Bac Path Exam 3

how toxins aid in bacterial survival:

• kill or impair the function of immune cells

• release vital carbon or iron sources

• dampen or modulate the immune response

Botulinum neurotoxin (BoNT)

produced by the Gram-positive bacterium Clostridium botulinum; attacks neurons in humans/animals causing paralysis and potentially leading to death

Clostridium botulinum

Gram-positive bacteria that produce Botulinum neurotoxin; typically survive as spores in the upper aerobic layers of the soil or as vegetative cells in lower anaerobic layers 

lysogenic bacteriophages

viruses that infect bacteria and integrate their DNA into the host’s chromosome; can carry toxin genes and produce toxins in bacteria, leading to the rapid spread of toxin-producing bacteria

endotoxin

embedded in the bacterial cell surface and released upon cell lysis; includes lipopolysaccharide (LPS), lipoteichoic acid (LTA), and phosphatidylglycerol (PG); small and lipid-like

exotoxins

proteins produced inside pathogenic bacteria that are then secreted; or exoenzymes released via cell lysis, delivery via injection

letter or number toxin designation examples:

• exotoxin A of Pseudomonas aeruginosa 

• epsilon-toxin of Clostridium perfringens

enzymatic/cellular activities toxin designation:

• Adenylate cyclase of Bordetella pertussis (whooping cough disease)

• Lecithinase of Clostridium perfringens (gangrene)

bacterial origin toxin designation:

• Shiga toxin

• Cholera toxin

host cell specificity toxin designation

• neurotoxin

• leukotoxin

• hepatoxin

• cardiotoxin

• enterotoxins

biochemical properties toxin designation

heat-stable and heat-labile toxins of E. coli

Jarisch-Herxheimer reaction

inflammatory response that develops due to an acute increase in pro-inflammatory cytokines, including IL-6-IL-8, and TNF-alpha

mycolactone toxins

lipid-like toxins that possess both cytotoxic and immunosuppressive properties

mycobacterium ulcerans

an emerging human pathogen that is harbored by aquatic insects; responsible for causing Buruli ulcer; prevalent in certain tropical and sub-tropical regions; produces several mycolactones as secondary metabolites

type I toxins

act on the target cell surface; disrupt host cells without entering them, however a few intracellular targets have been identified; includes superantigens (SAgs) produced by S. aureus and S. pyogenes

type II toxins

aka membrane-disrupting toxins; destroy host cell membranes from outside the cell; invade and disrupt host defense processes within the cell, includes hemolysis and phospholipases; host cell damage leads to DAMPs, releasing inflammatory cytokines

type III toxins

have an A-B structure with 2 functional components; bind cell surface or enter cell and target intracellular components; disrupt host cell defenses, facilitating dissemination to remote organs

toxic shock syndrome (TSS)

late 1970s/early 1980s affected women and caused by a rare but life-threatening bacterial infection; linked to use of tampons left in for a long time, allowing S. aureus to grow and produce exotoxin that can induce a shock response similar to LPS or LTA

superantigens (SAgs)

activate a massive number of T-cells by forging abnormal connections between macrophages and T cells to incite a cytokine storm; bind directly to MHC class II molecules on macrophages, bypassing the normal antigen processing pathway

alpha-hemolysis by Streptococcus pneumoniae

leads to the green pigment of red blood cells due to H₂O₂ production

beta-hemolysis

causes the lysis of surrounding red blood cells, leaving a yellowish zone

non-hemolytic strains of streptococci

do not cause hemolysis of red blood cells and result in whitish-gray colonies with surrounding red agar

pore-forming toxins

proteins form channels in the membrane, causing the cell to swell and lyse; binds to the cell surface, oligomerize, and insert into the membrane to form transmembrane channels

cytolytic enzymes

hydrolyze membrane phospholipids, leading to destabilization/lysis of host cell

alpha-forming toxins

highly alpha-helical and form pores using helices; includes cytolysin A, colicin A, and colicin la (E. coli)

beta-pore forming toxins

rich in beta sheets and form a beta-barrel in membranes; includes alpha-hemolysin (S. aureus

process of pore-forming toxins:

1. toxin targeting to the cell generally requires interaction with a cell surface receptor

