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High-Tech Biosensor Speeds Bacteria Detection

The Analyte 2000, a small, portable sensor based on devices used by the military to detect chemical and biological agents in the Gulf War, can detect, identify, and enumerate bacteria in food or water samples within 15 to 20 minutes. (Photo courtesy of Research International.)

Daniel Lim and his collaborators at the University of South Florida (USF) in Tampa are adapting a fiber optic technology originally developed for military applications to provide a portable and speedy means for detecting, identifying, and enumerating water- and food-borne microbial pathogens. A modified version of the military prototype biosensor can effectively detect microbes and their toxins "in 15 to 20 minutes," he says.

"I had always believed that more rapid detection technologies were possible and that a real-time/near-real-time detection procedure could benefit public health," Lim says. Those goals appeared more realistically within reach when he learned four years ago of a fiber optic biosensor that had been developed by Research International Inc., a small engineering company in Woodinville, Wash., near Seattle. A team at the Naval Research Laboratory had already adapted it for detecting chemical and biological toxins during the Persian Gulf War in 1991.

To adapt the biosensor for broader military and civilian public health applications, Research International fitted it with four strands of fiber optic cable (waveguides)—much like that used in phone lines—contained in a cassette about the size of a micro-audio tape. Each fiber optic waveguide is coated with antibodies, which are tailored to select and adsorb specific target microbial antigens. Samples from food or water sources can be injected into this cassette, which is slipped into an instrument about the size of a car battery. That instrument, in turn, squirts reagents containing similarly targeted antibodies but which are complexed with laser light-sensitive fluorescent molecules. When these reagent antibodies bind specific microbial antigens and are probed with an appropriate wavelength of laser light, they emit a light signal whose intensity can be converted and standardized to provide a reliable measure of pathogens present in the samples being tested.

"We are developing nucleic acid probes for use with the biosensor," Lim says. "In combination with immunoassays, such biosensor probes will make it possible to confirm the identity of a target analyte by both nucleic acid sequence and antigenic composition."

As it is currently configured, the instrument can detect bacterial pathogens such as Escherichia coli O157:H7 and Salmonella, Listeria, and Vibrio species in samples containing as few as 100 organisms per milliliter, according to Lim. Moreover, it is versatile in terms of types of sample—capable of directly analyzing microbes found in juice, drinking water, sewage-contaminated beach water, and foods such as ground beef. In addition to microbial pathogens, it also can detect specific toxins, including pseudexin, ricin toxin, Clostridium botulinum toxin A, and staphylococcal enterotoxin B,D-dimer. When equipped with multiple fiber optic waveguides coated with multiply targeted antibodies, it can also be used to detect and identify several target species simultaneously.

The speed, accuracy, versatility, and portability of the biosensor-based analytic system cannot be matched by other microbial pathogen detection techniques, according to Lim. Even relatively rapid-response analytic technologies such as ELISA, immunofluorescence, and metabolic fingerprinting require preliminary enrichment culturing, meaning they are slower and not very readily portable for field uses. Nucleic acid probes are sensitive and specific, but typically require clean samples and gene amplification by the polymerase chain reaction, a procedure usually done by trained personnel in specialized facilities.

The portable biosensor represents "a quantum leap ahead" over conventional technologies in water testing, says Mike Flanery, director of the environmental engineering department in Pinellas County, Fla. He and other officials in and near Tampa are keen to avoid prolonged closing of popular local beaches. "Currently, we take a sample, and by the time we hear back, tides and conditions change," he says, explaining how slow analytic techniques sometimes can keep beaches closed longer than they may need to be. "This method will be like taking a movie rather than a still picture, and it will helps us to pinpoint the source of the bacterium or virus."

"This is very exciting work," says microbiologist Joan Rose of USF, who specializes in water quality issues. "This means for beaches that when there is a storm or sewage spill, the site could be measured immediately." The biosensor is being field-tested as a monitor of beach water quality along the coastlines of Florida.

Despite such enthusiasm, the new biosensor technology faces a substantial challenge in terms of its high cost. Thus, direct testing costs $5-$6 for the biosensor compared to $3-$4 for most conventional microbiological techniques. Moreover, the $30,000 price tag for the instrument is pretty hefty, although Lim points out that this steep price is a short-term issue. "Computers were expensive when they first came out, but the prices really came down over time," he says. "As use [of the biosensor] increases, the cost will come down."

Brian Hoyle
Brian Hoyle is a freelance science writer based in Bedford, Nova Scotia, Canada.