process of colonization
• bacterial symbiont physiology
3. potential for engineering of new methods for bioremediation

Publications

Acidovorax-like symbionts in the nephridia of earthworms. Schramm, et al. (2003) Environmental Microbiology 5:804-9.

Transmission of nephridial bacteria of the earthworm Eisenia fetida. Davidson & Stahl (2006) Applied & Environmental Microbiology 72:769-75.

Isolation and characterization of Verminephrobacter eiseniae gen. nov., sp. nov., a nephridial symbiont of the earthworm Eisenia foetida (Savigny). Pinel, et al. In preparation.

Back