GENOMIC ANALYSES OF SALMONELLA-ENTERITIDIS PHAGE TYPE-4 STRAINS FROM AUSTRIA AND PHAGE TYPE-8 STRAINS FROM THE UNITED-STATES

Forty illness associated phage-type (PT) 4 and PT 8 strains of Salmone lla enteritidis were analyzed by the pulsed-field technique of clamped homogeneous electric fields (CHEF) electrophoresis. Using NotI and Xb aI, the 40 strains were subdivided by each enzyme into seven restricti on endonuclease digestion profiles (REDP). The 35 PT 4 isolates from A ustria were subdivided into six NotI and five XbaI REDP, while the fiv e PT 8 isolates from the United States displayed a single NotI and two XbaI REDP. When highly-concentrated, uncleaved genomic DNA was subjec ted to CHEF electrophoresis, plasmid DNA in the size range of 350 kb r elative to a linear DNA standard was discernible in 38 of the 40 strai ns. 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. en teritidis virulence plasmid pRQ29. Hybridization of the PE40 probe wit h S. enteritidis genomic DNAs identified a 54 kb fragment within the X baI REDP and two fragments, 20 and 34 kb, in NotI REDP of plasmid-posi tive strains. It was not possible to identify plasmid-specific bands i n NotI REDP without hybridization due to comigrating chromosomal and p lasmid DNA fragments. Regardless of PT, all 40 S. enteritidis strains showed highly related REDP. The similarity between PT 4 and PT 8 strai ns 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 effic ient spread of a single clone, or clusters of closely related clones, of S. enteritidis.