New Tool on Tap for Fighting Listeria

By Jan Suszkiw
May 27, 2003

A new tool could be at hand for "subtyping" strains of Listeria monocytogenes bacteria that cause food-borne illness, thanks to scientists with the Agricultural Research Service.

Subtyping determines the strain affiliation of Listeria specimens isolated in the lab. This is critical to epidemiologists tracing outbreaks back to their source, as well as to government and industry efforts to safeguard food supplies through environmental monitoring, disinfection, sanitation and other measures. In the United States, listeriosis sickens an estimated 2,500 people annually, and kills 500. Of the bacterium's 13 known strains, serotypes 1/2a, 1/2b and 4b are chiefly to blame.

In Pullman, Wash., ARS scientist Monica Borucki and Washington State University scientists Douglas Call and Thomas Besser devised a technique called mixed genome microarray analysis to examine L. monocytogenes' DNA for genes that differ among its strains. Identifying the genes will help the researchers learn why some strains cause disease epidemics, while others don't, and help them design subtyping methods for identifying the most pathogenic strains. These methods could then be used to check for genetic evidence of the strains in food, on farms or on food-processing equipment, according to Borucki, at ARS' Animal Disease Research Unit in Pullman.

In studies recently published in the Journal of Clinical Microbiology, the team extracted DNA fragments from 10 representative Listeria strains. They printed copies of them--in the form of hundreds of tiny dots, called microarray probes--onto special microarray slides. Next, they used fluorescence to label the DNA of the strains they wished to subtype, or genetically characterize. The labeled DNA was then applied to the slide, where it bound to probes with similar DNA. Computerized imaging software enabled the team to examine the slides for DNA illumination patterns signaling the presence of subtype-specific genes.

Eventually, the team hopes to parlay its microarray gene discoveries into a fast, standardized method of subtyping that public health labs can use to compare large amounts of data on strains that may cause local and/or national epidemics.

ARS is the U.S. Department of Agriculture's chief scientific research agency.