HOW MICROORGANISMS CAUSE
DISEASE-VIRULENCE
Assist Prof Microbiology
Dr. Syed Yousaf Kazmi
College of Medicine Majmaah
University
OBJECTIVES
1.
2.
3.
4.
Name methods of transmission of infection
Describe the process of infection
List bacterial virulence factors
Examples where bacterial virulence factors
play important role in virulence, antibiotic
susceptibility and host defence
PATHOGENESIS
• Pathogen- Org capable of
causing disease
• Opportunistic- Serious disease
in immuno-compromised
• Virulence-quantitative measure
of pathogenicity
• LD50
• ID50
• Shigella dysentrae and Vibrio
cholerae
VIBRIO CHOLERAE
SHIGELLA DYSENTRAE
PATHOGENESIS
Infection and Disease production
Microorganisms
Number
Body defenses
Virulence
Innate immunity
DISEASE PRODUCTION
Acquired immunity
PATHOGENESIS
COMMUNICABLE-inf spread from host to
host (e.g. TB)
CONTAGIOUS- highly communicable(Plague)
ENDEMIC- constantly present in low levels in
a population
EPIDEMIC-occurs more freq than usual
PANDEMIC- worldwide distribution
SUBCLINICAL – overt symptoms, only Ab
+ve(e.g. Hepatitis A)
LATENT inf- org lie dormant, reactivation
later(e.g. TB, Herpes simplex)
CARRIER STATE- Multiply with/without
producing symptoms in host(Typhoid Mary)
STAGES OF BACTERIAL PATHOGENESIS
1. Transmission from external
source into portal of entry
2. Evasion of primary host
defenses
3. Adherence
4. Colonization
5. Invasion/ inflammation or
Toxin production
6. Host responses
7. Progression/ resolution
TRANSMISSION
• Transmission of infection
– Human to human
– Nonhuman sources to
human (fomites, soil, water,
animals etc)
• The clinical manifestations
of diseases (e.g., diarrhea,
cough, genital discharge)
promote transmission of
the agents
• Endogenous vs Exogenous
(Normal flora)
TRANSMISSION
• Vertical and Horizontal
transmission
• Portal of entry: Respiratory, GI,
Urogenital tract or skin
• Diseases for which animals are
the reservoirs are called
zoonoses
• Animals can either
– be the source (reservoir) or
– mode of transmission (vector)
ADHERENCE
• Pili, capsules or glycocalyces allow
Bacteria to adhere to the surface of
human cells
• Pili of Ns gonorrhoeae and E.coli :
attach to urinary tract epith
• Glycocalyx of Staph epidermidis and
certain viridans streps : attach to
heart valves
• Adhesins: molecules mediating
attachment to surfaces
• IgA proteases→Cleaved Fc portion
bind to bact that bind to cellular Fc
rec
Ns gonorrhoeae attaching to
urinary epithelium via pili
ADHERENCE
 Biofilm: protective matrix of
polysaccharides and proteins
 Form on foreign bodies e.g.
prosthetic joints, prosthetic heart
valves, I/V catheters
 Native heart
 Protect bacteria from antibiotics
& host immune defenses e.g.
