FIG. 1.

Identification of 39 gene fragments specific for the epidemic L. monocytogenes strain F.4565 and cross-hybridization with other Listeria species, with reverse Southern blots of replica DNA arrays. PCR products were spotted four times each at 20 ng per spot onto positively charged nylon membranes and hybridized to whole-genome probes of Listeria strains. Membranes were washed under low-stringency conditions. The first row of the array contained eight known DNA fragments as hybridization controls. The second row contained two sets of four unrelated DNA sequences at 80, 40, 20, and 10 ng per spot as negative controls. The third row contained four sets with no DNA followed by three fragments common to the tester and driver strains (no. 64, 60, and 62). Epidemic-clone-specific fragments were spotted from left to right, starting with no. 17B and ending with no. 16 containing the 270-bp flanking vector sequence. Fragments 153 and 121 gave no signal and were excluded from the analysis. 65B is the same as U46. (A) A total of 39 fragments hybridized differentially with tester DNA. (B) Nine L. monocytogenes strain F.4565-specific fragments gave strong signals with L. innocua DNA, five gave signals with L. ivanovii DNA, and none gave signals with L. grayi DNA.