[
European Worm Meeting,
1998]
Two spontaneous variant strains of C. elegans have been isolated and studied, which exhibit similar tail-deformity phenotypes, with a conspicuous post-anal swelling. Investigation of the basis of this phenotype (called "Dar", for Deformed Anal Region) revealed that in both cases it is not genetically determined, but instead is caused by a bacterial infection. The bacteria are gram-positive rods, which adhere tightly to a small patch of cuticle immediately posterior to the anus, and elicit swelling and deformity of the underlying tissues. Bacteria from both strains can be cultured on nutrient media in the absence of worms, but grow much more slowly than E. coli. Two distinct colony types (CBX101 and CBX102, which appear to be phase variants of a single species) were isolated from one nematode strain, and a third (CBX103) from the other. All three elicit a similar Dar phenotype in wild-type N2 worms, when added in trace amounts to standard E. coli/C. elegans cultures. Taxonomic characterization (kindly carried out by Dr. G. Funke, University of Zurich) indicates that they belong to the coryneform genus Aureobacterium, a large and little-studied group of soil bacteria. The 13 described species of this genus were tested for the ability to induce a Dar phenotype in C. elegans. All were negative, so our isolates probably represent new species of Aureobacterium. Infection of nematodes by bacteria of this genus has not previously been reported. CBX102 can induce a Dar response in three other Caenorhabditis species, and in an Oscheius species, but has no obvious effect on representatives of several other genera of soil nematodes. The deformation of the anal region induced by CBX102 leads to constipation and slower growth in the infected worms, which is likely to be advantageous to the adherent bacteria. The bacteria do not kill the worms, nor are they otherwise resistant to being eaten by nematodes, because C. elegans will grow and reproduce on lawns consisting of pure CBX102. In mixed OP50/CBX102 lawns, used to feed worms, the coryneforms survive better than the E. coli. The Dar effect offers an opportunity to examine the details of a specific nematode-bacterium interaction, and to investigate the mechanism of a possible inflammatory response in C. elegans. A variety of standard mutant strains of C. elegans have been tested for resistance or hypersensitivity to Dar infection. Several srf mutants are conspicuously resistant, presumably because these mutants have altered cuticles to which the bacteria adhere less well.
[
International Worm Meeting,
2005]
Microbacterium nematophilum (M. nem) is a Gram-positive bacterium belonging to the coryneform group, which was discovered as a result of its ability to cause disease and morphological alteration in C. elegans (Hodgkin et al., 2000). Nematode cultures have been accidentally contaminated by M. nem and consequently become diseased on several independent occasions. The pathogenic bacteria colonize the rectum of susceptible worms and cause localized swelling, constipation and growth impairment, but they do not usually prevent survival and reproduction of infected worms. Similar pathology is seen after infection of C. briggsae and C. remanei. Recent taxonomic and genomic analyses (Kiontke et al., 2004) have defined new candidate species and a robust phylogeny for the Caenorhabditis genus. Ten species that can be easily grown on standard NGM agar/E. coli plates are now available (several were kindly provided to us by Karin Kiontke and David Fitch), and all have been tested for susceptibility to M. nem. In contrast to the effects on the four species of the 'elegans' group (C. elegans, C. briggsae, C. remanei, C. sp CB5161), M. nem is lethal to the other six species. Larvae exposed to either a mixed E. coli/M. nem lawn, or a pure M. nem lawn, are unable to mature, and usually experience paralysis, lysis and death after one or two days. For some but not all of the species, the dying larvae exhibit a Dar (swollen tail) phenotype. These observations suggest that M. nem is potentially a serious pathogen for rhabditid soil nematodes, and that members of the elegans group have evolved defenses that ameliorate the infection. Analysis of the C. elegans response has identified components of these defenses (see related abstracts). In most cases where more than one independent isolate of a species was tested, similar results were obtained. However, isolates of C. briggsae are noticeably heterogeneous, ranging from almost complete resistance to extreme sensitivity. Avirulent derivatives of the M. nem type strain, CBX102, have been generated, which are unable to cause disease in C. elegans. Some of these avirulent mutants have been tested on the more sensitive species, and found to be similarly non-virulent, which suggests that related mechanisms are involved in causing disease (in C. elegans) and death (in the more sensitive species).