L2 – How Bacterial Toxins Contribute to Disease and Dissemination Flashcards
1
Q
What are bacterial toxins and why are they important in disease progression?
A
Bacterial toxins are secreted substances that damage the host; they aid in nutrient acquisition, immune evasion, transmission, and competition.
2
Q
What are the two broad classifications of bacterial toxins?
A
Endotoxins and exotoxins.
3
Q
How does an endotoxin differ from an exotoxin?
A
Endotoxins are components of the Gram-negative cell wall (e.g. LPS) and typically cause non-specific inflammatory responses, whereas exotoxins are actively secreted proteins with specific targets.
4
Q
What structural components make up lipopolysaccharide (LPS) in endotoxins?
A
LPS consists of an O-antigen, core polysaccharide, and lipid A.
5
Q
What is the primary determinant of endotoxicity in LPS?
A
The acyl chain length and substitution pattern of the lipid A component.
6
Q
How are endotoxins released from bacteria?
A
They are released during cell division, bacterial death (often antibiotic-induced), or via immune-mediated lysis.
7
Q
What host responses are triggered by endotoxins?
A
Endotoxins activate toll-like receptors, inflammasomes, and complement systems, often leading to fever, septic shock, and organ failure.
8
Q
Why might endotoxin release be considered a double-edged sword for bacteria?
A
Although endotoxins trigger severe inflammatory responses, they can also assist in evading host defences and promoting transmission before host death.
9
Q
What is the structural organisation of cholera toxin?
A
It has a classical AB5 structure, with one active A subunit and a pentameric B subunit that binds to host cell receptors.
10
Q
Which receptors does cholera toxin target on host cells?
A
The primary receptors are GM1 gangliosides, with possible interaction with histo-blood group antigens.
11
Q
Describe the intracellular pathway of cholera toxin after internalisation.
A
It is trafficked from the endosome to the Golgi and then to the ER, where the A1 subunit is activated to ADP-ribosylate a G protein, ultimately increasing cAMP levels.
12
Q
How does the increase in cAMP caused by cholera toxin contribute to disease?
A
Elevated cAMP activates protein kinase A, which opens chloride channels (e.g. CFTR), leading to electrolyte and water efflux and the severe diarrhoea of cholera.
13
Q
What is the general structure of botulinum neurotoxin?
A
It is a binary AB toxin with a zinc-dependent metalloprotease domain (A) linked to a binding/translocation domain (B).
14
Q
How does botulinum toxin affect neuronal function?
A
It is internalised into neurons where the light chain cleaves SNARE proteins, preventing the release of acetylcholine at neuromuscular junctions leading to paralysis.
15
Q
What clinical effect does botulinum toxin have due to its mechanism?
A
It causes flaccid paralysis, which can lead to life-threatening respiratory failure.
16
Q
What is the role of leukotoxins, such as those from Staphylococcus aureus?
A
They target and kill immune cells, release nutrients from host tissues, and contribute to pus formation for transmission.
17
Q
What is a key structural feature of Shiga toxin?
A
It is an AB5 toxin with an enzymatically active A subunit and a pentamer of B subunits that bind to the glycolipid Gb3 on host cells.
18
Q
How does Shiga toxin disrupt host cell function?
A
It inactivates ribosomes by removing a specific adenine from 28S rRNA (component - large ribosomal subunit of eukaryoutic ribosome), thereby inhibiting protein synthesis and leading to cell death.
19
Q
What additional ecological role might Shiga toxin have aside from causing human disease?
A
It may play a role in intestinal colonisation and provide protection against protozoan predation in the natural environment.
20
Q
How do toxins contribute to the long-term survival of bacteria?
A
Toxins aid in immune evasion and facilitate transmission to new hosts by causing symptoms that promote spread (e.g. diarrhoea, pus formation).
21
Q
Why might some toxins have alternative roles beyond causing host damage?
