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Forty illness associated phage-type (PT) 4 and PT 8 strains of Salmonella enteritidis were analyzed by the pulsed-field technique of clamped homogeneous electric fields (CHEF) electrophoresis. Using NotI and XbaI, the 40 strains were subdivided by each enzyme into seven restriction endonuclease digestion profiles (REDP). The 35 PT 4 isolates from Austria were subdivided into six NotI and five XbaI REDP, while the five PT 8 isolates from the United States displayed a single NotI and two XbaI REDP. When highly-concentrated, uncleaved genomic DNA was subjected to CHEF electrophoresis, plasmid DNA in the size range of 350 kb relative to a linear DNA standard was discernible in 38 of the 40 strains. Subsequent isolation and restriction analyses of plasmid DNA from one strain (E40) revealed a single plasmid (pE40; ca. 54 kb) with one XbaI and two NotI cleavage sites that was similar in size to the S. enteritidis virulence plasmid pRQ29. Hybridization of the PE40 probe with S. enteritidis genomic DNAs identified a 54 kb fragment within the XbaI REDP and two fragments, 20 and 34 kb, in NotI REDP of plasmid-positive strains. It was not possible to identify plasmid-specific bands in NotI REDP without hybridization due to comigrating chromosomal and plasmid DNA fragments. Regardless of PT, all 40 S. enteritidis strains showed highly related REDP. The similarity between PT 4 and PT 8 strains as further revealed by Dice similarity coefficients was 90% to 95% for NotI REDP and 79% to 93% for XbaI REDP. These results support the hypothesis that the pandemic observed today is the result of the efficient spread of a single clone, or clusters of closely related clones, of S. enteritidis.

Twenty-five Listeria monocytogenes strains of human and food origin were studied, using low-frequency-cleavage restriction endonucleases and pulsed field gel electrophoresis. Sixteen different restriction patterns were obtained using ApaI and SmaI. The human isolates showed profiles different from those of the food isolates, thus strongly suggesting that the two human infections selected for this study did not originate from the contaminated foodstuffs examined at the same time. This method allowed it to clearly distinguish between strains apparently similar [i.e. belonging to the same serovar and same origin (cheese samples from the same dairy plant)].