Worm Breeder's Gazette 17(1): 45 (October 1, 2001)

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.

The C. elegans gastronome: differential palates for Mycobacterium smegmatis and Mycobacterium tuberculosis

Ming Chih Tsai, 1Jonathan Lipton, 1Scott Emmons, John Chan

Departments Medicine, Microbiology & Immunology, 1Molecular Genetics, Albert Einstein College of Medicine, 1300 Morris Park Avenue, Bronx, New York 10461

It has been observed that certain strains of Pseudomonas aeroginosa, when served as the only food source, are lethal to the nematode worm Caenorhabiditis elegans. This observation has led to the use of C. elegans for the expeditious screening of virulence factors of P. aeroginosa, based on lethality. Importantly, this screen has allowed identification of P. aeuroginosa factors that proved essential for virulence in plant and mice. We have initiated studies to test the suitability of the use of C. elegans to screen for virulence factors in Mycobacterium tuberculosis.


The relatively avirulent, rapid-growing M. smegmatis (MC2155) was initially used to examine the behavior of the nematode when offered mycobacteria as the sole food source. Preliminary studies revealed that C. elegans readily consumes M. smegmatis. Some worms, however, migrated away from the M. smagmatis lawn in the initial period following transfer to the center of the lawn, only to return later. This temporary avoidance was not observed on the Escherichia coli (OP50) lawn. Interestingly, though not quantified, enhanced sexual activities were observed in worms fed M. smegmatis as the sole diet, as assessed by the increased tendency of pairing of males and hermaphrodites. This apparent enhancement in sexual activities could be the result of aggregative behavior induced by mycobacterial noxious stimuli.


The aversion displayed by C. elegans to M. smegmatis was confirmed in another set of experiments. In these studies, eggs derived from bleach-treated gravid worms, deposited on a bacteria-free region of an agar plate seeded with the organism of interest (E. coli, M. smegmatis, or M. tuberculosis), were allowed to hatch and the migration of young larvae to the bacterial lawn monitored. While virtually all hatched larvae migrated to the E. coli lawn within 24 h, those seeded next to the virulent M. tuberculosis lawn avoided the tubercle bacillus completely. Trafficking of the larvae to the relatively avirulent M. smegmatis was apparent, although a significant number of worms still failed to reach the MC2155 lawn at the end of 48h.


Young larvae obtained by the method described above were also followed to evaluate the effects of different diets on overall C. elegans development and reproduction. On M. smegmatis plates, the ability of young larvae to develop and reproduce appeared to be comparable to those fed E. coli, although the number of eggs produced was not enumerated. In addition, as described above, C. elegans readily feeds on MC2155. By contrast, on M. tuberculosis plates, development was markedly stunted: the larvae remained small and moved sluggishly. This specific developmental pattern of M. tuberculosis-fed C. elegans is most likely due to the worm's aversion to the tubercle bacillus as a diet, and hence starvation.


In summary, the degree of shunning of C. elegans from the relatively avirulent M. smegmatis and the virulent M. tuberculosis differs significantly, as assessed by migration toward the food source, consumption of the specific mycobacterial species, and development. It is also noteworthy that C. elegans mating activities are enhanced when M. smegmatis is offered as the sole food source. Thus, although the C. elegans virulence screen may not be an expeditious or a suitable system for the study of M. tuberculosis pathogenesis, the species-specific effects of mycobacteria on some behaviors of C. elegans as complex as sexual activities and food consumption, and on the developmental process may deserve further investigation.