Molecular technique offers high degree of specificity.

Listeriosis, the illness caused by L. monocytogenes infection, affects around 2,500 people in the United States every year. It kills about 500. Newborns, seniors, pregnant women and individuals with compromised or weakened immune systems are particularly susceptible.

Most techniques used to detect foodborne bacteria rely on antibodies, proteins used by the immune system to fight infections and foreign bodies. Because these antibodies target very specific infections, researchers can use them to identify and locate specific pathogens.

Antibodies vary in their degree of specificity. Current antibody-based methods for detecting L. monocytogenes can't distinguish this bacterium from mixtures of harmless bacteria found in most foods. A molecular methodology, called phage display, uses bacteria and bacterial viruses, or phages, to quickly select antibodies to detect pathogens. USDA-ARS microbiologists and chemists have employed phage display to isolate an antibody fragment that binds specifically to L. monocytogenes.

This may be the first antibody to demonstrate the high degree of specificity required to accurately detect this pathogen. The researchers' success demonstrates that antibody phage display can be used to select antibodies for pathogen detection--even where traditional methods have proved inadequate. Investigators also are using phage display to select antibodies targeting other difficult-to-detect bacterial pathogens related to food safety and security.

Scientists have characterized the interaction of the antibody with the bacterium and identified the part of the L. monocytogenes cell to which this antibody binds. They linked the antibody fragments to microscopic magnetic beads, creating an L. monocytogenes-specific immunomagnetic bead (IMB). Because the antibodies bind to specific targets, the anti-L. monocytogenes IMBs can be used to capture just L. monocytogenes from the bacterial mix found in a product.

This is the first step in developing a significantly improved detection method for L. monocytogenes. This work will allow researchers to develop much-needed rapid tests for the pathogen, reducing the likelihood that contaminated food will reach consumers.

Further information. Shu-I Tu, USDA-ARS Microbial Biophysics and Residue Chemistry Research Unit, Eastern Regional Research Center. 600 E. Mermaid Lane, Wyndmoor, PA 19038; phone: [phone]; fax: [phone]; email: [e-mail].