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U.S. Department of Health and Human Services

Animal & Veterinary

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New Analytic Techniques in the Fight Against Foodborne Disease

FDA Veterinarian Newsletter 2007 Volume XXII, No VI

In 1993, a large outbreak of foodborne illness in the western United States affected some 800 people. Use of a new scientific technique may have kept this outbreak from growing larger and becoming more widespread.

During the 1993 outbreak, scientists at the Department of Health and Human Services’ Centers for Disease Control and Prevention (CDC) used the relatively new pulsed field gel electrophoresis (PFGE) technique. PFGE identified clinical and food isolates of the Escherichia coli strain found in patients sickened by the foodborne illness. The technique also linked those isolates to a strain of Escherichia coli isolated at the same time from hamburger patties served in a large, regional, fast-food chain. Prompt identification of the cause of this outbreak may have prevented many more cases of illness.

The basic gel electrophoresis technique developed in the 1930s can be used to separate nucleic acid and protein mole-cules using an electric current applied to a porous gel matrix. In most cases, the gel is a cross-linked polymer whose composition and porosity are chosen based on the specific weight and composition of the target to be analyzed. When proteins or small nucleic acids are involved, the gel is usually made up of varying concentrations of acrylamide cross-linked to produce different sized mesh networks of polyacrylamide.

Electrophoresis refers to the electromotive force that is used to move the molecules through the gel matrix. By placing the molecules in depressions in the gel and applying an electric current, the molecules move through the matrix at differ-ent rates, usually based on size. Standard gel electrophoresis techniques for separating DNA molecules provided huge advantages for molecular biology research. However, many limitations existed with the standard protocol because it was unable to separate very large molecules of DNA effectively.

In 1984, researchers developed a variation on the standard protocol by introducing an alternating voltage gradient to improve the resolution of larger molecules. This technique came to be known as PFGE.

PFGE is like a standard gel electrophoresis, except that the voltage is reversed periodically (pulsed) to make each band of DNA run in the opposite direction for a set time, rather than constantly running the voltage in one direction. The development of PFGE significantly expanded the range of resolution for DNA fragments.

Following the 1993 foodborne illness outbreak, CDC developed standardized PFGE methods that could be used for “fingerprinting” bacteria isolated from sick persons and from possible sources of contamination to determine if the bacteria are similar. In 1995, with the assistance of the Association of Public Health Laboratories, CDC selected the state public health laboratories in Massachusetts, Minnesota, Washington, and Texas as area labs for a national molecular subtyping network for foodborne bacterial disease surveillance. CDC transferred its newly standardized PFGE typing and pattern analysis technology to the area laboratories so the labs could assume responsibility for subtyping foodborne pathogenic bacteria from their states and provide subtyping service to neighboring states that requested assistance.

The PFGE process now provides one of the key tools for the operations of PulseNet, FoodNet, and NARMS.