Do You Want Pathogens with That?

Biosensors for Rapid Food Screening
Contributed editorial appearing in
Scientific Computing & Instrumentation 19:10, September 2002, pg. 16.

If asked to compile a list of least-desirable job titles, I would place “Royal Taste Tester” near the top of my list. Thankfully, the number of advertisements for this position has diminished since the Middle Ages; however, the fundamental approach to food testing has not experienced a palpable change until recently. The latest numbers compiled by the Centers for Disease Control and Prevention (CDC) report 76 million cases of food-borne illness each year in the United States. These are not covert food poisonings perpetrated by dissidents seeking a new regime, rather they are pathogenic microorganisms such as bacteria, viruses, and parasites present in our food having insidious names including Campylobacter, Salmonella, Listeria, and E. Coli O157. The U.S. Food & Drug Administration (FDA) colloquially refers to these disease-causing agents as “bad bugs.” After ingestion, disease symptoms can take as little as a few hours to more than a week to appear. Although very rarely fatal, insuring the safety of our food supply is a large and important endeavor.

As a defense against these bad bugs, a Royal Taste Tester would not be very effective unless the monarch they were protecting was willing to eat very cold leftovers. The most prevalent approach to food testing is to collect food samples and place them in an environment conducive to bug growth and reproduction. Thankfully, this is no longer the small intestine of the Taste Tester, but environmentally controlled laboratory glassware. After an appropriate incubation period lasting from two to seven days, the resulting culture is analyzed for bug identification and quantity. If bad bugs are present at unacceptable levels, the food producer is notified. Since most food is not quarantined during the testing period, positive detection necessitates a recall of food that has already shipped to market. In some unfortunate cases, the recall is initiated only after consumers have begun to show symptoms of the food poisoning. Obviously, a rapid and accurate food analysis technique that can be completed before shipping or serving is highly desirable.

In the method of “selective enrichment,” media components used to culture the bugs are added or deleted to promote the growth of specific bacteria. Biolog, Inc., has incorporated the selective enrichment concept into its MicroPlate technology. The sample is introduced into a 96-well plate that has been coated with an engineered array of carbon sources (read “bug food”). A dye present in each of the wells changes from colorless to violet when the bugs metabolize the carbon source of the well. An image of the well plate records the intensity of the dye and compares the intensity pattern of the plate to an identification database containing 1900 species of bacteria, yeast and filamentous fungi. The intensely-colored dye amplifies the presence of small amounts of the bugs and identification can be achieved in as little as four hours.

In addition to using the specific eating habits of each bad bug as a detection method, researches have turned to nature for models of efficient identification. After exposure to a bad bug, the body’s immune system develops molecules called “antibodies,” that specifically and uniquely bind to the invading bug, known as the “antigen.” After the bug has been marked with an antibody, the immune system destroys the antigen by various methods. Researchers have employed the identification function of antibodies in the creation of antigen-specific “biosensors.” A biosensor is a data acquisition transducer incorporating antibodies that displays a calibrated physical or chemical change when the antibodies bind with the specific antigen. Using this approach, a biosensor can be constructed that is specific for the bad bugs that are commonly present in the food being analyzed.

A convenient biosensor is one that exhibits a change in optical properties, such as fluorescence intensity, between the complexed and uncomplexed antibody bound to the sensor. The U.S. Naval Research Laboratory in cooperation with Research International, Inc., has developed the portable RAPTOR fiber optic biosensor that is capable of detecting four target pathogens in water and fruit in as little as 10 minutes. Four individual diode lasers are used to illuminate a biosensor coupon that has been coated with fluorescent dye-tagged antibodies. The coupon continues to exhibit a negative response during continuous monitoring until the target antigen is detected. After which, a fresh coupon can be inserted and analysis continued. Smaller and more sensitive biosensors are being developed using MEMS technology for the detection of optical and mass changes during antibody/antigen complexation. The CDC reports these nascent detection technologies have reduced the incidence of food poisoning to 23 percent of 1996 levels. Be on the lookout for biosensors incorporated into food containers. Perhaps this technology will push ads for Royal Taste Tester to the section containing Buggy Whips.