Download Mechanism of bacterial damage and bacterial toxins

Toipc Number Seven
Mechanism of Bacterial Damage and Bacterial Toxins
Microbial Damage
Pathogenicity = ability to cause disease
Virulence = degree of pathogenicity
ID50 (Infectious Dose) = number of microbes required to cause infection in half
the hosts. It depends on the virulence factors of the pathogen and the portal of
entry
For example, Shigella and Salmonella both cause diarrhea by infecting the
gastrointestinal tract, but the infectious dose of Shigella is less than 100
organisms, whereas the infectious dose of Salmonella is on the order of
100,000 organisms
LD50 (Lethal Dose) amount of toxin or pathogen necessary to kill half the hosts
Microbes cause damage to host cells by three major mechanisms: 1. Direct
damage of host cells 2. Inflammation 3. Bacterial Toxins
1. Direct damage of host cells - Siderophores
Iron required for electron
transport chain in both host and
pathogen. Host usually does not
have free iron available (free
iron leads to easy colonization
by pathogens)
Humans bind unused iron to
transport proteins by transferrin
or lactoferrin
Pathogens
can
produce
siderophores:
secreted
by
bacteria to compete iron from
host proteins, siderophore iron
complex then absorbed by
bacteria
2. Inflammation
 In most cases, focal infections are eradicated by an intense, localized
inflammatory response.
 By contrast, severe sepsis is characterized by dissemination of
inflammatory mediators (e.g. circulating cytokines) resulting in
widespread activation of the immune system referred to as the
systemic inflammatory response syndrome (SIRS).
 SIRS is often complicated by systemic hypotension and tissue hypoperfusion (shock), and direct (e.g. TNFα-mediated) cell injury, which
ultimately leads to multiple organ dysfunction syndrome (MODS), and
in many cases death
Cont
3. Bacterial Toxins
Main Features of Exotoxins and Endotoxins.
Comparison of Properties
Property
The table on the
right
compares
the main features
of exotoxins and
endotoxins
Exotoxin
Endotoxin
Source
Certain species of gram-positive and
gram-negative bacteria
Cell wall of gram-negative
bacteria
Secreted from
cell
Yes
No
Chemistry
Polypeptide
Lipopolysaccharide
Location of
genes
Plasmid or bacteriophage
Bacterial chromosome
Toxicity
High
Low
Clinical effects
Various effects
Fever, shock
Mode of
action
Various modes
Includes TNF and interleukin-1
Antigenicity
Induces high-titer antibodies called
antitoxins
Poorly antigenic
Vaccines
Toxoids used as vaccines
No toxoids formed and no
vaccine available
Heat stability
Destroyed rapidly at 60°C (except
staphylococcal enterotoxin)
Stable at 100°C for 1 hour
Cont
Exotoxins
Exotoxins are toxic proteins released from the
pathogen cell as it grows.
Exotoxins fall into three categories: the
cytolytic toxins, the AB toxins, and the
superantigen toxins.
Endotoxins
Endotoxins are part of the outer membrane of
the cell wall of Gram-negative bacteria.
Endotoxins are released in large amounts only
when the cells lyse.
Endotoxins consist of a core polysaccharide
chain, O-specific polysaccharide side chains (Oantigen) and a lipid component, Lipid A, which is
responsible for the toxic effects
Endotoxins and Exotoxins
Types of exotoxins
1. Cytolytic Toxins
 Cytolytic toxins damage the host cytoplasmic membrane, causing cell lysis and
death. Because the activity of these toxins is most easily observed with assays
involving the lysis of red blood cells (erythrocytes), the toxins are often called
hemolysins
 Some hemolysins attack the phospholipid lecithin (phosphatidyl choline) of the
host cytoplasmic membrane, these enzymes are called lecithinases or
phospholipases. An example is the α-toxin of Clostridium perfringens
 Streptolysin O, a hemolysin produced by streptococci, affects the sterols of the
host cytoplasmic membrane.
 Staphylococcal α-toxin is a pore-forming. It is released as a monomer, seven
identical protein subunits oligomerize in the cytoplasmic membrane of target
cells. The oligomer forms a pore, releasing the contents of the cell and allowing
the influx of extracellular material and the efflux of intracellular material.
Staphylococcal α-toxin
Conti
2. A-B toxins
A-B toxins are so named because they consist
of two parts, an A (catalytic) domain and a B
(receptor binding) domain.
The A domains of most A-B toxins catalyze a
reaction by which they remove the ADPribosyl group from the coenzyme NAD and
covalently attach it to some host cell protein,
a process called ADP- ribosylation
AB toxin enters cells via:
1) Receptor mediated endocytosis
2) Fusion of vesicle with lysosome
3) Acid environment of lysosome
reduces disulfide bonds and
releases A into cell
4) A has various cellular activities
Cont
3. Superantigens
 Superantigens are unusual bacterial toxins that activate very
large numbers of T-lymphocytes results in the secretion of
excessive amounts of cytokines.
 Excessive cytokine production leads to a number of symptoms,
including fever, nausea, vomiting, diarrhea, and sometimes
shock and even death.
