Bacterial virulence and Pathogenicity Mechanisms Flashcards
Lecture 5-6
What is the most common relationship between bacteria and humans?
Mutualism.
What are the key stages in bacterial pathogenesis?
Attachment and entry into the body, Localised or general spread within the body, Growth and multiplication, Evasion of host defences, Shedding (exit), Damage to host.
Define bacterial adhesion.
Measurable union between a bacterium and a substratum, requiring reversal energy.
What is a substratum?
A surface with receptors for adhesins.
What is an adhesin?
A ligand molecule capable of binding to a receptor, determining host and tissue specificity.
What is an adhesiotope?
The adhesion site of a receptor.
Define an invasin.
A bacterial ligand that induces a target cell to internalise the microbe, enabling bacterial invasion.
Explain the difference between an adhesin and an invasin.
An adhesin facilitates attachment, while an invasin facilitates internalisation. Some adhesins can also function as invasins, but not all adhesins are invasins.
What is phase variation?
Reversible on/off expression of certain genes, leading to changes in expressed proteins, through mutation or epigenetic regulation.
What is antigenic variation?
Changes in bacterial surface antigens to evade the host immune system, achieved through gene recombination, point mutations, and gene conversion.
What are two common mechanisms of bacterial adhesion?
Pili and outer membrane proteins on the bacterial capsule.
How does antigenic variation assist bacterial adhesion?
By modifying adhesins, bacteria can avoid recognition by antibodies while maintaining their ability to attach.
How does phase variation benefit bacterial adhesion?
It allows bacteria to adjust their ability to adhere to host tissues and evade immune detection.
Define tissue tropism.
The tendency of a bacterium to colonise a specific tissue.
Provide an example of tissue tropism.
Meningococci primarily reside in the nasopharynx.
What is avidity in bacterial adhesion?
The concept that multiple interactions between adhesins and receptors strengthen binding, increasing the likelihood of advantageous host signalling for bacteria.
Name 3 methods that bacteria use to evade the immune system.
Producing IgA proteases to cleave secretory IgA, Using a capsule to mask outer membrane antigens, Mimicking host molecules, e.g., N. meningitidis serogroup B capsule.
What are challenges of using animal models to study human-specific microbes?
There is no natural colonisation due to species-specific receptors, Poor survival of microbes in animal blood due to differences in Fe acquisition.
What are some approaches for studying human-specific bacterial adhesion?
In vivo: Taking swabs from naturally infected individuals, Ex vivo: Using organ cultures like biopsies, tonsils, umbilical cord, or placental tissue, In vitro: Culturing cells with purified components.
How can microscopy be used to study bacterial adhesion?
Techniques like light microscopy, immunofluorescence, confocal microscopy, and electron microscopy are used to visualise interactions.
What is haemagglutination?
A quantitative assay observing how bacteria interact with erythrocytes, causing them to clump together.
Describe the viable count assay for studying bacterial adhesion.
Cultured cells are infected, washed to remove non-adherent bacteria, and surviving colonies are counted to measure adhesion.
How does ELISA help study bacterial adhesion?
ELISA uses antibodies to detect and quantify binding between microbial adhesins and host substratum receptors.
How can antibiotics be used to study bacterial invasion?
Tissue is infected, submerged in antibiotics to kill surface bacteria, washed, and broken up to measure internalised bacteria quantitatively.
How can receptor density influence bacterial infection?
Higher receptor density can increase bacterial adhesion and infection, which may change during inflammation.
What are the possible advantages of tissue tropism for bacteria?
Enhanced colonisation of specific tissues, Avoidance of host defences in non-target tissues, Access to nutrients specific to certain tissues.
How can the host cell be damaged by the LPS?
Antigenic variation of the O antigen region ensures multiple serotypes: the immune system will not be able to recognise and neutralise all serotypes. When bacteria are lysed within the host organism, Lipid A can be released into the bloodstream. This cascade of events that follows leads to inflammatory response, coagulation and potentially shock.
What is a virulence factor?
A molecule or cellular structure that helps pathogens colonise a host and cause disease.
What is meant by the term ‘mitogen’?
A substance that induces mitosis.
What are haemolysins?
Lipids and proteins that cause lysis of erythrocytes by destroying their cell membrane.
What does toxigenicity refer to?
The capacity of microorganisms to produce toxins.
What is a cytokine storm?
A dramatic increase in cytokine concentrations.
What are the 2 main classes of bacterial toxins?
- Endotoxins (a.k.a. LPS), produced in Gram-Negative Bacteria only. 2. Exotoxins, produced in both Gram-Positive and Gram-Negative Bacteria.
Do all Gram-Positive Bacteria produce the same toxins?
