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family Enterobacteriacea. GeneraHuman, animal pathogensSalmonella, Yersinia, Aeromonas, E.coli/ShigellaCommensals/symbionts of animals E.coli, CitrobacterPlant pathogensErwinia Environmental bacteriaAeromonas, KlebsiellaWhy so different?Links to plant-related past?Most grow on plant-deriv
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1. Evolution of enteric pathogens
2. family Enterobacteriacea Genera
Human, animal pathogens
Salmonella, Yersinia, Aeromonas, E.coli/Shigella
Commensals/symbionts of animals
E.coli, Citrobacter
Plant pathogens
Erwinia
Environmental bacteria
Aeromonas, Klebsiella
Why so different?
Links to plant-related past?
Most grow on plant-derived sugars
Salmonella, E.coli cannot break down pectin. Yersinia, Klebsiella can
3. Evolution of Salmonella Serovars are differentiated based on antigens
O polysaccharide antigen
H1, H2 flagella
2,501 serovars are recognized
Some (sv Typhimurium, Newport) have a broad host range
Gallinarum and Pullorum --> chickens (fowl typhoid)
Typhi, Parathyphi --> primates (typhoid fever)
Cholerasuis --> pigs (pig paratyphoid)
Dublin --> cattle (bacteremia)
Other svs are host-adapted
Not all serovars are virulent
Sv Bongori
Sv Sophia
Common in chickens in Australia, not known to have caused disease in humans
All Salmonella have pathogenicity islands (SPIs)
4. SPI’s SPI’s = Salmonella pathogenicity islands. Absent in E.coli
SPI-1 carries genes involved in epithelial cell invasion. 40-kb, chromosomal
mutants are virulent only if delivered by direct injection
SPI-2 required for intracellular survival during infection
even when injected, mutants are not fully virulent
SPI-3 consists of 10 genes required for Mg2+ acquisition and growth in macrophages.
Present in some serovars, lost from others
SPI-4 carries 18 genes involved in survival within macrophages
SPI-5 contains 6 genes, 4 of which are required for enteritis in a calf model
5. Evolution of Salmonella: relatedness of subspecies
6. Evolution of Salmonella: relatedness of subspecies
7. Evolution of virulence in E.coli Normally a commensal in human gut
occupies large intestine and lower small intestine
the only gut inhabitant using oxygen and thus maintains anaerobiosis
virulent and commensal strains do not cluster separately (i.e. many commensal strains have closely related pathogens)
some E.coli sv are closely related to different Shigella sp
8. Virulent E.coli strains
Enterotoxigenic
plasmid-borne genes for enterotoxins
adhesin pili
a non-invasive pathogenic E.coli
Enteropathogenic.
carry LEE island encoding proteins involved in attachment, a protein injection system (Type 3)
EAF plasmid with genes for adherence
Enterohemorrhagic
LEE island carries a phage-encoded Shiga toxin
virulence EHEC plasmid
Enteroaggregative
Enteroinvasive (include Shigella)
carry pINV
Unlike “typical” E.coli, Shigella can’t utilize lactose, mannitol are non-motile.
Most enteroinvasive E.coli have the same deficiencies
Uropathogenic
9. EHEC O157:H7 Discovered in the 1980s
Common in cattle.
commensal in cattle
pathogen in humans
Virulence factors
LEE
stx (Shiga toxin) and EHEC plasmid are possibly recent acquisitions
Evolution:
gained O157 by recombination,
then split into two lineages: sorbital-negative and b-glucuronidase negative
both major lineages carry phage-encoded stx genes
10. How related are E. coli?
11. Evolution of Yersinia 11 species, only 3 are significant pathogens
Y. pseudotuberculosis, Y. enterocolitica
food-, waterborne pathogens that cause gastroenterocolitis
share virulence factors not found in non-pathogenic isolates
escape human immune system thanks to an outer membrane protein encoded by a 70-kB virulence plasmid
carry virulence determinants (production of siderophore) on a High Pathogenicity Island (HPI)
HPI is highly mobile and was found in other enterics
chromosomally encoded virulence factors also present
12. Evolution of Yersinia Y. pestis
causes plague
transmitted by fleas
a clone of Y. pseudotuberculosis. Indistinguishable by common methods
distinct from Y. pseudotuberculosis in its virulence strategy
colonizes flea gut.
genes on pFra plasmid are involved
is transmitted to a host through a bite
reverse blood flow during biting due to partial blockage of the flea’s proventriculus. Blood meal is regurgitated. hemin storage genes are found in both Y. pestis and Y. pseudotuberculosis
disseminates in blood from the infection site
13. Population genetics of enterics All bacterial populations are clonal to some extent
Recombination
important source of variation
for enterics, recombination appears more important then mutation
frequency differs for different species
Neisseria -- frequent, Salmonella -- rare
Through recombination clones adapt to specific niches
14. Conclusions Many virulence determinants are carried on mobile genetic elements
Pathogenicity islands, virulence plasmids are shared between clones and species
Surface antigen polymorphism
O antigen is the main target for immune system and phages (high selective pressure)
Gene decay
Genes mutate, and loss of function is OK
e.g. Shigella is non-motile, can’t utilize lactose or decarboxylate lysine.
Occupies a different niche (inside a eukaryotic cell) than commensal E.coli (in the gut of mammals where lactose is found). Making flagella is expensive, flagella is also an antigen