toxin mediated infection Flashcards
what are microbial toxins?
“Microbial toxins are components or products of microorganisms which, when extracted and introduced into host animals can reproduce disease symptoms normally associated with infection without infestation by those microorganisms.”
what is toxicity, toxigenicity toxemia, toxic, and toxoid?
Many bacteria (and fungi) produce toxins – more than 200 different bacterial toxins are known – and there are likely many more not yet characterized
Wide variety of structure, mechanisms of action and effects.
Toxins are important determinants of pathogenicity and virulence
In a given disease, toxins can be one of many pathogenetic factors, or can be the main / sole factor accounting for symptoms
Toxicity – degree to which a substance is toxic
Toxigenicity – ability of a microorganism to produce toxins
Toxemia – presence of toxins in the blood stream
Toxic – term used to describe the clinical presentation of a patient (as in “she looks toxic”)
Toxoid – inactivated toxin, used in vaccines to generate antibodies (antitoxins)
What are endotoxins?
Cell associated - present in both living and dead cells
E.g. Lipopolysaccharide
LPS of Gram Negative bacteria
Action is primarily at the site of bacterial growth (though some effects may be systemic – fever, shock)
Generic toxin – no specific receptors
Large doses are lethal
they are part of the outer portion of the cell wall of gram-negative bacteria. They are liberated when the bacteria die and the cell wall breaks apart.
What are exotoxins?
Toxins secreted by growing bacteria - present only with living bacteria
Usually protein / polypeptides
Many act remotely from the site of bacterial growth, though some may have local effects, especially in promoting invasion
Produce a wide range of different effects
Toxins are species specific, interact with specific receptors
Small doses are lethal
what do endotoxins do?
Endotoxin interacts with many different physiologic systems and functions producing a broad range of alterations to normal homeostasis.
e.g. interleukin, histamine, tumor necrosis factor, bradykinin, platelet activating factor.
Where is the site of action of exotoxins and what are the key modes of action of these toxins?
site of action
Membrane targets
Cytosolic targets
key modes of action of exotoxins
- Pore formation Membrane damage or dysfunction - Alteration of cytoskeleton - Inhibition of protein synthesis - Activation of second messenger pathways / signal transducation Guanylate cyclase: cGMP Adenylate cyclase: cAMP - Proteases Zinc metalloprotease Serine proteases
what is one additional “super” mode of action of exotoxins?
Activation of immune response by superantigens
Superantigens differ from regular antigens
They are not processed into peptides and displayed in the usual binding site
Although they require MHC class II molecules for presentation they are NOT MHC restricted
Interact with T cells only via the Vβ segment
Result in massive activation of T cells rather than selective activation.
How does pore formation occur with exotoxins?
- Toxin binds to cell surface receptors to create a pore
- Small pores allow transit of small ions and nucleotides (e.g. alpha toxin of Staph aureus)
Loss of vital molecules (ATP)
Dissipation of transmembrane potentials and ion gradients
Irreversible osmotic swelling - Large pores can be made by cholesterol dependent toxins such as streptolysin and pneumolysin
allow transit of larger molecules such as proteins
how does the inhibition of protein synthesis occur?
- ADP ribosylation of elongation factor 2 (EF-2).
EF-2 is essential for transfer RNA to insert new amino acids into the growing peptide chain dysfunction of EF-2 stops protein synthesis
Inhibition of protein synthesis causes cell death
Ex: Diphtheria toxin, Pseudomonas exotoxin A - N-glycosidase removes adenosine from 28S rRNA
Ex: Shiga toxin & Shiga like toxins
How does the second messenger/signal transduction system work for exotoxins?
- Adenylate cyclase (AC) = membrane protein that makes cyclic AMP (cAMP)
- cAMP is a key intracellular second messenger that mediates a variety of processes
- ADP ribosylation of AC activates AC leading to marked increase in cAMP, which in turn causes excess secretion of Cl-
Na+ and water follow chloride - Examples
Cholera toxin
Edema factor (anthrax) - GTPase - Rho signaling pathway
- Many important functions – more than 60 targets affecting many cellular activities.
Cytoskeletal structure
Cytokinesis
Cell differentiation
Gene expression
Cell cycle progression
Apoptosis
Maintenance of tight junctions
How does alteration of cytoskeleton work with exotoxins?
- Several mechanisms
Enzymatic modification of actin (eg: ADP ribosylation)
Modification or inactivation of regulatory proteins (GTPase-Rho pathway) - Results:
Dissociation of actin filaments
Altered cell permeability
Disruption of intercellular junctions
Disrupted cell signaling
Disruption of transmembrane transport systems
What are proteases due to exotoxins?
Zinc Metalloproteases (ZMPs) - Zinc is essential for catalytic activity - ZMPs are important virulence factors Necrosis and hemorrhage Increased vascular permeability Triggers inflammatory mediators - Wide range of pathogens Bacteroides enterotoxin Anthrax Lethal factor Clostridial neurotoxins Vibrio vulnificans, Vibrio cholera Pseudomonas aeruginosa Legionella….
Serine Proteases (SPs)
- Exfoliative toxin of Staphylococcus aureus is an SP
Highly specific cleavage of desmosomal cadherins in the superficial layers of skin
Staphylococal Scalded Skin syndrome
what are superantigens?
caused by exotoxin
Cause massive activation of T cells – polyclonal response rather than the focused monoclonal response of a normal antigen Intense cytokine storm Fever, shock, erythema, edema Multisystem failure DIC High mortality
what is the structure of toxin?
- Many (but not all) bacterial toxins have a similar structure: A – B subunit
B subunit binds to cell surface and mediates entry into cell.
A subunit mediates toxin action
Remember…. B = binding / A = action - Receptor mediated endocytosis
Fusion with lysosome
Acidification of lysosome reduces disulfide bonds and releases A subunit into cell
What are the three main categories of exotoxins? NB: this does not refer to mechanisms of toxin action but rather the effects of toxin action.
cytotoxins - disrupt host cells
enterotoxins - disrupt epithelial cell function (gut)
neurotoxins - disrupt nerve cell function