No, Toxins are generally specific to a particular bacterial species that produces the disease associated with the toxin. e.g. Clostridium tetani produces Tetanus toxin. e.g. Corynebacterium diphtheriae produces Diphtheria toxin.
Traditionally how were toxins discovered and understood? How has this process improved in more recent years?
Based on the disease they cause. A process involving identification and cultivation of the pathogen, purification of the toxin then study of the mechanism of action of the toxin. Nowadays, we can use bioinformatics and genomics to compare new toxin sequences to a database.
Describe the process of host cell damage from the release of Lipid A into the bloodstream.
Lipid A forms a complex with lipid binding protein. This complex travel through the bloodstream and binds to CD14 and TLR4 receptors on host cell membranes. This triggers cascade of events in host cell leading to inflammatory response, potentially coagulation and shock.
How have the effects of LPS on hosts been studied?
In Vivo Approach: Mice injected with purified LPS, after 2 hours blood is collected and analysed for presence of cytokines (molecules expressed by immune cells that cause inflammation). Latent (asymptomatic) period, then physiological distress and death of the mice.
In studies of the effect of LPS on hosts, what 3 factors influenced the time interval between injection of LPS and subsequent death of the host organism?
Dose of LPS (greater dose = quicker death), route of administration and species of animal (some are more susceptible than others.
Often pharmaceutical drugs are obtained using engineered bacteria. This could result in drugs becoming contaminated with LPS. How is this problem overcome?
Labs test small portion drugs on amoebocytes (amoeba-like cells found in Limulus; very mobile) If LPS present they will die and stop moving.
What is a key use of LPS in developing therapeutic drugs?
Can test efficiency of drugs that inhibit inflammation - more effective drugs will inhibit inflammatory effects of LPS.
Which of the following statements are true? A Lipid A is part of Gram-positive bacterial cell wall, B O-polysaccharide is found on all bacteria, C Portions of the LPS from several bacterial strains have been shown to be chemically to human host cell surface molecules, D O-antigen binds Lipid Binding Protein, E LPS cannot be eliminated from contaminated drugs.
C Portions of the LPS from several bacterial strains have been shown to be chemically to human host cell surface molecules.
What is the chemical composition of exotoxins and how are they produced?
They are proteins that are secreted from the bacterial cell.
What are the 3 main classes of exotoxins?
Type I Toxins: Bind to membrane of host, do not enter host cell. Type II Toxins: Damage membranes: create pores / destroy structure. Type III Toxins: Intracellular acting. Can enter by endocytosis or injection.
What are the 2 sub-types of Type I Exotoxins?
Heat Stable Toxins and Superantigens
What are the 2 sub-types of Type II Exotoxins?
Pore-forming toxins and enzymatically activated toxins.
What are the 2 sub-types of Type III Exotoxins?
Type 3 Secretion System Cytotoxins and AB Toxins.
Heat Stable toxins are very short peptides containing lots of disulphide bonds, giving them a very strong structure. What are 3 types of these toxins produced by different types of E. Coli bacteria, and what is the result of them binding to host cell Guanylyl cyclase receptors?
STa, STb, EAST-1. Cascade of processes that cause host cell to secrete water and electrolytes (diarrhoea)
Describe Superantigens and their mechanism of action.
Small peptides, can be inactivated by heat. There are mitogens. Bind outside of MHC T-cell receptor, inducing proliferation of far more T-cells than in regular antigen presentation. Results in a cytokine storm, causing shock (so much inflammation, body cannot cope).
How do pore-forming toxins cause cell death?
Cell death by osmotic lysis.
Give an example of a pore-forming exotoxin.
S. aureus α-haemolysins made up of 7 subunits, inserted as a channel into erythrocyte membrane.
Enzymatically active exotoxins cause cell death by damaging the plasma membrane of host cells. What are the 2 main types of enzymes that they can exist as?
Phospholipases and Proteases.
What types of bacteria are Type 3 Secretion System Cytotoxins found in?
Gram-Negative bacteria: they are embedded into the outer plasma membrane of the cell wall.
Describe the mode of action of Cholera Toxin.
It is an AB toxin made up of 1 x A subunit, 5 x B subunits. Secreted in the intestine. 5B domain binds to the GN1 receptor found on enterocyte cells (intestinal epithelium cells). This induces endocytosis of the receptor-toxin complex, when travels to and fuses with the endoplasmic reticulum. The ‘A’ subunit is liberated into lumen of ER, then released back into cytoplasm, where it binds to and blocks activity of adenylate cyclase, leads to high levels of cAMP → transport of electrolytes outside of the cell (water will follow) (diarrhoea)
What is pyrogenicity?
Ability to induce fever.
