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Many of the parasites that plague humans have life cycles that are positively baroque, hopping between species and hiding out in tissues for years before setting off a damaging infection. These habits can make them extremely difficult to study, since it can be hard to tell what tissues and cells the parasites pass through on their way to causing disease. But a clever bit of genetic engineering has now forced one parasite, Toxoplasma gondii, to leave telltale signs of its progress.

The work took advantage of some basic understanding of Toxoplasma biology. Upon infecting a cell and taking up residence, proteins in a specific organelle get exported into its hosts' cells. The researchers took the gene for one of the proteins that is known to be shipped into hosts, toxifilin, fused it to a site-specific DNA recombinase called cre, and injected the fusion gene into Toxoplasma cells. The resulting cells were called secreted Cre, epitope-tagged, presumably so that the authors could use the abbreviation SeCreEt to refer to them.

When a SeCreEt expressing parasite infects a mouse cell, the recombinase will catalyze DNA rearrangements at any sites that match a specific sequence. So, for example, the researchers used a DNA construct that normally expresses a red fluorescent protein, but switches to green following cre-based rearrangement. When mouse cells carrying this construct were infected with SeCreEt parasites, 95 percent of them switched from glowing red to glowing green. Mice that expressed a cre-dependent luciferase gene (the protein that helps fireflies glow) could be infected, and the progress of the infection tracked over the course of a week.

The authors suggest that SeCreEt cells will be useful for eliminating various host genes during infection, so that we can test whether different mouse proteins are essential for Toxoplasma to grow. But the general approach could potentially be used simply to follow the parasite during infection, since it could be used to create a trail of glowing green cells behind it. It might also be possible to engineer systems that don't actually require the parasite to enter cells.

In any case, the CDC calls Toxoplasma "the third leading cause of death attributed to foodborne illness in the United States," so knowing more about it can't be a bad thing.

Nature Methods, 2010. DOI: 10.1038/Nmeth.1438 (About DOIs).

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