2. membrane insertion is facilitated by acidic pH, leading to changes in protein structure

3. alpha-pore forming toxins do not always require oligomerization, but beta-pore forming toxins do

phospholipases

remove the charged head group from the lipid portion of the molecule, causing membrane destabilization and cell lysis

A domain

confers toxic activity, usually an enzymatic activity

B domain

binds to host cell receptors and facilitates translocation of A domain

single-chain A-B toxins

proteolytic cleavage separates the A and B domains; optional T domain then mediates the transfer of the A domain across the membrane and into the cytosol

multi-subunit A-B toxins

have separate polypeptides for the enzymatic subunit (A) and the binding subunit (B); proteolytic cleavage separates the catalytic domain of the A subunit (A1 domain) from the tail portion of the A subunit, it interacts with the B subunit complex (A2 domain)

bacterial toxins

what are well-known among virulence factors because they can be easily purified from bacterial supernatant?

horizontal gene transfer

what allows the rapid spread of toxin genes among bacteria in the evolution and spread of toxin-producing bacteria?

Shiga toxin

what toxin is primarily associated with Shigella bacteria but can also be produced by certain strains of E. coli?

lipid A

what is the primary component of LPS endotoxin found in Gram-negative bacteria?

Mycolactone toxins

what class of toxins that possesses both cytotoxic and immunosuppressive properties?

Mycobacterium ulcerans

what bacterium is responsible for causing Buruli ulcer?

host cell surface

type I toxins act primarily on the:

binds to the host cell surface

type III toxins have an A-B structure with 2 functional components, where the B component:

T-cell receptors (TCRs)

superantigens exert their effect by binding to:

beta-hemolysis

__________ causes the lysis of the surrounding red blood cells, leaving a yellowish zone

forming channels in the membrane

pore-forming toxins cause cell lysis by:

gangrene

Clostridium perfingens produces an alpha-toxin that causes:

conferring toxic activity, usually enzymatic

A-B toxins, also known as type III toxins, typically have 2 functional domains of subunits, the A domain is responsible for:

facilitates the translocation of the A domain across the membrane

B domain of A-B toxins:

disulfide bond

proteolytic cleavage separates the A and B domains in A-B toxins, which remain connected by a:

the B domain

the host cell specificity of A-B toxins is determined by:

Clostridium botulinum

single-chain AB-type neurotoxin

Bordetella pertussis

single-chain AB-type dermonecrotic toxin

E. coli, Yersinia enterocolitica, and Yersinia pseudotuberculosis

single-chain AB-type cytotoxic necrotizing factors

Corynebacterium diphtheriae

single-chain AB-type Diphtheria toxin

Shigella and certain strains of E. coli

single-chain AB-type Shiga toxins

Clostridium difficile

single-chain AB-type Clostridial toxins

Vibrio cholerae

multisubunit AB-type Cholera toxin (CT)

Bordetella pertussis

multisubunit AB-type Pertussis toxin

E. coli, Campylobacter, and Shigella

multisubunit AB-type Cytolethal distending toxins 

Bacillus anthracis

multisubunit AB-type Anthrax toxins

T domain

translocation domain structurally related to pore-forming toxins

Corynebacterium diphtheriae

gram-positive, non-spore-forming aerobic rod with a distinctive club-like appearance

diphtheria pathogenesis

bacterial infection primarily transmitted through inhalation of aerosolized droplets, but can be spread through contact with open wounds or sores; colonizes the throat, produces DT, which causes inflammation and fibrin accumulation

diphtheria toxin (DT)

potent heat-labile A-B polypeptide; A subunit catalyzes ADP-ribosylation of EF-2 for protein translation in the host cell, which can result in cessation of protein translation and thus cell death

diphtheria toxin portion A

responsible for the enzymatic activity of the toxin, specifically mediating ADP-ribosylation of the host cell’s elongation factor protein, EF-2

diphtheria toxin portion B

binds specifically to the heparin-binding epidermal growth factor-like growth factor (HB-EGF) present on host cells

diphtheria toxin T domain

facilitates A subunit translocation across endosome membrane; similar to pore-forming toxins

