Escherichia coli

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Escherichia coli

• Ubiquitous, but a pathogen of major importance
  when equipped for it
• Infects many organ systems
• septicemia, often with meningitis
• most common as pathogen of urinary, GI tracts
• mastitis (domestic animals)
• Serotyping O (LPS), H (flagellar)

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General Characteristics

• Virotypes
• enterotoxigenic E. coli (ETEC)
• enteroaggregative E. coli (EaggEC)
• enteropathogenic E. coli (EPEC)
• enterohemorrhagic E. coli (EHEC)
• enteroinvasive E. coli (EIEC)

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Virotypes - Pathogenic E. coli
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Enterotoxigenic E. coli

• Disease similar to cholera, usually less severe
• adhere, elaborate toxins, cause diarrhea
• no apparent histologic changes in host cells
• diarrhea accompanied by vomiting and fever
• ETEC diarrhea can be fatal, especially in infants
  and young children
• non-native adults gt travelers diarrhea

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• Two Types of Enterotoxin
• Heat-labile toxin (LT)
• structure and mechanism very similar to cholera
  toxin, same receptor
• A-B type ADP-ribosylating toxin
• 1 A (enzymatic) subunit, 5 identical B (binding)
  subunits
• intact A not enzymatically active until nicked to
  Al, A2
• Al subunit released by reduction of disulfide
  bond

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• enters host cell, ADP ribosylates membrane
  protein Gs
• regulates activity of host cell adenylate cyclase
  (AC)
• active (GTP-bound) form of Gs increases activity
  of AC
• inactive GDP-bound form renders adenylate cyclase
  inactive
• ADP-ribosylation of Gs short-circuits off-on
  control by locking Gs in "on" form
• cAMP accumulates, alters activities of Na and Cl
  transporters
• ion imbalance leads to water loss diarrhea

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• Heat-stable toxin a (STa)
• not inactivated by 100C, 30 min
• acts on guanylate cyclase, yields excess of cGMP
  (not cAMP)

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Virotypes - Pathogenic E. coli
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Cholera

• Serious epidemic disease caused by Vibrio
  cholerae
• killed millions, continuing major health problem
  worldwide
• endemic in India, severe epidemics elsewhere
  (Americas)
• 1911 last epidemic-related case in US sporadic
  past 15 years
• Source water contaminated with human feces
• Mortality rate
• without adequate therapy 60
• with replacement of fluids, electrolytes 1
• Treatment and prevention
• oral, IV rehydration therapy
• vaccine

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Pathogenesis

• Gastric acidity is important defense
• Attachment (small intestine) via pili
• organisms unable to attach are avirulent
• Production of cholera toxin (CT)
• normal net flow of ions from lumen to tissue,
  net uptake of water from lumen
• CT decreases net flow of Na into tissue, causes
  net flow of Cl (and water) into lumen

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Pathogenesis

• CT is an A-B type ADP-ribosylating toxin
• ctxA, ctxB form an operon transcribed together
• 1 A (enzymatic) subunit, 5 B (binding) subunits
• same structure, mechanism as E. coli LT
• In small intestine (continued)
• massive diarrhea, electrolyte imbalance
• net loss of 20 l of water daily
• Dilute feces with mucus called rice water stool
• Lesions very mild, no invasion

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Regulation of Virulence Genes

• Transcription of ctxAB operon affected by
  environmental factors
• pH (transcription higher at pH 6 than at pH 8.5)
• temperature (transcription higher at 30C than at
  37C)
• 20 other genes regulated similarly regulon
• 3 controlling proteins ToxR, ToxS, ToxT
• ToxS receives signal, changes conformation,
  phosphorylates ToxR, converts to DNA binding form
• ToxR spans membrane, binds to operator region of
  ctxAB, activates expression
• ToxR also activates expression of ToxT
• activates expression of other genes, including
  tcpA

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Enteroaggregative E. coli

• Persistent diarrhea in children
• Resemble ETEC strains bind to SI cells,
  noninvasive
• Differ from ETEC strains
• do not adhere uniformly to mucosal surface
• clump in small aggregates, pilus-mediated
• Produce ST-like toxin
• called EAST (enteroaggregative ST)

