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Chapter 15 : Evolution of Enteric Pathogens
Category: Bacterial Pathogenesis; Microbial Genetics and Molecular Biology
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This chapter focuses on three significant human pathogens in the family Enterobacteriaceae: Salmonella enterica, Escherichia coli (including Shigella), and Yersinia spp. S. enterica has been assigned to serovars based on the combination of antigenic properties of the polysaccharide O antigen and flagellar H1 and H2 antigens. S. enterica is found in reptiles and warm-blooded vertebrates. The most common diseases caused by subspecies 1 serovars are acute enterocolitis and enteric fever. Virulence factors in S. enterica are commonly in pathogenicity islands (PAIs). Early studies on evolution and population genetics of the species were centered on isolates from healthy humans and animals. Enteropathogenic E. coli (EPEC) strains carry the locus of enterocyte effacement (LEE) PAI encoding a type III secretion system, with proteins for effacing and attachment, and the EAF plasmid with genes for adherence. The EHEC O157:H7 clone was discovered in the early 1980s when several outbreaks of hemorrhagic colitis were associated with the serotype O157:H7, and has since been implicated worldwide in outbreaks of food and waterborne disease. Bacteria reproduce by binary fission, and as a result, all bacterial populations are clonal to some extent. The Shigella clones of E. coli, the Pestis clone of Yersinia pseudotuberculosis, and serovar Typhi of S. enterica all have many pseudogenes, and are clearly losing many of their more general species properties as they become adapted to what appears to be in all three cases a new niche.
Key Concept Ranking
- Type III Secretion System
Evolutionary tree of Salmonella enterica. The tree shows phylogenetic relationships of the subspecies as determined by sequences of five housekeeping genes by Selander et al. ( 123 ), with the external edge and strain names removed. After the subspecies number is the number of serotypes reported for each subspecies ( 101 ), given in parentheses. Two subspecies IV serotypes were allocated to subspecies VII by genetic means (see text), and some of the remaining 69 subspecies IV serotypes may belong to subspecies VII. Major events are indicated, including the gain of SPI-1, SPI-2, and H2 antigen genes. The numbers at each node indicate the numbers of genes gained as determined by microarray analysis of LT2 genes ( 103 ).
Genetic relationships of commensal E. coli strains represented by the ECOR set strains and representative pathogenic E. coli strains as resolved by MLEE ( 104 ). For clarity, all strain names were removed from the tree (see original paper for details). External branches, where pathogenic E. coli strains are represented or the sole member, are marked with the pathogenic form, while other branches without names are ECOR set strains.
The evolutionary tree of Shigella and other E. coli based on housekeeping genes ( 105 ), with distribution of key characteristics of E. coli and Shigella (adapted from reference 75 ). The pINV form is indicated as A or B ( 74 ), or as a “+” if not known to be A or B, or non-A/non-B. The common properties of commensal E. coli are represented by K-12 (top row). Other rows correspond to the cluster or strain on the left. The Shigella strains in the three clusters are shown in the original paper. It can be clearly seen that Shigella, with the exception of Boydii 13, are clones of E. coli.
Distribution of E. coli proteins: core and unique genes among E. coli K-12, O157: H7 (EDL933), and uropathogenic E. coli CFT073 (adapted from reference 139 ). Areas represent categories of genes but are not proportional to the number of genes. The percentage within a category is based on the total of 7,638 proteins. The number of proteins used for comparison in each strain is indicated under the strain name, together with the percentage of core genes.
Serotypes associated with pathogenic E. coli categoriesa