antibodies and neutrophils
 Persistence of Pseudomonas in
the lungs of cystic fibrosis and
Streptococci in dental plaques
ADHERENCE
• Bacteria can adhere to foreign body surfaces
• Phagocytes adhere poorly: lack of selectins
and adhesion molecules etc
COLONIZATION & MULTIPLICATION
• After adherence to the
surfaces, bacteria colonize
and multiply
• Thereafter, many possible
outcomes
– Local damage and
inflammation
– Invasion and spread to distant
sites
– Toxin release
– Cytolysins release
INVASION & INFLAMMATION
• Invasion followed by inflammation
• Invasive bacteria secrete enzymes
–
–
–
–
Collagenase and hyaluronidase
Coagulase
Immunoglobulin A (IgA) protease
Catalase
AVOIDING IMMUNE DESTRUCTION
ANTIPHAGOCYTIC COMPONENTS
1. Capsule & Slime layer
– Strep pneumoniae
– H influenzae
2. Antiphagocytic surface
components
– Strep pyogenes M proteins
– Ns gonorrhoea pili
– Staph aureus Protein A
3. IgA proteases
Structure of Strep pyogenes
AVOIDING IMMUNE DESTRUCTION
ANTIGENIC VARIATION
– Ns gonorrhoeae outer
membrane protein & pili
– Enterobacteriaceae capsular &
flagellar protein
SURVIVING INTRACELLULARLY
– Evading killing mechanisms
• M tuberculosis inhibits
phagolysosome fusion
• Listeria escapes phagosome before
phagolysosome fusion
TOXIN PRODUCTION
1. EXOTOXINS
• Extracellular
• Secreted
• Protein
2. ENDOTOXINS
• Integral part of cell
• Lipopolysaccharide
• Toxic moiety is Lipid A
FEATURES OF EXOTOXINS & ENDOTOXINS
Property
Exotoxin
Endotoxin
Source
Certain species of Gram +ve &
Gram –ve bacteria
Cell wall of gram-negative
bacteria
Secreted by cell
Yes
No
Chemistry
Polypeptide
Lipopolysaccharide
Location of genes Plasmid or bacteriophage
Bacterial chromosome
Toxicity
High (fatal dose ~1µg)
Low (fatal dose ~1000µg)
Clinical effects
Various effects
Fever, shock
Mode of action
Various modes
Includes TNF & IL-1
Antigenicity
Induces high-titer antitoxins
Poorly antigenic
Vaccines
Toxoids used as vaccines
No toxoids formed
Heat stability
Destroyed at 60°C(except staph entero)
Stable at 100°C for 1 hour
Typical diseases Tetanus, botulism, diphtheria
Meningococcemia, sepsis
EXOTOXINS
• Secreted by bacteria
• Polypeptides
• Certain Gram +ve & Gram –ve
org
• Genes are on plasmid or
lysogenic bacterial viruses (e.g.
diphtheria toxin, cholera toxin,
and botulinum toxin)
• Very potent
• Induce formation of Antitoxins
• Toxins treated with formalin→
Toxoid retain antigenicity
(Vaccines)
EXOTOXINS
• Most Exotoxins have A–B
subunit structure
– A subunit is active
• possesses pathogenicity
• Usually an Enzyme
– B binds to the receptor
• Exotoxins cause protein
synthesis inhibition,
neurotoxicity or can be cytolytic
EXOTOXINS-PROTEIN SYNTHESIS
INHIBITORS
ORGANISM
TOXIN
MECH OF ACTION Primary
Target
C. diphtheriae Diphtheria ADP Ribosyl
Heart,
toxin
transferase EF2
nerve,
epithelium
P. aeroginosa
Exotoxin A ADP Ribosyl
transferase EF2
Liver
S. dysentreae
Shiga
toxin
Interfere 60S
ribosome
Kidney,
Endothelium
of Blood ves
EHEC
Verotoxin
Interfere 60S
ribosome
Intestine
EXOTOXIN-NEUROTOXINS
ORGANISM
TOXIN
MECH OF
ACTION
C. tetani
Tetanus
Blocks release
toxin
of inhibitory NT
Glycine & GABA
C. botulinum Botulinum Blocks release
toxin
of Acetylcholine
at NMJ
EFFECT
Spastic
contraction
of muscles
Flaccid
paralysis
EXOTOXINS-cAMP INDUCERS
ORGANISM
TOXIN
MECH OF
ACTION
EFFECT
V cholerae
Cholera toxin
ADP Ribosylate Gs
Protein , that
activates Adenyl
cyclase—›↑cAMP
Profuse diarrhea
and vomiting
ETEC E coli
Heat labile toxin
Similar to above
Secretion of fluid
and electrolytes
B anthracis
Anthrax toxin
Edema factor
Lethal factor
Protective antigen
Edema formation
B pertussis
Pertussis toxin
ADP Ribosylate Gi
Protein—›
inactivates it &
↑cAMP levels
Whooping cough
symptoms
MNEUMONIC: cAMP (cholera Anthrax Ecoli Pertussis)
EXOTOXINS-SUPERANTIGEN
• TSST of Strep pyogenes &
Staph aureus
