Host Pathogen Interactions Lecture 4

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Host Pathogen InteractionsLecture 4

• MRSA, Group A Streptococci,
• and TB

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Methicillin Resistant Staphylococcus aureus (MRSA)

• Group of Gram positive bacterial strains
• S. aureus
• Colonizes skin and moist squamous epithelium of
  the anterior nares (nasal)
• 20 colonized, 60 intermittent carriers, 20
  dont have it
• Can infect through skin abrasions, abscesses,
  burns or surgical sites
• Generally a limited cause of pneumonia in the
  public
• Hospital acquired infection (HA-MRSA) is more
  associated with pneumonia
• Ventilator associated pneumonia
• MRSA - Highly resistant to methicillin class of
  antimicrobials
• Misnomer since this class of antibiotics is not
  used to treat this pathogen (within the last 10
  years)

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Community acquired MRSA(CA-MRSA) a major issue

• Primarily associated with skin and soft tissue
  infections
• A serious issue in prisons
• Outbreaks in closed areas and certain populations
• Can be seen in sports associated infections
• Wrestling
• Rubgy
• Contact sports
• Noted by physiotherapists and sports medicine
  physicians by increased occurences

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Antimicrobial resistance

• (CA-MRSA)-Strong resistance to b-lactams
• SCCmec cassettes
• Mobile genetic elements that are passed among
  strains
• Five different types
• Most strains have SCCmec type I-III
• CA-MRSA typically has type IV (not often found in
  hospital strains)
• SCCmec carries mecA gene that encodes PBP2a
• PBP2a penicillin binding protein
• Slowly gaining resistance to vancomycin
• VISA, VRSA strains are appearing
• Linezolid good activity, good serum levels,
  active against MRSA, VISA, VRSA
• , very expensive.

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Mechanisms leading to infection and disease

• Express capsule
• Extra polysaccharide layer
• Capsule displays anti-opsonic action
• prevents opsonization (phagocytosis) by
  neutrophils
• BUT MRSA strains are known to survive within
  neutrophils
• MRSA possess many surface associated proteins and
  secreted toxins that promote adhesion and damage
  tissues
• Sortase mediates anchoring to the cell wall
• Protein A binds to IgG
• ClfA (clumping factor) binds fibronectin (host
  surface protein)
• Strains also have an immunomodulatory features
  that promote colonization, allowing for disease

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• Immunosuppression
• Failure to mount an appropriate immune response
  (anergy)
• Mediated by secretion of superantigens, Map MHC
  class II analogue protein (aka Eap)
• Lack of antibody response seen in TSS patients
• TSS toxin-1

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Group A Streptococci

• Known as a human pathogen (exclusively)
• Streptococcus pyogenes
• Rheumatic Heart Disease (RHD)
• CD4 T cells recognize GAS, but also heart
  tissues resulting in heart lesions
• Sore throat pharyngitis (non-invasive)
• Toxic shock syndrome
• Respiratory infection and skin/soft tissue
  infection in children
• Necrotizing fasciitis
• Bacteremia
• Meningitis, septic arthritis, surgical site
  wounds
• Transmissable to people in close contact

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Pathogenicity islands and virulence factors

• Genes within PAI are upregulated during infection
  to promote colonization and disease
• This is achieved with dedicated transcription
  regulators
• Mga (multigene activator)
• Two component regulatory systems
• Transcriptome variation during progression of
  disease
• Entrance, survival, colonization, invasiveness
• Toxins (a variety of produced)
• Surface proteins for adhesion, target destruction
• ScpA (C5a peptidase), Sic (streptococcal
  inhibitor of complement)
• ScpC (cleaves IL-8) during invasive disease
• Avoid the host immune response by reducing
  neutrophil recruitment to sites of infection