A
They can also be involved in niche competition, colonisation, or even in microbial interactions outside the human host.
22
Q
What is the evolutionary dilemma associated with toxin-induced host death?
A
Killing the host can be an evolutionary dead-end, so toxins may also function to modulate the immune response rather than solely cause damage.
23
Q
How does understanding toxin structure assist in designing therapeutic interventions?
A
Detailed structural knowledge enables the rational design of inhibitors and vaccines that can block toxin activity.
24
Q
What are the different functions bacterial toxins can perform beyond host damage?
A
Bacterial toxins can aid in niche competition, facilitate colonisation, and act as signalling molecules in microbial communities.
25
How do bacterial toxins contribute to immune evasion?
They can kill or disable immune cells, interfere with signalling pathways, and modulate inflammatory responses.
26
What role do toxins play in bacterial competition?
Some bacterial toxins target competing bacteria by disrupting their membranes or metabolic processes.
27
How does the structural organisation of LPS contribute to its function as an endotoxin?
The three main components—lipid A, core polysaccharide, and O-antigen—each play a role in immune recognition and pathogenesis.
28
Explain the structure of lipid A and why it plays a crucial role in endotoxin activity?
Disaccharide backbone which is highly conserved. Acyl chain length/substitution pattern is the primary determinant of endotoxicity. Lipid A is the toxic component that interacts with host receptors, triggering strong immune responses.
29
How can endotoxins lead to septic shock?
Endotoxins bind to TLR4 on immune cells, triggering excessive cytokine release, leading to systemic inflammation and organ failure.
30
What is the function of the O-antigen in LPS?
The O-antigen helps bacteria evade host defences by varying its structure to prevent immune recognition.
31
How does the immune system recognise endotoxins?
Endotoxins are recognised by Toll-like receptor 4 (TLR4) on immune cells, leading to activation of inflammatory pathways.
32
What intracellular signalling pathway is activated by LPS?
The NF-κB signalling pathway is activated, resulting in the production of pro-inflammatory cytokines.
33
How does cholera toxin differ from Shiga toxin in its mechanism of action?
Cholera toxin increases cAMP levels, causing severe diarrhoea, while Shiga toxin inhibits protein synthesis, leading to cell death.
34
Why is cholera toxin particularly effective at promoting bacterial transmission?
By causing profuse diarrhoea, cholera toxin ensures efficient shedding and transmission to new hosts.
35
How does botulinum toxin enter neurons?
It binds to neuronal receptors and enters via endocytosis, eventually reaching the cytosol.
36
What molecular target does botulinum toxin cleave to cause paralysis?
It cleaves SNARE proteins, preventing synaptic vesicles from fusing with the neuronal membrane.
37
What makes botulinum toxin one of the most potent toxins known?
It has extreme potency due to its enzymatic activity, requiring only a few molecules to cause paralysis.
38
How does Shiga toxin gain entry into host cells?
It binds to Gb3 receptors on host cells, triggering receptor-mediated endocytosis.
39
What is the role of the Gb3 receptor in Shiga toxin-mediated disease?
The Gb3 receptor determines tissue tropism, particularly affecting kidney and intestinal cells.
40
How does Shiga toxin damage host cells at the molecular level?
It enzymatically cleaves ribosomal RNA, preventing protein synthesis and inducing cell death.
41
How can bacterial toxins contribute to disease persistence?
Some toxins suppress immune responses, allowing bacteria to persist in host tissues for extended periods.
42
What are some potential applications of bacterial toxins in medicine?
Bacterial toxins have been adapted for therapeutic uses, including Botox for medical treatments and vaccine adjuvants.
43
How can bacterial toxin structures be exploited for vaccine development?
By identifying neutralising epitopes, vaccines can be designed to block toxin activity before it affects the host.
44
Why might a toxin-producing bacterium evolve to regulate its toxin production?