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Virotypes - Pathogenic E. coli
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Enteropathogenic E. coli

• Patchy adherence
• aggregates different than EAEC strains
• nonintimate binding, mediated by pili
• alterations in mucosal cell structure
  attaching-effacing
• destruction of microvilli, formation of
  pedestal-like structure in cytoplasm
• composed of dense mat of actin fibers
• No LT, ST, other known diarrheal toxin
• possible undetected toxin
• possible loss of absorptive capacity of mucosal
  cells
• inflammation, subversion of signal transduction
  systems

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Virotypes - Pathogenic E. coli
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Enterohemorrhagic E. coli

• Bind tightly to cells
• same type of attachment-effacement as EPEC
• invade, not as readily as Shigella
• Difference from EPEC produce Shiga-like toxin
  (SLT)
• Disease more similar to Shigella infection than
  ETEC or EPEC diarrhea
• dysentery hemolytic-uremic syndrome can follow
  (SLT)
• ETEC, EPEC diarrhea most common in developing
  countries
• EHEC disease mainly in developed countries so far
• One predominant serogroup and serotype (O157H7)

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Virotypes - Pathogenic E. coli
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Pathogenesis of Edema Disease
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Edema Disease of SwineF18 pili, Shiga-like toxin
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Gram Stain Impression Smear
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Enteroinvasive E. coli

• Disease indistinguishable from dysentery caused
  by Shigella sp.
• actively invade colonic cells, spread laterally
  to adjacent cells
• steps virtually identical to those in Shigella
  sp.
• HUS not reported as complication in EIEC
  dysentery
• do not produce Shiga toxin (could explain lack
  of HUS)

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Shigella spp.General Characteristics

• Gram negative, nonmotile
• no flagella, no H (flagellar) antigens
• Taxonomy
• 4 species cause disease
• S. flexneri, S. sonnei, S. boydii, S. dysenteriae
• similar virulence factors, mechanisms
• narrow distribution, natural disease rare in
  nonhumans

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Shigellosis

• ID for humans 100-200 cfu
• Dysentery (large intestine, blood, mucus)
• colon, terminal ileum ulcers, pseudomembrane
• infections of proximal ileum watery diarrhea
  only
• penetration beyond submucosa rare, no bacteremia,
  mainly self-limiting
• severe dehydration may occur, fatality rates to
  20
• 3-50 followed by neurological (severe headache,
  convulsions) or renal (HUS) complications
• damage to blood vessels (Shiga toxin, LPS)

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Pathogenesis

• Invasion normally describes active penetration
  of host cell membrane by bacteria
• Shigella sp. causes induced phagocytosis, host
  cell plays active role
• Entry through Peyer's patches, via M cells
• naturally phagocytic, role is antigen sampling
• invade through M cells, invade enterocytes from
  below
• Binding induces actin rearrangement, ingestion
• Escape endocytic vesicle, multiply in cytoplasm
• Move on actin filaments, infect adjacent mucosal
  cells

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Pathogenesis

• Intracellular movement
• along host actin filaments, form microcolonies
  near nucleus
• spread to adjacent cells Ics (intercellular
  spread)
• actin filaments polymerize, cometlike tails,
  propel bacteria through cytoplasm
• contact membrane, push into adjacent cell
• Plaques in cultured cells
• cell death due to cessation of protein synthesis

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PathogenesisShiga Toxin

• A-B (15) toxin, released during cell lysis
• B subunits bind to host cell glycolipid
• Internalized by endocytosis, nicking and
  translocation of A occurs inside host cell
• Inhibits protein synthesis (inactivation of 60S
  ribosomal subunit)
• enterotoxin in rabbit ileum (provokes fluid loss)
• neurotoxin in mice/ rabbits (causes paralysis)
• stxA and stxB chromosomal operon

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Pathogenesis

• Role in shigellosis unclear
• not essential for invasion, killing mucosal cells
• mutants lacking Shiga toxin invade, spread, cause
  diarrhea
• Main role hemolytic uremic syndrome
• HUS strains produce large amounts of Shiga toxin
• primary effect may be damage to blood vessels
• vascular damage causes renal failure, paralysis,
  occasional neurological complications

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Virotypes - Pathogenic E. coli
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THE END