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Mycobacterium tuberculosis

• Major issue for the global population
• Infection rates are rising in less developed
  countries
• Extremely low infectious dose (1-10 bacteria)
• Cough, sneezing (droplets)
• Endemic where population levels are higher and
  medicine is less available
• Drug resistant strains are appearing
• Treatment of infected individuals is a long
  process often with failure due to non-compliance
• Asymptomatic carriers can spread TB
  (tuberculosis) to others individuals
• Estimated that 2 billion are infected (1/3 of
  global population!)
• Co-infection with M. tuberculosis and HIV is the
  major issue worldwide
• HIV patients often die due to tuberculosis
  related complications

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The tubercule

• A hallmark of TB disease
• A granuloma that exists after M. tb replication
  within a macrophage (phagosome)
• Composed of recruited T cells, bacteria and
  macrophage
• Develops necrotic cells
• Caseum (cheese like structuretubercule)
• Most of the bacteria are killed, although some
  survive in a latent (dormant) form
• Avoid an active immune response
• Carrier, asymptomatic, in equilibrium with host
• Can be activated and then become infectious
  again

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Prevention of phagosome maturation

• Phagosome maturation is critical for host defence
• Mice (and humans) deficient in IFN-g (macrophage
  activating molecule) are highly susceptible to M.
  tb infection
• IFN-g induces LRG47 (a host GTPase) that is then
  recruited from Golgi to phagosomes
• IFN-g cannot stimulate the maturation of LRG47
  deficient macrophages that are infected with M.
  tb.
• Suggests a causative link between these host
  proteins in innate immunity
• M. tb prevents phagosome maturation
• Disrupts phagosome membrane constituents, alters
  maturation and signaling, blocks transition of
  early to late endosome fusion
• - SapB secreted M. tb protein with acid
  phosphatase actvity, thought to prevent membrane
  signaling
• PknG protein kinase G
• A secreted protein with serine/threonine kinase
  activity
• Involved in preventing phagosome maturation
• pknG mutants are immediately killed upon
  phagocytosis but exist outside of cells.
• The activity of PknG can be blocked with kinase
  inhibitors (in vitro)
• Major therapeutic area secreted protein is a
  convenient target as the inhibitor does not have
  to enter the bacterial cell (tends to be
  impermeable to many compounds)

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Autophagy

• Self-eating
• Directed degradation of intracellular compounds
  for
• Catabolites (induced by starvation)
• Innate immune response (against intracellular
  pathogens)
• Seen for Salmonella, Legionella and M. tb among
  others
• IFN-g stimulation of macrophages (in vitro)
  stimulates autophagy
• Results in M. tb phagosome fusion with
  degradative lysosomes
• Therapies to boost the innate immune response are
  of considerable interest given the
  ineffectiveness of antibiotic treatments.
• Overstimulation of TLR signaling to promote
  phagosome maturation?
• Balance of inflammation (good and/or bad) and
  disease

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PAIs within the M. tb genome
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Cfp10-ESAT-6 protein complex

• Secretion system Esx is involved in secreting
  Cfp10-ESAT-6
• Located in the RD-1 locus (region of difference)
• Other genes within the chromosome (not in PAI)
  are involved in complex secretion (?)
• Deletion of RD-1 locus results in increase in DNA
  transfer (ESAT-6 secretion link to type IV
  secretion?)
• RD-1 locus is not required for intracellular
  growth but is absolutely required for cell to
  cell spreading and invasion of other tissues
• RD1 mutants are reduced in their ability to form
  granulomas (do not elicit a strong inflammatory
  response)
• Supporting evidence that the M. tb induced
  tubercule is involved in spreading of this
  intracellular pathogen

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Esx secretion what now?

• Many members of ESAT-6 family in different PAI
• Cfp10-ESAT-6 for a tight protein complex
• Unclear what role the complex has but together
  they are known as potent T-cell antigens
• The complex paradigm of the tubercule
• Anti-M.tb (secludes bacteria which are potent
  stimulators of T cells
• Tubercule is not truly bactericidal and M. tb
  resides there, eventually using RD1 locus to
  spread and infect other cells

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Class review and practice quiz

• Individual and interactive
• Review your notes for secretion in addition to
  the Host-pathogen lectures
• Meant to be an open discussion to promote
  learning and to answer questions
• A list of references will be provided after the
  quiz for additional reading
• To support the lecture material