Overproduction of toxins can be harmful to the bacterium itself, so regulatory mechanisms control expression based on environmental conditions.
45
How do environmental factors influence bacterial toxin expression?
Toxin production can be influenced by host signals, temperature, pH, and nutrient availability.
46
What are some common strategies used to neutralise bacterial toxins therapeutically?
Antitoxins, neutralising antibodies, and receptor mimetics can prevent toxin-mediated damage.
47
Why are AB toxins particularly effective at targeting host cells?
AB toxins have a dual structure: the A subunit exerts enzymatic activity, while the B subunit ensures cell entry.
48
What determines the tissue specificity of bacterial toxins?
The presence of specific host cell receptors determines which tissues are affected by a given bacterial toxin.
49
How does GWAS contribute to understanding bacterial pathophysiology?
GWAS helps identify genetic factors that contribute to bacterial virulence, antibiotic resistance, and infection outcomes, improving our understanding of bacterial pathophysiology.
50
What types of genetic variations are typically analyzed in bacterial GWAS?
Single nucleotide polymorphisms (SNPs), insertions, and deletions are commonly analyzed to find associations between genotype and phenotype.
51
Why is whole genome sequencing crucial for bacterial GWAS studies?
Whole genome sequencing allows comprehensive analysis of genetic variations, ensuring that all potential mutations contributing to a phenotype are considered.
52
What is the significance of a Manhattan plot in GWAS research?
A Manhattan plot visualizes genetic associations by plotting genomic locations (x-axis) against statistical significance (-log10 P-value) (y-axis), highlighting strong associations.
53
How does GWAS help in identifying antimicrobial resistance genes?
By identifying genetic mutations associated with resistance traits, GWAS helps uncover the genetic basis of antimicrobial resistance and potential drug targets.
54
Why is population structure important to consider in bacterial GWAS?
Population structure must be accounted for to distinguish true genetic associations from spurious correlations due to clonal expansion.
55
What are the main challenges of performing GWAS on bacteria compared to humans?
Unlike human populations, bacterial reproduction is clonal and asexual, leading to high linkage disequilibrium and challenges in pinpointing causal mutations.
56
How does linkage disequilibrium affect the interpretation of bacterial GWAS results?
Linkage disequilibrium causes nearby genetic variants to be inherited together, making it difficult to separate causative mutations from linked but non-functional variations.
57
What role do phylogenetic approaches play in bacterial GWAS?
Phylogenetic methods help correct for evolutionary relationships between bacterial strains, reducing confounding effects in GWAS analyses.
58
How can GWAS be used to predict clinical outcomes of bacterial infections?
GWAS can reveal genetic markers linked to infection severity and patient outcomes, aiding in risk assessment and personalized treatment strategies.
59
What is homoplasy, and why does it complicate bacterial GWAS studies?
Homoplasy occurs when the same mutation arises independently in different bacterial strains, making it difficult to determine whether a genetic variant is truly associated with a trait.
60
How do mixed models help address challenges in bacterial GWAS?
Mixed models account for population structure by incorporating genetic relationships between bacterial strains, improving the accuracy of GWAS findings.
61
How has GWAS been applied to studying Staphylococcus aureus?
In Staphylococcus aureus, GWAS has identified mutations linked to antibiotic resistance and virulence factors, helping to understand its pathogenic potential.
62
What genetic factor was linked to vancomycin resistance in Staphylococcus aureus GWAS studies?
A GWAS study found a mutation in the orpB gene associated with vancomycin resistance, providing insight into antibiotic resistance mechanisms.
63
What strategies are used to validate GWAS-identified genetic variants in bacteria?
Functional validation strategies include the use of transposon mutant libraries and knockout experiments to confirm the role of identified genetic variants.
64
Why is Mycobacterium tuberculosis a useful model for bacterial GWAS?
Mycobacterium tuberculosis has a well-documented population structure and extensive genomic data, making it a suitable model for GWAS in bacterial infections.
