Selective enrichment medium offers simultaneous detection of bacteria.

When using a detection system, a proper selective enrichment medium is needed to detect multiple pathogens in a food sample. Currently, there is no single selective enrichment medium that can support the growth of E. coli, L. monocytogenes and Salmonella concurrently while suppressing the growth of other common food contaminants.

Researchers at Purdue University have developed a selective enrichment medium that will allow the simultaneous growth of E. coli O157:H7, L. monocytogenes and S. enteritidis for use with a suitable detection system. A selective medium designated SEL (Salmonella, E. coli and Listeria) was formulated by scientists who compared its performance with other common selective media--modified EC broth with novobiocin for E coli O157:H7, Fraser broth for Listeria, and Rappaport-Vassiliadis broth for Salmonella. The scientists used a lateral flow-based immunoassay and viable plate counting to make the comparison.

In addition, the growth of mixed cultures of E. coli O157:H7, L. monocytogenes V7 and S. enteritidis PT1 was determined in the SEL broth. Each was detected in a bacteria-specific lateral flow immunoassay strip to evaluate the ability of SEL to facilitate the detection of several foodborne pathogens simultaneously.

Each sample of E. coli O157:H7, Listeria and Salmonella grew well in SEL individually or in a mixture. The growths were comparable to those of commercially available selective enrichment broths for individual organisms. Each organism gave a strong signal in the lateral flow immunoassay after growth in the SEL. The signal was also comparable or better when grown separately in respective selective enrichment broths.

The SEL medium supported the growth of the three major foodborne pathogens individually or in a mixture and made it possible to detect each by immunoassay. This suggests that this medium, coupled with a suitable detection system, can facilitate the rapid detection of multiple pathogens concurrently from a product, thus reducing the overall cost and time of pathogen testing.

Further information. Arun Bhunia, Department of Food Science, Purdue University, 1160 Food Science Building, West Lafayette, IN 47907; phone: [phone]; fax: [phone]; email: [e-mail].