The Silkworm Mouse
Legends are tricky things. Some of them have so many versions that one wonders if one of them was really passed down through the years or someone just made it up on the spot. The Chinese legend about the discovery of the silkworm is one such story. Was it discovered by an ancient empress named Lei Zu as she took an afternoon walk? Was it the Huang Di when he observed a silkworm cocoon dropping into his tea? Or was it Xi Lingshi, a Chinese princess who lived five thousand years ago? Nonetheless the silkworm Bombyx mori has been the center of sericulture for a very long time.
Silkworms also have the distinction of being the only domesticated insect to date. B. mori is entirely dependent on humans for survival as they have lost the ability for flight. Aside from the silk industry, silkworms have also been useful for basic research and biotechnology–particularly on the study of pheromones.
Certainly, Bombyx mori has been one of those fringe model organisms–at least to my thinking. Even as an invertebrate, it doesn’t hold the same cachet as Caenorhabditis elegans or Drosophila melanogaster. To put it bluntly, it doesn’t have nearly as much scientific precedence as the worm and the fly. No one has ever followed the silkworm cell lineages from egg to adult. No one looked for white eyed mutants in a colony of silkworms. But maybe today, it’s slowly changing.
But why use B. mori at all? Well, for one thing, it’s easy to breed. People have been doing it for thousands of years after all. Generation time is definitely faster than that of a mouse (and they probably cost less to house too). And I’m pretty sure no animal rights activist would break into a lab to free all the silkworms.
In a recent issue of Molecular Microbiology, Kaito et al. have taken the silkworm and put it to a quite different use–as an infection model. Most scientists are more familiar with the mouse, worm, and fly as models for pathogen-host interaction and in a way it’s easier to examine how a pathogen infects a host using those organisms because their genetics are more understood. But mice are expensive and worms and flies–compared to the silkworm–are too tiny to work with easily.
To test if the silkworm could be used as an infection model, the researchers injected silkworm larvae with Staphylococcus aureus and a staph mutant for a known virulence gene. The larvae injected with the wildtype staph died off much faster than the larvae injected with the mutant staph–a strong indication that the silkworm could be used as a model. But the real test was, could they use the silkworm to identify new virulence genes in staph?
Staph has over 500 genes whose functions are still not known. In order to search for virulence genes among those unknown genes, Kaito et al. constructed staph mutants for 100 of those genes and injected these mutants into silkworms. Only three batches of silkworms survived–those that were injected with the staph mutants of the cvf (conserved virulence factor) genes. Two of these genes were confirmed to affect staph’s virulence by injecting these into mice. The cvf genes were later found to control the bacteria’s production of toxins.
So the results look promising. But will the silkworm ever be the new laboratory mouse? It’s certainly an interesting model that has some advantages, but I would think not. There are vast differences between invertebrates and mammals, and rodents (and monkeys) are currently still the best models we have for human disease. Sure, virulence genes can be pulled out by using the silkworm–but there will always be false positives while other genes might be missed entirely. It would make sense that a bacterium would use different weapons if it finds itself in a silkworm or a human.