New system detects active Shiga toxin

Scientists at USDA’s Agricultural Research Service Western Regional Research Center have come up with a new way to detect biologically active Shiga toxin, a product of pathogenic Escherichia coli serotype O157:H7, through the use of a camera and a light-emitting source that biologically determines the active toxin.

Affordable, sensitive devices like this are needed to reduce the sources and incidence of foodborne illness, Reuven Rasooly, who works in the center’s Foodborne Toxin Detection and Prevention Research Unit, said in a recent article. It is estimated that E. coli O157:H7 causes 73,000 cases of food poisoning and more than 60 deaths in the U.S. each year and current immunological tests, such as the ELISA, cannot distinguish between the active and inactive form of Shiga toxin.

“For example, in food processing, a heat-treatment method inactivates the toxin, but the ELISA cannot tell whether the toxin is active or inactive in the processed food,” Rasooly said. “Determining the activity of the toxin is very important, because the active form poses a threat to humans. The inactive form is nontoxic.”

Rasooly added that while the cost of technology used to detect Shiga toxin and other pathogens is not a concern for wealthy countries, the equipment is typically too expensive for developing ones, where the risk of foodborne illness and outbreaks is greatest. The camera system makes it easy and affordable for diagnostic labs that cannot afford a fluorometer, which is typically used to detect toxins.

“We demonstrated that our system is effective in measuring Shiga toxin activity compared with equipment costing 100 times more,” Rasooly said in the article. “A fluorometer costs about $35,000, whereas the camera in this experiment costs $300.”

Scientists constructed a fluorescence detection system using a camera and light source to measure GFP (green fluorescent protein) in a cell-based assay. A portion of a Shiga toxin-containing food sample was incubated with cells designed to produce GFP. The toxin in the sample inhibited the synthesis of GFP — reducing GFP production in relation to the amount of toxin present. The greater the toxicity, the less fluorescent the cells were.

Filters used with the light source and camera blocked wavelengths (signals) that would interfere with precise measurements of fluorescence.

“We obtained these signal levels by taking a picture with a camera and analyzing the image with a free, available computer software that determines average pixel intensity,” Rasooly explained.

The camera method, which can easily be adapted for detecting other foodborne toxins, was compared to a commercial fluorometer for detecting active Shiga toxin, Rasooly said. Both instruments had the same level of toxin detection.

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