New tool on tap for fighting listeria

Pullman, Wash.-

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 (ARS) of the United States Department of Agriculture.

Subtyping determines the strain affiliation of Listeria specimens isolatedin the lab. This is critical to epidemiologists tracing outbreaks back totheir source, as well as to government and industry efforts to safeguardfood supplies through environmental monitoring, disinfection, sanitationand 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, 1/2band 4b are chiefly to blame.

ARS scientist Monica Borucki and Washington State University scientistsDouglas Call and Thomas Besser devised a technique called mixed genome microarrayanalysis to examine L. monocytogenes' DNA for genes that differ among itsstrains.

Identifying the genes will help the researchers learn why some strainscause disease epidemics, while others don't, and help them design subtypingmethods for identifying the most pathogenic strains. These methods couldthen be used to check for genetic evidence of the strains in food, on farmsor on food-processing equipment, according to Borucki, at ARS' Animal DiseaseResearch Unit.

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, calledmicroarray probes-onto special microarray slides. Next, they used fluorescenceto label the DNA of the strains they wished to subtype or genetically characterize,ARS explains. The labeled DNA was then applied to the slide, where it boundto probes with similar DNA. Computerized imaging software enabled the teamto examine the slides for DNA illumination patterns signaling the presenceof subtype-specific genes.

Eventually, the team hopes to parlay its microarray gene discoveriesinto a fast, standardized method of subtyping that public health labs canuse to compare large amounts of data on strains that may cause local and/ornational epidemics, USDA reports.