Host Pathogen Interactions Lecture 2

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Host Pathogen InteractionsLecture 2

• Salmonella, Vibrio, Listeria

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Typhoid Fever

• Salmonella typhi primate (human) restricted
• Other species as well (S. typhimurium, S.
  enterica, S. bongorii)
• Over 2500 serotypes Salmonella species
• Gram negative, bacillus, highly motile,
  food/water borne pathogen
• Salmonellosis (disease)
• Systemic infection, fever, diarrhea, in severe
  cases death
• Drug resistance
• Likely due to use of antibiotics in animal feed
• Mediated by plasmids, microevolution (change over
  short time often due to selective pressure)

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Mortality and Morbidity

• Salmonella sp. infections account for 17 million
  cases/yr with 600,000 fatalities
• 3.5 fatality rate (underestimated)
• Major burden, socioeconomic force
• Salmonellosis can be endemic (regional) and
  infect naïve hosts (children, immune compromised,
  elderly)

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Salmonella (non-typhoid)

• Transmission
• Highly efficient through faecal matter, and
  contaminated water sources
• Water low dose can cause infection
• Food Higher doses are required for infection
• Why this difference?
• Consider the growth and metabolic state of the
  pathogen
• Transmission from infected animals including
  cats, dogs and reptiles (yes reptiles)
• S. Enteridis and S. Typhimurium are commonly
  transmitted from animals to humans
• Zoonotic pathogen
• Also food borne (chicken, cantaloupe)

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Typhoid Mary (Mallon)

• Asymptomatic, carrier and chronic shedder of
  Salmonella in early 1900s
• Irish immigrant. She worked as a cook, and
  inadvertently infected many people
• Appeared healthy, fled law officials, finally
  reprimanded
• Stool samples confirmed that she was a Salmonella
  shedder.
• Released with agreement not to be a cook
• Still cooked years later and infected more people

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Asymptomatic Carriers

• Disease and infection are two very different
  situations
• Carriers are often free from disease but are
  infected (maybe symptomatic initially, but become
  asymptomatic with time)
• S. typhi 1-5
• Mary Mallon likely had Salmonella colonization of
  her gall bladder, which seeded her stool with
  the pathogen.

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Salmonella Infection

• Enters via faecal-oral route
• Invades (active) intestinal M cells (primarily)
• Taken up my resident macrophages
• Salmonella resists killing and replicates in an
  intracellular vacuole (SCV)
• The processes of invasion and intracellular
  survival are mediated by two pathogenicity
  islands on the chromosomes
• SPI-I and SPI-II (there are many more)
• Other virulence factors are associated with
  prophage (within the chromosome)
• Found using genetic/phenotypic screens using
  transposon mutagenesis
• Bacteria that cannot invade or bacteria that do
  not replicate (survive) within macrophages
• T3SS genes located in both of these pathogenicity
  islands

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Salmonella SPI-1 effector function-rapid invasion
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Once inside a vacuole.

• Disruption of vesicular trafficking
• SifA (accumulation of membrane)
• Inhibition of Nf-kB signaling
• Delayed cell death (survival)
• Actin accumulation in vicinity to intracellular
  vacuole
• Recruitment of trans-Golgi network to the vacuole

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Vibrio cholerae (Cholera)

• Cholera
• Violent vomiting, purging with watery
  rice-coloured evacuations, severe cramps, and
  collapse, death often occurring in a few hours
• Acutely dehydrating
• Before proper therapy case-fatality rate of
  40!
• Oral Rehydration Therapy (ORT) with salts (to
  maintain ion balance)
• Monitor rice stool volume cholera cot.
• Main goal is to prevent acute dehydration
• Prevent bacteremia, organ failure
• Two main serogroups O1 and O139
• Serogroup 01 has two biotypes
• El Tor and classical. El Tor is prevalent today
• Currently a major problem in Africa (Angola)
• Seasonal
• Association with various copepods?

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Two main virulence factors

• Cholera toxin (enterotoxin, AB type)
• B binds to ganglioside, A is internalized and
  disrupts cyclic AMP levels
• Linked to a filamentous phage CTXf that encodes
  the toxin (transmission)
• Type IV pilus
• Genes for pilus in 40 kilobase pathogenicity
  island
• CTXf uses this pilus as receptor and then
  integrates into the chromosome
• Linked evolutionarily- new biotypes in cholera?
• Pilus is co-regulated with CT expression, might
  mediate predominant binding to intestinal cells
• -other toxins exist (RTX toxin, type II secreted)
  but do not display dramatic phenotypes upon
  infection
• -still a concern for vaccine development

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Chitin binding protein of V. cholerae

• Vibrio gene identified in a screen of mutants
  deficient in adherence to intestinal cells
• Initially identified as a chitinase
• chitin subunit N-acetylglucosamine (GlcNAc)
• Therefore gene called GbpA
• GbpA is found to be cell surface associated and
  depends on Eps (Type II protein secretion)
• GbpA found to be critical for adherence to
  Daphnia and epithelial cells
• Survival in environment (Daphnia)
• Link between environmental host and human
  colonization
• Support for seasonal incidence of disease and
  outbreaks
• Linked to floods, copepod populations

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Listeria

• Listeriosis
• Food borne infection caused by the Gram positive
  bacterium L. monocytogenes
• Septicaemia, meningitis, gastroenteritis,
  encephalitis, mother to fetus transmission
• Indicators of a highly virulent pathogen,
  sophisticated mechanisms to interact with host
  cells during infection
• Three examples Internalin (InlA/B), ActA, LLO

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Binding to host receptors internalization

• InlB
• Induces internalization
• Binds to cell surface protein Met (tyrosine
  kinase)
• Met is normally the receptor for hepatocyte
  growth factor (HGP)
• Use membrane clathrin to mediate endocytosis
• Clathrin mediated endocytosis is used to acquire
  nutrients and recycle membrane

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InlA-actin based internalization-fewer cell types
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InlA
E-cadherin
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Listerolysin O (LLO)

• Pore forming toxin
• Expression believed to be tightly regulated so
  that timing occurs correctly
• Replication in cytoplasm after internalization
• Active at pH 6 (phagosome) and less so within the
  host cell cytoplasm (pH7, neutral)
• pH sensing protein?
• Activity is critical for cell replication within
  host cell

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Moving along between cells the journey continues

• Disseminating pathogen
• Correlates with the different disease types
• ActA (like Shigella IcsA)
• Listerio-pod
• Actin based motility mediates directed
  intracellular push for eventual intercellular
  movement
• Double membrane
• Upon invasion of Listeriopod into an adjacent
  cell, LLO is activated again (movie)

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Proof of intermediates!