New technique differentiates among genes of Listeria DNA.

A new tool may be at hand for subtyping strains of L. monocytogenes which cause foodborne illness. 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 it kills 500. Of the bacterium's 13 known strains, serotypes 1/2a, 1/2b and 4b are chiefly to blame for the illness. Epidemiological studies and analysis of putative virulence genes have shown that L. monocytogenes has diverged into several phylogenetic divisions. The researchers believe that similar divergence has occurred for many genes that influence niche-specific fitness and virulence. Identifying these differences may offer new opportunities for the detection, treatment and control of this important pathogen.

In Pullman, WA, USDA-ARS scientist Monica Borucki and Washington State University scientists Douglas Call and Thomas Besser devised a technique--mixed genome microarray analysis--to examine the DNA of L. monocytogenes 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. This approach also will help them design subtyping methods for identifying the most pathogenic strains. These techniques could then be used to check for genetic evidence of the strains in food, on farms or on food-processing equipment.

In studies recently published in the Journal of Clinical Microbiology, the research team extracted DNA fragments from 10 representative Listeria strains. The investigators printed copies of them--in the form of hundreds of tiny dots, called microarray probes--onto microarray slides. Next, they used fluorescence to label the DNA of the strains they wished to subtype, or genetically characterize.

The labeled DNA was 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 scientists hope to parlay their microarray gene discoveries into a fast, standardized method of subtyping that health labs can use to compare large amounts of data on strains that may cause local or national epidemics.

Further information. Monica Borucki, USDA-ARS Animal Disease Research Unit, 3003 ADBF, Washington State University P.O. Box 646630, Pullman, WA 99164; phone: [phone]; fax: [phone]; email: [e-mail].