65
How can GWAS provide insights into bacterial virulence mechanisms?
GWAS can identify genetic determinants of bacterial virulence by associating specific mutations with pathogenic traits like toxin production and immune evasion.
66
What is the relationship between bacterial genetic variation and infection severity in patients?
Genetic variations in bacterial strains can influence disease severity by affecting toxin expression, immune system interactions, and antibiotic susceptibility.
67
How can integrating genotypic and phenotypic data improve disease outcome predictions?
Combining genomic and phenotypic data allows for more accurate predictive models of disease progression, improving clinical decision-making.
68
In what ways can GWAS findings contribute to the development of new antimicrobial therapies?
By identifying key resistance and virulence genes, GWAS can guide the development of novel antimicrobial agents and targeted therapeutic strategies.
69
How is cholera toxin (CTX) internalized into host cells?
CTX is internalized by endocytosis.
70
After internalisation, what is the intracellular trafficking pathway of CTX?
CTX moves from the endosome to the Golgi apparatus, then to the endoplasmic reticulum (ER).
71
What happens to CTX in the ER?
CTX dissociates; the A1 subunit (CTX-A1) is exported into the cytosol.
72
Which host factor activates CTX-A1 in the cytosol?
ADP ribosylation factor 6 (ARF6) activates CTX-A1.
73
What does the ARF6-CTX-A1 complex do?
It catalyzes ADP-ribosylation of a G protein-coupled receptor (GPCR), activating adenylyl cyclase (AC).
74
What is the function of adenylyl cyclase (AC) in CTX’s mechanism?
AC converts ATP into cyclic AMP (cAMP), increasing intracellular cAMP levels.
75
What is the downstream effect of increased cAMP in host cells?
Activation of protein kinase A (PKA).
76
What does activated PKA do in cholera pathogenesis?
It phosphorylates CFTR chloride channel proteins.
77
What is the result of CFTR activation in cholera?
Electrolyte (Cl−, HCO₃⁻, Na⁺, K⁺) and water secretion into the intestinal lumen, causing secretory diarrhoea.
78
What bacteria produces Cholera toxin and it's Gram status?
Gram-negative Vibrio cholerae
79
Name 5 possible structures of Exotoxins
Single polypeptide toxins (AB), Binary toxin, Oligomeric toxins, Protoxins, and Pore-forming toxins
80
Provide 2 examples of Single polypeptide toxin (AB)
Diphtheria toxin, Pseudomonas Exotoxin A
81
Describe the structure of Binary toxin and provide an example.
consists of two independent polypeptide chains (Non-associated AB toxins) e.g. Anthrax toxin
82
Describe the structure of Oligomeric toxins, and provide an example.
multimeric complex consisting of two or more non-covalently linked subunits/domains (ABn) e.g. cholera toxin (AB5)
83
Describe the structure of protoxins and provide an example.
secreted in an inactive form (proenzyme) which can be converted to an active form by proteolytic enzymes e.g. Iota toxin of Clostridium perfringens
84
Describe the structure of Pore-forming toxins.
monomeric or bi-component molecules which bind to specific cells and oligomerise on the surface resulting in cell membrane pore
85
What are the 4 structural classifications of exotoxins?
target/binding site, mode of action, structure or generic function
86
Name 2 classifications under mode of action with examples
ADP-ribosyltransferase e.g. cholera toxin.
𝛽-barrel pore forming toxins e.g. Staphylococcus aureus alpha haemolysin.
87
Name 2 classifications under target or binding site with examples
Cholesterol e.g. streptolysin O
Elongation factor 2 e.g. diphtheria toxin
88
Give 6 examples of where/what exotoxins can act on
Cytolytic, dermonecrotic, enterotoxin, leucocidin, neutrotoxin, superantigen
89
How are bacterial toxins structurally organised?
They can be single proteins or oligomeric protein structures.