• Erythrogenic toxin of Strep
pyogenes
• Enterotoxin of Staph aureus
& C perferingens
• Superantigens binds directly
to class II MHC on antigenpresenting cells
(macrophages) without
intracellular processing
• Release of large amounts of
interleukins, esp IL-1 & IL-2
EXOTOXINS-CYTOLYSINS
ORGANISM
C perferingens
Staph aureus
TOXIN MECH OF ACT EFFECTS
α Toxin Lecithinase
Cell
membrane
damage,
myonecrosis
α Toxin Pore forming Leaky cell
toxin (PVL)
membrane→
death
ENDOTOXINS
• Integral parts of the cell walls
• Endotoxins are
lipopolysaccharides
• Toxic moiety is lipid A
• Produce the same
generalized effects of fever
and shock
• Weakly antigenic
• No toxoids
ENDOTOXINS-BIOLOGICAL EFFECTS
• Fever due to IL-1 & IL-6
• Hypotension, shock, and impaired perfusion
due to TNF, NO, Bradykinins
• DIC due to release of tissue factor
• Inflammation & tissue damage due to
activation of alternate pathway of
complement
MCQs
• A 24-years-old man was admitted with sepsis
due to a Gram negative bacillus. Patient
exhibits high grade fever. Which component of
Gram negative bacillus is responsible for high
grade fever in this patient?
• Enterotoxin
• Hyaluronidase
• Siderophore
• Endotoxin
• All of the following are correct
concerning endotoxin EXCEPT:
– They are part of the outer portion of the cell wall.
– They are liberated if the bacterial cell wall breaks
apart.
– They are only present in gram positive cells.
– They can lead to disseminated intravascular
clotting.
• Exotoxins have all the following characteristics
EXCEPT:
–
–
–
–
They are present in the LPS of the outer cell membrane.
They are usually destroyed by heat.
A small dose can be lethal.
They can be converted to toxoids.
• What cell structure does Neisseria
gonorrhoeae use to attach and enter host
epithelial cells?
–
–
–
–
–
Cell wall waxes
Fimbriae
M proteins
Capsules
Flagella
• Capsules play a role in the virulence of all of
the following except:
–
–
–
–
–
Mycobacterium tuberculosis
Bacillus anthracis
Klebsiella pneumoniae
Haemophilus influenzae
Yersinia pestis
• Which of the following bacterial toxins binds
to nerve cells, preventing chemical
communication between nerve and muscle
cells?
–
–
–
–
–
Erythrogenic toxin
Staphylococcal enterotoxin
Botulinum toxin
Diphtheria toxin
E. coli endotoxin
• Which is true of endotoxins?
–
–
–
–
–
They are proteins.
They are produced by gram-positive bacteria.
They are disease-specific.
They increase blood pressure.
They are released upon cell lysis.
• Which is not true of exotoxins?
– They can produce protective antibodies called antitoxins.
– The genetic material that encodes most of them is plasmid
or prophage associated.
– They can be converted to toxoids.
– The exotoxin can work by binding and entering the host
cell.
– They rarely have enzymatic activity.
• A laboratory developed antibody that reacts
with mitochondrial antigen. This antibody was
treated with different micro-organisms and
Ag-Ab reaction was read with special
equipment. Which of the following microorganism will be negative for this reaction?
–
–
–
–
–
Filamentous fungus
Protozoan parasite
Virus
Yeast
Cestode
• A 45-years-old lady reported with non healing
ulcer on leg. The histopathology report of the
ulcer margin revealed granuloma. If the causative
agent is a microorganism, which of the following
will be true of this agent?
–
–
–
–
–
It has a lipopolysacharride
It has pili
It is an exotoxin producer
It is a superantigen
It is intracellular
• A cancer chemotherapy patient has to have her
intravenous catheter replaced after it was found
to be blocked. The catheter was found to contain
bacteria. Which of the following attributes is
most likely to be a factor in this pathogenesis?
–
–
–
–
–
Ergosterol containing membrane
Peptidoglycan layer
Biofilm production
Possession of IgA protease
Possession of pili