Use of specific oligonucleotides for direct enumeration of Listeria monocytogenes in food samples by colony hybridization and rapid detection by PCR.

Two 18-mer oligonucleotides derived from the sequence of hly, the gene coding for listeriolysin O, were shown to be specific for Listeria monocytogenes in the genus Listeria in colony hybridization tests. The oligonucleotides did not hybridize with any of the bacterial species found in food and co-isolated with Listeria on selective media. They were used in colony hybridization tests for enumeration of L. monocytogenes present in food samples after direct plating on selective media plates. In addition, two 24-mer oligonucleotides, each including the sequence of one of the 18-mers, were successfully used for the PCR-based detection of L. monocytogenes bacilli present in food samples after 48-h enrichment period. Using this technique, as little as 10(2) bacteria per ml of enrichment broth can be detected.