90
What structural motif is common in many bacterial toxins?
The AB structure-function organisation.
91
What is the role of the A domain in AB toxins?
It encodes the active, often catalytic, component of the toxin.
92
What types of activities can the A domain perform?
Ribosylation, glucosylation, proteolysis, non-covalent modification, or direct binding to host proteins.
93
What is the function of the B domain in AB toxins?
It facilitates receptor binding and translocation into the host cell.
94
What are the two general functions of the B domain?
Receptor binding
Translocation of the A domain into the host cell
95
What are the side effects of Cholera infection?
Voluminous diarrhoea, rapid dehydration, acidosis and hypovolemic shock
96
What is the structural classification of cholera toxin (CTX)?
CTX has a classical AB5 structure.
97
What are the components of the CTX-A subunit?
CTX-A is a heterodimer composed of two polypeptide chains: CTX-A1 and CTX-A2.
98
What is the composition of the CTX-B subunit?
It is a pentamer made of five identical polypeptide chains.
99
What is the function of the CTX-B subunit?
It binds specifically to receptors on intestinal epithelial cells.
100
What is the primary receptor for CTX-B on host cells?
GM1 gangliosides.
101
What is a secondary receptor for CTX-B?
Histo-blood group antigens.
102
What is the result of CTX-B binding to its receptor?
It promotes endocytosis of the toxin.
103
What are the prevention methods for Vibrio cholerae infection
Safe drinking water, sanitation and hygiene (WASH)
104
What is the potential secondary role of CTX?
Inhibit growth of gut bacteria by targeting ganglioside-like glycoconjugates
105
What host interaction does CTX exhibit with intestinal bacteria like Campylobacter jejuni?
CTX shows GM1-dependent binding to C. jejuni, relying on LOS-mimicking GM1 structures.
106
What neurotransmitter is released at the neuromuscular junction under normal conditions?
Acetylcholine.
107
What is the function of acetylcholine in the parasympathetic nervous system?
It enables smooth muscle contraction.
108
What receptors are recognized by Botulinum neurotoxin (BoNT) for entry into neurons?
1) Polysialogangliosides, 2) Synaptic vesicle proteins (e.g., synaptotagmin, synaptic glycoprotein 2).
109
What is the next step after BoNT binds its receptors?
Endocytosis of BoNT into the neuron.
110
What is the role of the SNARE complex in neurotransmission, and how does botulinum toxin interfere with it?
The SNARE complex enables acetylcholine release at the neuromuscular junction. Botulinum toxin cleaves specific SNARE proteins via its light chain, preventing their assembly and blocking acetylcholine release, resulting in flaccid paralysis.
111
What is the functional result of BoNT activity at the neuromuscular junction?
Inhibition of acetylcholine release, causing flaccid paralysis.
112
What is the causative agent of botulism?
Clostridium botulinum, a Gram-positive, anaerobic, spore-forming bacterium.
113
Where is C. botulinum commonly found?
In soil and marine sediments, mainly as spores.
114
How can botulism be acquired?
Through food contamination (e.g., canned, preserved, or fermented foods) or wound infection.
115
Why is spore eradication important in preventing botulism?
Because spores are the environmentally persistent form that can cause contamination.
116
What type of toxin is botulinum toxin (BoNT)?
A binary AB toxin.
117
What are the structural components of BoNT?
N-terminal zinc-dependent metalloprotease (A domain) linked by disulphide bond to C-terminal binding domain (B fragment).
118
Leukocidins Name examples of receptor-mediated pore-forming toxins.
α-toxin, bi-component leukotoxins (e.g., PVL, LukAB, LukGH, LukDE).
119
Leukocidins What is the first step in receptor-independent pore formation?
Attachment to the membrane without a specific receptor.
120
Leukocidins What are the consequences of receptor-independent binding?
Membrane disintegration followed by formation of short-lived pores.
