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Analytical Chemistry

A New Detector For Endotoxin

Sensors: At low levels, bacterial lipid causes visible reordering of liquid crystals

by Lauren K. Wolf
May 19, 2011 | APPEARED IN VOLUME 89, ISSUE 21

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Credit: Science
When added to liquid-crystal droplets (top panels), endotoxin induces a structural change (bottom panels) visible with both a bright-field microscope (left panels) and a polarized microscope (right panels).
8921NOTW_pg10_250.jpg
Credit: Science
When added to liquid-crystal droplets (top panels), endotoxin induces a structural change (bottom panels) visible with both a bright-field microscope (left panels) and a polarized microscope (right panels).

It might one day be possible to visually detect endotoxin, a molecular indicator of bacterial infection, with a little bit of liquid crystal, thanks to researchers at the University of Wisconsin, Madison, and the University of California, Davis (Science, DOI: 10.1126/science.1195639). The research team reports that the bacterial lipopolysaccharide, when added at concentrations less than 1 pg/mL to micrometer-sized droplets of the liquid-crystal 4'-pentyl-4-cyanobiphenyl, causes a structural change in the droplets. This change is visible with a light microscope.

The standard assay for endotoxin is based on horseshoe crab blood, which clots in the molecule's presence. But the heavily armored crabs have to be caught and bled to give up their blue blood for the assay material. The sensitivity of the liquid-crystal method rivals that of the standard assay. That observation hints that this new method may offer the basis of an assay for endotoxin, says UW Madison chemical engineer Nicholas L. Abbott, the leader of the study. And it doesn't involve any crab catching.

Abbott and coworkers show that endotoxin from Escherichia coli reorders the liquid-crystal droplets, causing a visible defect to form at their centers. By doing calculations and using confocal fluorescence microscopy, the team determined that each droplet's structure changes because endotoxin, a six-tail lipid from the outer membrane of gram-negative bacteria, aggregates at a point defect in the liquid crystals. These defects—nanoscopic melted areas in the liquid crystals—stem from the droplet's spherical geometry.

Other lipids also change liquid—crystal orientation on surfaces, says Noel A. Clark, a physicist at the University of Colorado, Boulder, but "the striking observation here, and key to the method's potential utility, is the extremely small concentration of endotoxin required to produce the change—six orders of magnitude lower than other lipids."

Although it's "fairly straightforward" to assess the optical appearance of pure endotoxin in droplets of liquid crystal dispersed in water, Abbott says, many issues must be addressed before this new method can be used with complicated samples such as blood. Those studies, he adds, are under way.

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