Submitted to: United States Animal Health Association Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 6/18/2008
Publication Date: 10/29/2008
Citation: Guard, J.Y., Morales, C., Gast, R.K. 2008. Evolutionary trends of Salmonella Enteritidis linked to subpopulation biology and virulence attributes. United States Animal Health Association Proceedings. 480.
Technical Abstract: Salmonella enterica serovar Enteritidis (S. enteritidis) is currently the world’s leading cause of food borne salmonellosis. It is the only serotype out of over 1400 within Salmonella enterica I that contaminates the internal contents of the egg by vertical transmission from the reproductive tract of otherwise healthy hens. Epidemiological studies have shown that this exceptionally invasive pathogen with an unusual tissue tropism has a more clonal population structure than most other broad-host range Salmonella serotypes. In contrast to its clonal genomic structure, cell surface analysis and hen infection studies indicate that S. enteritidis generates more phenotype heterogeneity than does S. typhimurium. To resolve the conundrum of how a genome can look the same but yet generate heterogeneous subpopulations that vary in their ability to interact with the avian reproductive tract, comparative genome sequencing (CGS) of 3 whole genomes of S. enteritidis was performed in conjunction with high-throughput phenotype microarray (PM) and hen infection studies. Application of CGS revealed a genome that harbored approximately 200 small scale evolutionary events per strain as well as evidence of homologous recombination, insertion of uropathogenic genes from E. coli, and acquisition of single genes from S. typhi and other serotypes. PM analysis revealed striking divergence in physiological capabilities between strains, including the ability to metabolize D-serine and to grow well in the presence of a number of nitrogenous compounds, antibiotics and other compounds, some of which are host-derived. Results from hen infection studies that recovered bacteria from internal organs and measured egg production following infection supported the concept that some cultures could harbor three major subpopulations were sometimes present in both ST64b (PT4) and Fels2 (PT13a) bacteriophage lineage strains. These results indicate that the most virulent isolates of S. enteritidis are at least triphasic, which means that three prevalent phenotypes are inherently expressed from a single genome in response to environmental conditions. Strains that vary in their ability to contaminate eggs and to grow to high cell density are likely to vary in their ability to express all three developmental pathways because of the accumulation of small scale evolutionary events over time.