High-Tech Tools for Food Safety Sleuths

by Food and Drug Administration (FDA)

A geneticist couples two DNA strands, one natural from bacteria and one synthetic, and forms a hybrid that positively identifies a species of food-borne bacteria responsible for causing severe illness and death. In another laboratory, a chemist, using mass spectrometry, tracks the amount of an unwanted chemical in fermented products to parts per billion.

Both are FDA food scientists working on the persistent problems caused by contaminants that sometimes creep into what we eat and drink. Often the problem is bacterial, causing rapid development of illness. Other times it's chemical, with illness occurring immediately or at some future time because of cumulative effects.

Health problems from contaminants in foods are well documented. For example, a particularly virulent species of Listeria bacteria causes listeriosis, a disease the national Centers for Disease Control estimates results in 1,850 illnesses and 425 deaths annually in the United States. An example of a chemical problem is the presence of ethyl carbamate, a byproduct of fermentation, in alcoholic beverages and other fermented products. This chemical has been known to cause cancer in laboratory animals.

Up-to-the-minute technology helps food scientists lessen the dangers from these and other food contaminants.

The Need for Speed

Listeria monocytogenes, the species that causes listeriosis, was the culprit in one of the United States' most tragic food-borne illness outbreaks. In 1985, 48 people from the Los Angeles area who had eaten soft cheese contaminated with the bacteria died, and 94 others became ill.

More recently, last July food and agriculture ministries in France reported an outbreak of listeriosis that had started four months earlier. At the time of the report, there were 108 cases from all regions in France, including 21 deaths of newborn or elderly persons and five spontaneous abortions. The source of the outbreak had not yet been identified.

Serious complications of listeriosis include meningitis (brain infections) and septicemia (bacteria in the bloodstream). For pregnant women, the disease can be transmitted to the fetus, resulting in similar complications in the newborn, or miscarriage or stillbirth.

At the time of the Los Angeles outbreak, the deadly potential of the bacteria made quick identification imperative so that whatever food was causing the illness could be removed from grocery shelves immediately. But it took nearly a month using traditional laboratory methods to positively determine that L. monocytogenes had caused the illness.

Recognizing the need for a speedier laboratory technique to identify the bacterial species without sacrificing accuracy, Atin R. Datta, Ph.D., a geneticist with FDA's division of microbiology, and his FDA associates went to work on the problem. Datta's team developed a synthetic gene probe to positively identify L. monocytogenes, and today, food scientists can identify L. monocytogenes in only two to four days.

FDA's DNA probe has been accepted worldwide. After researchers in France find the source of the recent listeriosis outbreak there, it is expected that they will use the DNA probe to save time in positively identifying the bacteria.

How the DNA Probe Works

All food, unless it has been sterilized and packaged in a sterile container, contains many types of bacteria. Most are harmless to healthy people, but some, like L. monocytogenes, are capable of causing serious problems. In this procedure, the Listeria class of bacteria is isolated and if L. monocytogenes is present, it can be identified and counted.

In the first part of the DNA probe procedure, Datta spreads a diluted sample of food suspected of containing Listeria on the surface of a selective agar medium (a gelatinous substance). This allows Listeria bacteria to multiply by suppressing the growth of most other bacteria normally present in many foods. Within two days, the Listeria forms colonies on the agar.

When the colonies are formed, Datta presses membrane filter paper onto the agar plates containing the bacterial colonies. This transfers the bacterial colonies to the filter paper, giving the colonies a firm support base for the next step — colony hybridization.

In hybridization, bacterial colonies on the filter paper are treated with microwaves and strong alkaline solutions to break open the cells and release the DNA, uncoupling the natural double-stranded DNA into single strands. Next, synthetically produced, radioactive-labeled, single strands of L. monocytogenes DNA (called gene probes) are added to the bacterial colonies on the filter paper.

If L. monocytogenes is present, the synthetic gene probe finds it among the natural single strands of DNA and binds with it, forming a hybrid DNA molecule. The probe has been designed in such a way that only L. monocytogenes DNA will form the hybrid molecule; other bacterial colonies will not bond to this probe. (See accompanying diagram.)

In the last step of the DNA probe procedure, Datta places a sheet of x-ray film on the filter paper holding the hybrid DNA molecules. Each colony containing hybrid molecules leaves a dark spot on the film. Because each colony has grown from a single cell, the number of dark spots tells Datta how many L. monocytogenes cells were in the original sample.

Other types of DNA probes are used in FDA labs for problems with Shigella, Escherichia coli, and other bacteria that cause food-borne illnesses. They're all labeled with radioactive material.

Handling radioactive material is a health hazard, and disposal is a costly environmental problem. (Currently, only three locations in the United States have appropriate disposal facilities for low-level radioactive waste.) So Datta and his associates are developing and testing a method that labels DNA probes with a non-radioactive material — horseradish peroxidase.

DNA probes labeled with horseradish peroxidase could be used to detect L. monocytogenes in foods in the same way as radioactive-labeled probes. Once this technique is standardized, Datta believes it will replace radioactive probes, not only for Listeria but also for other food-borne pathogens.

About the Author

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FDA is A United States government body that oversees medical devices, including contact lenses, intraocular lenses, excimer lasers and eyedrops. In the US, these products must be approved by the FDA before they can be marketed.