Transcript Mechanisms of Bacterial Virulence

Mechanisms of Bacterial
Virulence
Or what separates the pathogens from
the wannabes
Summary
• Serum resistance capsule, IGBP,LOS host protein
binding
• Intraphagocytic survival
• Modulation of phagocytic function:
phagocytic activity, NO, respiratory burst inhibition
• Immunoglobulin binding proteins
• Antigenic variation
Level of virulence factor expression will vary with the
type of virulence factor and environmental factors.
Constitutive factors like cell wall component are expressed
continually. Virulence factors such as toxins, adhesin,
capsules, immunoglobulin binding proteins, anticomplement
proteases, and alterations of external antigen have variable
expression. Expression of these virulence factors comes at a
high metabolic cost to the bacteria. The only way they are
favorable to the bacteria’s survival is if they in some way
confer a competitive advantage to the bacteria’s survival invivo. These genetically encoded virulence factors are usually
expressed in a coordinated manner influenced by metabolic
and genetic regulator elements that respond to environmental
signals.
Or…said another way if given a choice
bacteria often do not express encoded
virulence factors until they sense the
environment is right for its favorable
application. Once they “sense” the right
environment (ie proximity to a mucous
membrane, or presence within a phagosome)
the genes coding for the particular virulence
factor are transcribed and expressed.
Virulence factors of H. somnus
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Complement resistance
Resistance to phagocytosis
Intraphagocytic survival
Antibody resistance
Modulation of phagocytic function
Lipooligosaccharide
Enhancer bacteria
Innate host defense
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Mucosal and epithelial barriers
Secretory and circulating immunoglobulins
Innate serum antimicrobial factors
Secretory protease
Professional phagocytic cells
Natural Killer cells
Acute phase response
Adaptive host immune response
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Secretory and circulating immunoglobulin
Helper and cytotoxic T cell response
Opsonization
Adapted acute phase response
– enhanced phagocytic function
– enhanced serum mediated killing
Virulence factors . . . . the
competitive edge
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Adhesins
Antigen cloaking
Antigen variation
Complement
resistance
• Antibody resistance
• Antiphagocytic
properties
• Resistance to
intracellular killing
• Bacterial proteases
• Invasiveness
• Exotoxins
• Endotoxins
• Iron regulating
proteins
Virulence factors affecting innate
immunity
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Capsular polysaccharide
Protease
Adhesins
Induced cellular entry
Resistance to phagocytosis
Intaphagocytic survival
Virulence factors affecting
adaptive immunity
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Intracellular survival
Immunoglobulin binding proteins
Antigenic variation
Proteases
Capsular polysaccharide
Adhesins
• Pili
• Type IV Pili
• Non Pilus adhesion (afimbrial)
Masking Bacterial Recognition
• Capsule
• Surface binding of host proteins
• Non-neutralizing epitopes
Toxins
• Pore forming toxins (RTX toxins)
• IgA proteases
• Heat stable toxins (guanylin cyclase
activators)
• Cytoskeletal toxins
Complement Resistance
• Confers serum resistance
• Favors septicemic disease
• Serum resistance is usually one of the major
virulence factors differentiating invasive
strains from carrier mucosal isolates.
Capsular polysaccharide and complement lysis
and nonfunctional assembly
protease
nonfunctional
assembly or
remote
assembly, subject
to degradation
Capsular polysaccharide
Invasion
• Directed uptake
• Phagocytic uptake
– intraphagocytic survival
Outcomes of Phagocytosis
Blocked fusion
lysosome
X
Phagocytosed
bacteria
fusion
survival
Escape prior to
fusion
killing
Resistance to phagocytic killing
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Escape from the vacuole
Prevention of phagolysosomal fusion
Ability to survive in the lysosome
Prevention of oxidative burst activity
Virulence factors affecting
adaptive immunity
• Antibody lysis
• Immunoglobulin binding proteins
– surface Ig binding proteins
– soluble Ig binding proteins
• Antigenic cloaking
• Antigenic variation
• Modulation of the immune response
IgG Isotypes and resistance to
disease
• IgG2 Most important in controlling
pyogenic infections
• IgG2a
• IgG2b
– Most easily bound by H. somnus Ig binding
proteins.
– Pathogenic strain of H somnus with IGB fibrils
are C resistant
Immunoglobulin Binding Proteins
Normal Ig binding
Opsonization
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Ig mediated C
fixation
x
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YYY
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Soluble IGBP
IGBP fixed
Antigenic variation
• Genetic mechanisms
• Post translational antigen modification
• Selection pressure
– culture + cattle usually are seronegative to
strains isolated from their lungs, but may be
seropositive to previous infections strains.
– Molecular Cloning and Mutagenesis of a DNA Locus Involved in LOS
Biosynthesis in H somnus. Wu et al Infection and Immunity vol 68 no 1 Jan 2000
p310-319
Start codon
20 repeats
AUGCAATCAATCAATCAAT//////CAATCAATCAATGCCTACCGGTATATT
antigen
Slip Strand Mispairing
19 repeats
AUGCAATCAATCAATCAAT/////CAATCAATGCCTACCGGTATAT
New antigen
Expression of virulence factors
• Genetic control
– Environmental stimuli
– Induced expression
• Innate virulence factor expression
• Host specific virulence factors
• Tissue specific virulence factors
Genetic control of virulence
factors
• External environment triggering gene
transcription
• Multiple different regulatory systems
• Bacterial population density.
NEGATIVE CONTROL
INDUCER
POLYMERASE
gene
REPRESSOR
Active repressor protein in place preventing transcription
Polymerase
gene
Inducer binds to repressor, causing
alteration releasing repressor and allowing
polymerase to transcribe gene
Inducer
activator
Inducer activates activator protein
and stimulates transcription
RNA POL
pro
gene
TRANSCRIPTION PROCEEDS
Act
RNA POL
pro
ACTIVATOR
POLYMERASTE
gene
INDUCER
PROMTER
Selective pressure
• Immunologic pressure
• Competitive pressure
• Nutrient restriction
Summary
• Serum resistance capsule, IGBP,LOS, host protein
binding
• Intraphagocytic survival
• Modulation of phagocytic function:
phagocytic activity, NO, respiratory burst inhibition
• Immunoglobulin binding proteins
• Antigenic variation
Metabolic alterations of virulence
factors
• Temperature
• Chemotherapeutic agents