Bacterial virulence and Pathogenicity Mechanisms

Question 1

What is the most common relationship between bacteria and humans?

Answer 1

Mutualism.

Question 2

What are the key stages in bacterial pathogenesis?

Answer 2

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.

Question 3

Define bacterial adhesion.

Answer 3

Measurable union between a bacterium and a substratum, requiring reversal energy.

Question 4

What is a substratum?

Answer 4

A surface with receptors for adhesins.

Question 5

What is an adhesin?

Answer 5

A ligand molecule capable of binding to a receptor, determining host and tissue specificity.

Question 6

What is an adhesiotope?

Answer 6

The adhesion site of a receptor.

Question 7

Define an invasin.

Answer 7

A bacterial ligand that induces a target cell to internalise the microbe, enabling bacterial invasion.

Question 8

Explain the difference between an adhesin and an invasin.

Answer 8

An adhesin facilitates attachment, while an invasin facilitates internalisation. Some adhesins can also function as invasins, but not all adhesins are invasins.

Question 9

What is phase variation?

Answer 9

Reversible on/off expression of certain genes, leading to changes in expressed proteins, through mutation or epigenetic regulation.

Question 10

What is antigenic variation?

Answer 10

Changes in bacterial surface antigens to evade the host immune system, achieved through gene recombination, point mutations, and gene conversion.

Question 11

What are two common structures implicated in bacterial adhesion?

Answer 11

Pili and outer membrane proteins on the bacterial capsule.

Question 12

How does antigenic variation assist bacterial adhesion?

Answer 12

By modifying adhesins, bacteria can avoid recognition by antibodies while maintaining their ability to attach.

Question 13

How does phase variation benefit bacterial adhesion?

Answer 13

It allows bacteria to adjust their ability to adhere to host tissues and evade immune detection.

Question 14

Define tissue tropism.

Answer 14

The tendency of a bacterium to colonise a specific tissue.

Question 15

Provide an example of tissue tropism.

Answer 15

Meningococci primarily reside in the nasopharynx.

Question 16

What is avidity in bacterial adhesion?

Answer 16

The concept that multiple interactions between adhesins and receptors strengthen binding, increasing the likelihood of advantageous host signalling for bacteria.

Question 17

Name 3 methods that bacteria use to evade the immune system.

Answer 17

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.

Question 18

What are challenges of using animal models to study human-specific microbes?

Answer 18

There is no natural colonisation due to species-specific receptors, Poor survival of microbes in animal blood due to differences in Fe acquisition.

Question 19

What are some approaches for studying human-specific bacterial adhesion?

Answer 19

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.

Question 20

How can microscopy be used to study bacterial adhesion?

Answer 20

Techniques like light microscopy, immunofluorescence, confocal microscopy, and electron microscopy are used to visualise interactions.

Question 21

What is haemagglutination?

Answer 21

A quantitative assay observing how bacteria interact with erythrocytes, causing them to clump together.

Question 22

Describe the viable count assay for studying bacterial adhesion.

Answer 22

Cultured cells are infected, washed to remove non-adherent bacteria, and surviving colonies are counted to measure adhesion.

Question 23

How does ELISA help study bacterial adhesion?

Answer 23

ELISA uses antibodies to detect and quantify binding between microbial adhesins and host substratum receptors.

Question 24

How can antibiotics be used to study bacterial invasion?

Answer 24

Tissue is infected, submerged in antibiotics to kill surface bacteria, washed, and broken up to measure internalised bacteria quantitatively.

Question 25

How can receptor density influence bacterial infection?

Answer 25

Higher receptor density can increase bacterial adhesion and infection, which may change during inflammation.

Question 26

What are the possible advantages of tissue tropism for bacteria?

Answer 26

Enhanced colonisation of specific tissues, Avoidance of host defences in non-target tissues, Access to nutrients specific to certain tissues.

Question 27

How can the host cell be damaged by the LPS?

Answer 27

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.

Question 28

What is a virulence factor?

Answer 28

A molecule or cellular structure that helps pathogens colonise a host and cause disease.

Question 29

What is meant by the term ‘mitogen’?

Answer 29

A substance that induces mitosis.

Question 30

What are haemolysins?

Answer 30

Lipids and proteins that cause lysis of erythrocytes by destroying their cell membrane.

Question 31

What does toxigenicity refer to?

Answer 31

The capacity of microorganisms to produce toxins.

Question 32

What is a cytokine storm?

Answer 32

A dramatic increase in cytokine concentrations.

Question 33

What are the 2 main classes of bacterial toxins?

Answer 33

1. Endotoxins (a.k.a. LPS), produced in Gram-Negative Bacteria only. 2. Exotoxins, produced in both Gram-Positive and Gram-Negative Bacteria.

Question 34

Do all Gram-Positive Bacteria produce the same toxins?

Answer 34

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.

Question 35

Traditionally how were toxins discovered and understood? How has this process improved in more recent years?

Answer 35

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.

Question 36

Describe the process of host cell damage from the release of Lipid A into the bloodstream.

Answer 36

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.

Question 37

How have the effects of LPS on hosts been studied?

Answer 37

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.

Question 38

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?

Answer 38

Dose of LPS (greater dose = quicker death), route of administration and species of animal (some are more susceptible than others.

Question 39

Often pharmaceutical drugs are obtained using engineered bacteria. This could result in drugs becoming contaminated with LPS. How is this problem overcome?

Answer 39

Labs test small portion drugs on amoebocytes (amoeba-like cells found in Limulus; very mobile) If LPS present they will die and stop moving.

Question 40

What is a key use of LPS in developing therapeutic drugs?

Answer 40

Can test efficiency of drugs that inhibit inflammation - more effective drugs will inhibit inflammatory effects of LPS.

Question 41

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.

Answer 41

C Portions of the LPS from several bacterial strains have been shown to be chemically to human host cell surface molecules.

Question 42

What is the chemical composition of exotoxins and how are they produced?

Answer 42

They are proteins that are secreted from the bacterial cell.

Question 43

What are the 3 main classes of exotoxins?

Answer 43

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.

Question 44

What are the 2 sub-types of Type I Exotoxins?

Answer 44

Heat Stable Toxins and Superantigens

Question 45

What are the 2 sub-types of Type II Exotoxins?

Answer 45

Pore-forming toxins and enzymatically activated toxins.

Question 46

What are the 2 sub-types of Type III Exotoxins?

Answer 46

Type 3 Secretion System Cytotoxins and AB Toxins.

Question 47

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?

Answer 47

STa, STb, EAST-1. Cascade of processes that cause host cell to secrete water and electrolytes (diarrhoea)

Question 48

Describe Superantigens and their mechanism of action.

Answer 48

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).

Question 49

How do pore-forming toxins cause cell death?

Answer 49

Cell death by osmotic lysis.

Question 50

Give an example of a pore-forming exotoxin.

Answer 50

S. aureus α-haemolysins made up of 7 subunits, inserted as a channel into erythrocyte membrane.

Question 51

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?

Answer 51

Phospholipases and Proteases.

Question 52

What types of bacteria are Type 3 Secretion System Cytotoxins found in?

Answer 52

Gram-Negative bacteria: they are embedded into the outer plasma membrane of the cell wall.

Question 53

Describe the mode of action of Cholera Toxin.

Answer 53

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)

Question 54

What is pyrogenicity?

Answer 54

Ability to induce fever.