dtxR

transcriptional repressor that binds to the operator region of the DT tox gene only in the presence of Fe2+; under iron limitation complex dissociates and allows transcription to proceed

diphtheria vaccine

DTP vaccine protects against diphtheria, tetanus, and pertussis

Clostridium botulinum

gram-positive, rod-shaped bacterium that produces the toxin that causes botulism

types of botulism

intoxication and active infection

intoxication of botulism

toxin is ingested without the presence of the bacteria

active infection of botulism

bacteria infects body and produces the toxin internally

structure of BoNT

complex molecule composed of a heterodimer and the progenitor toxin complex; synthesized and secreted from the bacteria as 150-kDa protein; cleavage into 2 fragments: heavy and light chain; includes nontoxic components to protect derivative toxin from stomach acid/proteases

peripheral neurons

BoNT targets ______ _______

BoNT mechanism

binds to ganglioside and protein receptors only on neuronal cells, then is internalized; inhibits the release of the neurotransmitter acetylcholine (ACh), which is involved in initiating action potential for muscle movement at neuromuscular junction; leads to muscle paralysis

A and B portions

AB-type toxins consist of:

Clostridium botulinum

what bacteria produces neurotoxins?

enzymatic activity, often ADP-ribosylation

AB-type toxins have 2 main portions: A and B, what is the A portion responsible for?

involved in toxin transfer inside the host cell

T domain of AB-type toxins is:

grayish pseudo membrane

what is the hallmark symptom of diphtheria?

through endocytosis and translocation through the endosome membrane

how does diphtheria toxin enter the host cell’s cytoplasm?

catalyzing the ADP-ribosylation of EF-2

what is the function of the A subunit of diphtheria toxin?

the chromosomal gene dtxR

how is diphtheria toxin expression regulated in Corynebacterium diphtheriae?

to reduce the depth of wrinkles, to treat urinary incontinence, to prevent severe scarring during wound healing

purposes of using Botox injections:

the nontoxic components of the progenitor toxin

botulinum toxin is protected from stomach acids and protease enzymes in the GI tract by:

inhibition of neurotransmitter release

what is the specific effect of the botulinum neurotoxin on peripheral neurons?

SNARE proteins

in normal neurotransmission what proteins help mediate vesicle docking and fusion?

an antitoxin

what treatment for botulism works by neutralizing the botulinum neurotoxin?

secretion systems

deliver virulence factors either into the medium or directly to target cells

translocation

trafficking of proteins from the cytosol through the cell membrane

export

translocating proteins from the cytosol to the periplasm in Gram-negative bacteria

secretion

translocating proteins from the cytosol to the extracellular milieu, other cells, or the bacterial surface

excretion

extracellular transport of non-proteinaceous compounds

Sac secretion system

N-terminus secretion signal; single step; unfolded proteins; present in Gram-negative and Gram-positive

Tat secretion system

N-terminus secretion signal containing twin arginine residues; single step; folded proteins; present in Gram-negative and Gram-positive

general secretory (SEC) system

found in both Gram-negative and Gram-positive bacteria, the only universally conserved protein translocation pathway; precursor proteins have N-term signal sequence that contains protease cleavage site; membrane-embedded SecYEG translocon

Gram-positive general secretory (SEC) system

mature protein folds as it emerges through SecYEG channel; released directly to surface of the bacteria or directly into medium

Gram-negative general secretory (SEC) system

translocated protein folds in the periplasmic space; remains transported across the outer membrane through additional transport mechanisms

2 major routes of SEC translocation system

post-translational system and co-translational system

SEC post-translational system

1. SecA binds to signal peptide of secretion substrate protein → SecYEG translocase serves as channel in the inner membrane

2. SecA undergoes ATP-dependent conformational change → drives translocation through pore of the translocase or SecYG

3. SecB protein helps translated proteins unfold before delivery to SecA-SecYEG complex

4. protease cleaves signal sequence from proteins and allows protein to be folded upon delivery to periplasm

SEC co-translational system

predominant pathway in eukaryotes and haloarchaea, not as common in bacteria; SRP recognizes signal peptide and SRP-bound peptide is targeted to the SecYEG complex; at SecYEG complex signal sequence is cleaved by signal peptidase and mature protein is released into periplasm