121
Leukocidins
122
Leukocidins What toxins typically act in a receptor-independent manner?
α-type phenol-soluble modulins (PSMs).
123
Leukocidins
124
Leukocidins What is the initial interaction in α-toxin mechanism?
The α-toxin monomer binds to its receptor ADAM10 on host cells.
125
Leukocidins What follows monomer binding in α-toxin action?
Oligomerization of monomers into a pre-pore complex.
126
Leukocidins How does α-toxin complete pore formation?
Insertion of the transmembrane channel into the membrane, facilitated by CAV1.
127
Leukocidins What is the final effect of α-toxin on the host cell?
Disruption of membrane integrity → cell lysis.
128
Leukocidins What are the two components of bi-component leukocidins?
S-component (binds first) and F-component (binds second).
129
Leukocidins What is the stepwise process of leukocidin pore formation?
Monomer secretion
S-component binding to membrane
F-component dimerization
Oligomerization into pre-pore
Transmembrane channel insertion
130
Leukocidins What structural form do leukocidin pores take?
Typically a heptameric or octameric ring, allowing cytotoxic ion flux.
131
Leukocidins What cells are commonly targeted by leukocidins?
Leukocytes—hence their name (leuko-cidins).
132
Leukocidins add extra ones here
133
How might botulinum toxin benefit C. botulinum in the environment?
BoNT may support C. botulinum by killing grazing animals that ingest spores, releasing nutrients and spores back into the soil to promote plant growth. Alternatively, BoNT might have an unknown soil-specific function driving its retention, as mammalian toxicity may be an accidental byproduct.
134
Why are leukotoxins clinically significant?
They contribute to antibiotic-resistant infections and support colonization and disease progression.
135
How does WHO categorize bacteria producing leukocidins in terms of antibiotic resistance?
Priority 2 - High for research and development of new antibiotics.
136
What organism produces Shiga toxin?
Enterohaemorrhagic Escherichia coli (EHEC), such as O157:H7.
137
Where is EHEC commonly found as a commensal organism?
In the bovine gut.
138
How is EHEC transmitted to humans?
Through contaminated food, water, animals, infected persons, and surfaces.
139
What encodes the Shiga toxin in EHEC?
A prophage, expressed during the lytic cycle.
140
How is Shiga toxin secreted from bacterial cells?
Via phage-mediated bacterial lysis.
141
What regulates Shiga toxin expression?
Iron levels; production is restricted at high Fe concentrations.
142
Where in the host does Shiga toxin production typically occur?
In the distal small intestine and colon.
143
What is the first host compartment Shiga toxin traffics to after entry?
Early and recycling endosomes.
144
What pathway does Shiga toxin follow inside host cells?
Retrograde trafficking through the Golgi and ER.
145
What cellular machinery aids in Shiga toxin trafficking?
Clathrin and retromer, which generate membrane curvature and tubules.
146
What process liberates the A domain of Shiga toxin in the ER?
Reduction of disulphide bonds.
147
What enzyme cleaves the C-terminal A domain of Shiga toxin?
Furin, in the endosome.
148
How does Shiga toxin inhibit host cell function, and what is the outcome?
Shiga toxin acts as an N-glycosidase, cleaving the N-glycosidic bond at adenosine 4324 on 28S rRNA. This blocks aminoacyl-tRNA binding at the A site of the 60S ribosomal subunit, halting protein synthesis and causing cell death.
149
What does Shiga toxin inhibit in host cells?
Binding of aminoacyl-tRNA to the A site of the 60S ribosomal subunit.
150
What is the cellular outcome of Shiga toxin action?
Inhibition of protein synthesis and cell death.
151
What are the possible environmental functions of Shiga toxin, and what drives its selection?
Shiga toxin may help EHEC establish a niche in environments like soil, water, or the bovine gut, and could protect against protozoan predators. These environmental advantages likely maintain toxin production in nature, while human infection is rare and incidental.
