Differences in Gene Content betweenSalmonella entericaSerovar Enteritidis Isolates and Comparison to Closely Related Serovars Gallinarum and Dublin

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Abstract
Salmonella entericaserovar Enteritidis is often transmitted into the human food supply through eggs of hens that appear healthy. This pathogen became far more prevalent in poultry following eradication of the fowl pathogenS. entericaserovar Gallinarum in the mid-20th century. To investigate whether changes in serovar Enteritidis gene content contributed to this increased prevalence, and to evaluate genetic heterogeneity within the serovar, comparative genomic hybridization was performed on eight 60-year-old and nineteen 10- to 20-year-old serovar Enteritidis strains from various hosts, using aSalmonella-specific microarray. Overall, almost all the serovar Enteritidis genomes were very similar to each other. Excluding two rare strains classified as serovar Enteritidis in theSalmonellareference collection B, only eleven regions of the serovar Enteritidis phage type 4 (PT4) chromosome (sequenced at the Sanger Center) were absent or divergent in any of the other serovar Enteritidis strains tested. The more recent isolates did not have consistent differences from 60-year-old field isolates, suggesting that no large genomic additions on a whole-gene scale were needed for serovar Enteritidis to become more prevalent in domestic fowl. Cross-hybridization of phage genes on the array with related genes in the examined genomes grouped the serovar Enteritidis isolates into two major lineages. Microarray comparisons of the sequenced serovar Enteritidis PT4 to isolates of the closely related serovars Dublin and Gallinarum (biovars Gallinarum and Pullorum) revealed several genomic areas that distinguished them from serovar Enteritidis and from each other. These differences in gene content could be useful in DNA-based typing and in understanding the different phenotypes of these related serovars.