New method of subtyping Listeria

A new tool for subtyping strains of Listeria monocytogenes bacteria that cause food-borne illness has been developed in America by USDA Agricultural Research Service (ARS) scientists.

Subtyping is a process that determines the strain affiliation of Listeria specimens. This process is vital for epidemiologists attempting to trace outbreaks back to their source and for governments attempting to safeguard food supplies through environmental monitoring disinfection and sanitation.

The new method can be used to check for genetic evidence of the strains in food, on farms or on food-processing equipment.

The new technique, called mixed genome microarray analysis, examines Listeria monocytogenes' DNA for genes that differ among its strains. Identifying these genes helps researchers understand why some strains cause disease epidemics, while others don't.

In studies recently published in the Journal of Clinical Microbiology, the ARS 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 characterise. The labeled DNA was then applied to the slide, where it bound to probes with similar DNA.

Computerised imaging software enabled the team to examine the slides for DNA illumination patterns, signaling the presence of subtype-specific genes.

USDA​ hopes to standardise this method of subtyping so that public health labs can compare large amounts of data on strains that may cause local and/or national epidemics.

Cold leftover meat and undercooked chicken have been linked to causing listeriosis. This food-borne illness affects an estimated 2,500 people a year in the US, and kills 500.

Of the bacterium's 13 known strains, serotypes 1/2a, 1/2b and 4b are chiefly to blame.