Listeria monocytogenes is a human and animal pathogen responsible for severe and sometimes fatal infections. Several outbreaks have been associated with contaminated commercial foodstuffs such as raw milk, soft cheese, fresh and frozen milk, poultry, seafood, fruits and vegetable products. Currently, the official method recognized by the Government of Canada for the detection and isolation of L. monocytogenes can take up to six days without confirmation, which can require two more days. An approach based on molecular beacons that fluoresce upon hybridization was developed and tested to detect L. monocytogenes and the genus Listeria in food. Two different beacons were created: one specific to species L. monocytogenes (MG1) and another for the genus Listeria (MG2). Each of these molecular beacons was used with two separate sets of primers: MG1-1f/MG1-2r, MG1-7f/MG1-4r, MG2-2f/MG2-2r and MG2-3ft/MG2-2r. (Abstract shortened by UMI.)

Cryptic plasmids ranging from 2.0 to 10C kb in size were isolated from 25 out of 122 Listeria monocytogenes strains, and from 7 out of 11 strains of other Listeria species. / Of 2500 clones of a genomic library of L. monocytogenes 81-861 generated in Escherichia coli cells, 5 clones were identified in which $ beta$-hemolytic activity was stably expressed. Testing by intraperitoneal injection showed that these clones were lethal to mice. Restriction mapping of the inserts of the recombinant plasmids showed that, apart from a 650-bp internal Hind III fragment in 2 inserts, there were no other common sites. No homology was demonstrated between the DNAs of the inserts when Southern blots of restriction digests of the 5 plasmids were probed, though homology was demonstrated between the L. monocytogenes listeriolysin O gene and the DNA of one insert. The evidence suggests that at least one additional $ beta$-hemolysin, other than listeriolysin O, exists in this strain of L. monocytogenes, and that it may be a virulence factor. / Using a direct colony hybridization procedure on hydrophobic grid-membrane filters (HGMFs), the inserts of the recombinant plasmids were screened, and a DNA probe specific for L. monocytogenes was identified. After labelling with horseradish peroxidase and colour development of the chromogen, a commercial counter (HGMF Interpreter) was able to detect and count the organism electronically. When the efficacy of the chromogen-labelled DNA probe method on HGMFs was compared with the conventional method for three artificially-inoculated foods, there were no significant differences ($ alpha$ = 0.05) shown in the recovery of L. monocytogenes from the foods.

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