Pathogenomics and Source Dynamics of Salmonella enterica Serovar Enteritidis

Open Access
Author:
Moreau, Matthew R
Graduate Program:
Pathobiology
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
March 06, 2015
Committee Members:
  • Subhashinie Kariyawasam, Dissertation Advisor
  • Bhushan M Jayarao, Dissertation Advisor
  • Mary J Kennett, Committee Member
  • Matam Vijay Kumar, Committee Member
Keywords:
  • Salmonella Enteritidis
  • Pathogenomics
  • Source dynamics
  • Molecular mechanisms
  • host-pathogen interactions
Abstract:
Salmonella enterica serovar Enteritidis (SE) is one of the most frequent common causes of morbidity and mortality in humans due to consumption of contaminated eggs and egg products. The association between egg contamination and foodborne outbreaks of SE suggests egg derived SE might be more adept to cause human illness than SE from other sources. Therefore, there is a need to understand the molecular mechanisms underlying the ability of egg-derived SE to colonize the chicken intestinal and reproductive tracts and cause disease in the human host. To this end, the present study was carried out in three objectives. The first objective was to sequence two egg-derived SE isolates belonging to the PFGE type JEGX01.0004 to identify the genes that might be involved in SE colonization and/or pathogenesis. Both genomes were almost identical (99% identity) being approximately 4.67Mb in size and the GC content was 52%. Both genomes contained about 4,600 open reading frames, of which 600 (or 12.5% of the genome) were related to virulence. Nine genes contained single nucleotide polymorphisms (SNPs) when these virulence-associated genes of egg isolated SE were compared with human isolated SE providing evidence of host-adapted microevolution of SE. Among these SNPs, two resulted in non-conservative changes in a fimbrial usher and a lipopolysaccharide biosynthesis gene, whereas four SNPs were located in promoter regions. The second objective of the study was to identify the host-specific genes (poultry vs. human) by pan-genomic analysis of the newly sequenced genomes and the genomes of Salmonella serovars published in the NCBI database. There were approximately 2, 800 cluster of orthologous genes (COGs) conserved among all serovars tested of which 247 were associated with Salmonella virulence. These core virulence genes may be associated with colonization and/or subsequent infection in all host species whereas the ‘host-specific’ genes most likely determine the host-specific mechanisms. This analysis identified 10 poultry-specific potential virulence genes, including the genes of two fimbrial operons, lpf- and sti- and four genes with hypothetical functions. Twelve other genes (e. g. iroD, ttrB/C, and sthA) were present only in Salmonella serovars that can infect the human host. The third objective of the study was to elucidate if the source attributes have an effect on SE virulence. Here, the colonization ability and virulence potential of SE grown in Luria broth (LB) medium and egg yolk in the laboratory and SE recovered from feces of mice experimentally infected with SE) were compared using a mouse colitis model of SE infection. The results demonstrated that SE grown in the egg yolk possesses enhanced colonization, shedding, and virulence capabilities as compared to the SE grown in the LB medium or SE recovered from mouse feces as determined by clinical signs, gross pathology, histological lesion scoring, and bacterial enumeration of feces, small and large intestines, and internal organs of the infected mice. These data suggest that egg yolk may condition SE to be better ‘primed’ for transmission to and infection of a second host by upregulating the expression of certain genes. Future studies should be directed towards understanding the mechanisms involved in microevolution of SE virulence in egg yolk using approaches, such as microarray and RNA sequencing. In conclusion, this study provides new insights into the current understanding of SE virulence and identifies potential targets for rational development of vaccines and antimicrobials to minimize human foodborne illness due to SE.