 Bacterial superantigens include the staphylococcal toxins that
cause food poisoning and toxic shock syndrome
Diphtheria toxin
Diphtheria: Infection of upper respiratory tract by
Corynebacterium diphtheria bacteria grow on throat
tissues
Characterized by the formation of pseudomembrane
(greyish membrane of bacteria, damaged host cells) as a
result of host’s inflammatory response
Diphtheria toxin is encoded by the tox gene in a lysogenic
bacteriophage called phage β. Toxigenic, pathogenic
strains of C. diphtheriae are infected with phage β and
encode the toxin.
Nontoxigenic, nonpathogenic strains of C. diphtheriae can
be converted to pathogenic strains by infection with phage
β, a process called phage conversion
Mechanism of Action of
diphtheriae toxin
Cholerae toxin
Cholera toxin is released from bacteria in the gut
lumen and binds via the B subunit to GM1 receptors
on enterocytes, triggering endocytosis.
The A subunit enzymatically activates a G protein
and locks it into its GTP-bound form through an ADPribosylation reaction.
G protein activity leads to activation of adenylyl
cyclase and increased cAMP levels.
 High cAMP levels then go on to activate the
membrane-bound CFTR protein, leading to dramatic
efflux of chloride, sodium, and water from the
intestinal epithelium.
Anthrax toxin
Bacillus anthracis, the causative agent of anthrax, secretes three
monomeric, plasmid-encoded proteins that are collectively called
anthrax toxin.
Two are enzymes: Lethal Factor, a Zinc protease that specifically
cleaves and inactivate MAP kinase kinases, and Edema Factor (EF), a
Calcium and calmodulin dependent adenylyl cyclase.
The third, Protective Antigen (PA83), named for its effectiveness in
inducing protective immunity against anthrax. It is also binds to
receptors and promotes translocation of LF and EF to the cytosol.
Mechanism of Action of anthrax
Edema F
Lethal F
EDEMA
Increased expression
of pro-inflammatory
mediators
B
cAMP
EF
Endosome
B
Acidic
Environment
MAPK
Mitogen activated protein kinase
IMMUNE SUPPRESSION
WBCs do not divide in
the presence of
pathogens; overall
decrease in phagocytosis
Tetanus and Botulinum toxins
 Clostridium tetani and Clostridium botulinum are endospore forming bacteria
commonly found in soil.
 These organisms occasionally cause disease in animals through potent AB exotoxins
that are neurotoxins—they affect nervous tissue.
 C. botulinum sometimes grows directly in the body, causing infant or wound
botulism
 Death from botulism is usually from respiratory failure due to flacid muscle
paralysis.
 C. tetani grows in the body in deep wounds that become anoxic, such as punctures.
Although C. tetani does not invade the body from the initial site of infection, the
toxin can spread via the neural cells and cause spastic paralysis
 Botulinum toxins, the most potent biological toxins known, are seven related AB
toxins. One milligram of botulinum toxin is enough to kill more than 1 million
guinea pigs.
Mechanism of Action of botulinum toxin
A-Upon
stimulation
of
peripheral and cranial nerves,
acetylcholine
is
normally
released from vesicles at the
neural side of the motor end
plate. Acetylcholine then binds
to specific receptors on the
muscle, inducing contraction.
B-Botulinum toxin acts at the
motor end plate to prevent
release of acetylcholine from
vesicles, resulting in a lack of
stimulus to the muscle fibers,
irreversible relaxation of the
muscles, and flaccid paralysis.
Mechanism of Action of tetanus toxin
(a) Muscle relaxation is normally induced by glycine (G) release from
inhibitory interneurons. Glycine acts on the motor neurons to block
excitation and release of acetylcholine (A) at the motor end plate.
(b) Tetanus toxin (tetanospasmin) binds to the interneuron to prevent
release of glycine from vesicles, resulting in a lack of inhibitory signals to
the motor neurons
Blockage of release of the inhibitory transmitter leads to convulsive
contractions of the voluntary muscles best exemplified by spasm of the
jaw and neck muscles ("lockjaw").
Mechanism of Action of tetanus toxin
Mechanism of the endotoxin
Begins with CD14 binding of receptors on Macrophages that:
1. Induces cytokine production: IL-1, IL-6, IL-8, TNF, PAF, PG
2. Activation of complement cascade (C3a, C5a or alternate pathway)
3. Activation of coagulation cascade (Hageman factor; Factor XII)
The clinical effects of endotoxin
Clinical Findings
Mediator or Mechanism
Fever
Interleukin-1
Hypotension (shock)
Bradykinin and nitric oxide
Inflammation
Alternative pathway of complement (C3a, C5a)
Disseminated intravascular coagulation (DIC)
Activation of Hageman factor
Activation of macrophages and activation of many clones of B lymphocytes resulting in
increasing antibody production. (Endotoxin is a polyclonal activator of B cells, but not T cells.)
Septic shock and death
Mechanism of the endotoxin
Endotoxins and the pyrogenic response
The cytokines induce the hypothalamus to release lipids called
prostaglandins, which reset the thermostat in the hypothalamus
at a higher temperature