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RESEARCH PROFILES Colorimetric assay for field detection of cholera lactose, a disaccharide composed of gluImproved sanitation in developed coun- that can be done out in the field,” says Russell. cose and galactose, and this sugar is tries has relegated cholera outbreaks The first challenge in designing the what the researchers ultimately chose. mostly to the poorer nations of Asia and assay was choosing the type of nanoparCompared with galactose, “we found Africa. The same lack of infrastructure ticle to use. Four factors determine the that the lactose was more sensitive and that facilitates the spread of cholera in behavior of the particles in solution: sol- more reliable,” Russell says. these countries also severely hampers Through a series of straightforward doctors’ ability to detect Vibrio cholerae, vent, size, composition, and coating. experiments, the researchers demonthe organism that causes this deadly dis- The size and composition of the particles were factors that the researchers strated that their particles do indeed ease. In the February 15 issue of Anaproduce an easily perceptible, rapid lytical Chemistry (pp 1356–1361), color change in aqueous solution David Russell and colleagues at the upon addition of CTB. UV–vis abUniversity of East Anglia (U.K.) sorption spectra show that a direct describe a new, simple, robust, colcorrelation exists between redshift orimetric detection system for choland broadening of the absorption era toxin, the organism’s major virand the concentration of CTB ulence agent. added. The researchers also showed Cholera toxin gains entry into that most of the aggregation occurs cells by binding to the GM1 ganwithin 30 seconds and that it is glioside receptor. Cholera toxin Bcomplete within 10 minutes. subunit (CTB) recognizes the To assess how their particles branched pentasaccharide of the rewould perform in a clinical sample, ceptor and helps cholera toxin Athe researchers measured aggregasubunit (CTA) to cross the cell tion in the presence of a series of membrane. CTA triggers a cascade potentially interfering ions and a of cellular signaling that eventually protein. They found that their manifests in a massive loss of cellunanoparticles were unaffected by lar electrolytes into the bowel and biologically relevant concentrations the watery diarrhea that is the clasof these species. The researchers sic deadly symptom of cholera. The Lactose-stabilized gold nanoparticles before (left) and also showed that nanoparticles that recognition between the GM1 ganafter (right) the addition of 1.5 µM CTB. have been freeze-dried and resusglioside receptor and CTB is the pended perform as well as fresh ones. had previously optimized. To be used basis for many previous detection Freeze-drying could ultimately aid in for biological testing, the particles must schemes for cholera toxin. transport and storage of these particles undergo an observable color change in Russell and his colleagues suspected in the field. that part of the receptor pentasaccharide aqueous solution. Gold nanoparticles In countries that lack reliable electriwith a 16-nm diameter display a bright alone could be exploited as a simpler cal systems, tests that require complex red color in water and shift to blue and more stable recognition target for instrumentation or refrigeration of CTB. They had been studying silver and upon aggregation. Silver nanoparticles reagents are obviously not feasible, Ruscould be used, but they display a much gold nanoparticles stabilized with varisell notes. “The whole idea of using a less perceptible yellow-to-orange color ous sugars, such as mannose, galactose, simple molecule like lactose is that you change when they aggregate. “For the and lactose. They had found that in socould use this, and it’s not labile so it lution, aggregation caused a broadening eye, the red-to-blue color [change] is wouldn’t break down in extremes of much more dramatic,” Russell explains. and redshift of the particles’ surface heat,” he says. “Previous methods are The final choice the researchers had plasmon absorption band, resulting in a very good, very sophisticated, and have to make was the sugar to be attached to distinct color change. The researchers exceptional detection limits, but the exthe surface of the particles. In vivo, reasoned that the multivalent interactremes of temperature or the variation cholera toxin recognizes galactose, sialic tion between CTB and the sugars disin temperature would destroy some of acid, and N-acetylgalactosamine, the played on their nanoparticles might also the more fragile molecules, and the dethree terminal monosaccharides of the cause aggregation and that the ensuing vice won’t work at the end of the day.” a branched GM1 pentasaccharide. Studies color change could be used as a visual —Jennifer Griffiths test for cholera. “It’s really a simple test have shown that the toxin will also bind 1772
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