Worm Breeder's Gazette 16(1): 38 (October 1, 1999)

These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.

Microbacterium nematophilum -- not just a Cambridge disease

Jonathan Hodgkin¹, Patricia Kuwabara², Marie-Anne Felix³

A novel coryneform bacterial pathogen of C. elegans has been isolated twice at MRC-LMB, in 1986 and 1997, as a chance plate contaminant (see WBG 15.1, p. 73). 16S rRNA sequencing has now been carried out on these isolates and confirms their assignment to the genus Microbacterium. Previous typing, based on phenotypic characters, was generously carried out by Dr. Guido Funke (University of Zurich) who assigned them to the genus Aureobacterium, recently incorporated into the genus Microbacterium. The two isolates (CBX103 and CBX102) are 100% identical in 16S rRNA sequence, and 97-98% identical to eight other species of Microbacterium. The sequence divergence and distinctive properties of the pathogen justify definition of a new species, which has been named Microbacterium nematophilum.

As previously reported, tests on other soil nematode species indicate that M. nematophilum is pathogenic to Caenorhabditis species and to Oscheius sp. CB5162, but not to representatives of seven other genera. Infection is not lethal, but causes a distinctive deformation of the tail, with conspicuous swelling of the post-anal region (the Dar, or Deformed Anal Region phenotype). This was illustrated in a report given by J.H. at the 1998 European Worm Meeting, after which M-A. F. recalled seeing a similar phenotype in an apparent mutant strain of Oscheius sp. CEW1, isolated by Mark Viney in a screen carried out at Caltech. The abnormal strain was thawed and examined, revealing a similar bacterial infection. As with the previous isolates from C. elegans, it proved possible to grow the bacterium in the absence of the worms, establishing a new strain CBX105. This was found to be pathogenic when tested on C. elegans. 16S rRNA sequencing revealed 100% identity to CBX102, so CBX105 is a third isolate of M. nematophilum.

The three strains so far isolated are distinguishable in certain respects. The 1997 sample is unusual in exhibiting phase variation between two different forms, which can be grown as stable strains (CBX101 and CBX102). CBX101 cells are markedly sticky, and tend to form clumps when grown in liquid media. Plate colonies appear very dark, when examined by transmitted light, and after growing for a few days they form peculiar concave shapes, like tiny vol-au-vents. CBX102 cells are less sticky, and form normal translucent round colonies when grown on plates. Conversion between the two different forms almost never occurs during axenic growth on standard rich nutrient media. However, after infecting C. elegans, interconversion occurs at much higher frequency, with a bias towards the CBX102 phase. No obvious advantage to the phase variation has been noticed, except that the sticky phase seems better at surviving on old culture plates. Neither of the other two isolates has exhibited any phase variation so far. CBX101, CBX102 and CBX105 colonies are white when grown in the dark, but develop a golden-yellow color when illuminated. In contrast, colonies of the 1986 isolate, CBX103, are always white. M. nematophilum has now been isolated twice in Cambridge, England and once in Pasadena, California, so it probably has a world-wide distribution, and may turn up in other C. elegans labs. Look out for worms with funny tails and strange genetics!