2- Bacterial Pathogens I

Question 1

define a pathogen

Answer 1

a microorganism capable of causing disease – adapted to cause infection, colonises host and causes damage

Question 2

define pathogenicity

Answer 2

the ability of an infectious agent to cause disease

Question 3

define virulence

Answer 3

the quantitative ability of an agent to cause disease

Question 4

define toxigenicity

Answer 4

the ability of a microorganism to produce a toxin that contributes to the development of disease

doesn’t necessarily kill the host but interacts with it to promote disease development

Question 5

define an endotoxin

Answer 5

lipopolysaccharide component of the outer membrane of gram-negative bacteria, released upon bacterial cell death or lysis

Question 6

define an exotoxin

Answer 6

heterogenous group of highly toxic molecules produced and secreted by gram-negative & positive bacterial cells

each type acts through specific modes of action to cause disease

Question 7

define a virulence factor

Answer 7

specific trait of a pathogens that enables them to cause disease or infection in a host organism

Question 8

list examples of virulence factors

Answer 8

adherence factors
biofilms
endotoxins and exotoxins
ability to invade host cells/ tissues

Question 9

what is a biofilm?

Answer 9

a complex community of microorganisms, mainly bacteria, encased within a self-produced extracellular matrix

Question 10

how do biofilms form? how do they contribute to disease as virulence factors?

Answer 10

formation:
micro-organisms adhere to surfaces and produce extracellular substances that form a protective matrix. the matrix embeds organisms, enhancing cell-surface attachment for colonisation

contribute to disease:
- reservoir for infection
- contribute to bacterial persistence
- facilitate attachment of cells to surfaces for colonisation and forming niches
- virulence factors promote biofilm production

Question 11

what are adherence factors? how do they contribute to disease as virulence factors?

Answer 11

adherence factors:
- bacterial molecules, allow attachment of bacteria to surfaces/ cells

contribute to disease:
- important in initial colonisation = enable colonisation and forming a favourable niche
- interact with host cells and promote establishing the infection

Question 12

how do bacterial virulence factors promote invasion of host cells and tissues?

Answer 12

bacterial cells produce molecules which interact with bacterial cells and host surfaces/cells

they promote destruction of the immune barrier and inhibit mechanisms keeping the barrier in place

Question 13

concerning general bacteria, why have exotoxins?

Answer 13

facilitate bacterial transmission through selective advantages:
- evade immune response by interfering with the host immune system
- enable biofilm formation and allow bacteria to adhere to surfaces
- facilitate bacterial attachment to host cells, promoting colonisation and infection
- help in escaping phagosomes within host cells, evade immune destruction

Question 14

why are exotoxins considered an evolutionary dead end?

Answer 14

exotoxins help in disease transmission and causing disease symptoms, but interacting with the host can cause severe disease and kill the host

comprising the host’s survival affects bacteria’s transmission

Question 15

list the various haemolytic toxins produced by Staphylococcus aureus. why are they important?

Answer 15

haemolytic toxins:
- alpha, beta and gamma toxins
- PVL
- LukAB, LukED and MF
- PSMs (phenol soluble modulins)

importance:
- contribute to pathogenicity - e.g. alpha, beta and gamma toxins cause red cell lysis by forming pores in the cell membrane

Question 16

what is the role of phenol-soluble modulins (PSMs) in S. aureus infections?

Answer 16

help S. aureus escape phagosomes = allow it to survive and propagate within host cells

assist in biofilm formation = enable bacterial colonization and stability

Question 17

How do toxins released by S. aureus in the nose contribute to its ecological role? - examples of toxins, what they do?

Answer 17

alpha toxins:
- inhibit lysosome-phagosome fusion = allows bacteria to evade immune destruction

PSMs:
- helps S. aureus movement through surfaces as it’s a non-flagellated bacteria = can reach areas of the respiratory tract
- allow S. aureus to establish a bacterial community in the nose as its main ecological niche

Question 18

how do different S. aureus exotoxins contribute to biofilm formation?

Answer 18

alpha toxins:
- important for initial attachment of bacterial cells to mucosal surfaces in the upper resp. tract

PSMs & beta toxins:
- with cellular DNA, contribute to stablising biofilm structures

Question 19

what are chromosomal encoded exotoxins? how are they spread within bacterial populations?

Answer 19

these encoded exotoxins are fixed within the bacterial chromosome - produced and released as part of the bacterial life cycle

they spread through bacterial division

Question 20

examples of chromosomal gene-encoded exotoxins (2)

Answer 20

Shiga toxin
TcdA and B in C. difficile

Question 21

what are plasmid-encoded exotoxins?

Answer 21

these exotoxins are carried on plasmids

can be transferred between bacteria through plasmid sharing or incorporation into bacteriophages

Question 22

examples of plasmid-encoded exotoxins (2)

Answer 22

Bacillus anthracis toxin
tetanus toxin

Question 23

how do some exotoxins have a dual genetic origin? what is an advantage of this?

Answer 23

some exotoxins have a mixed genetic origin = encoded by both chromosomal genes and fragments of DNA carried on plasmids

this dual origin allows for additional mechanisms of spread and regulation within bacterial populations

Question 24

list the three classifications of exotoxins

Answer 24

type 1 - membrane acting toxins
type 2 - membrane damaging toxins
type 3 - intracellular toxins

Question 25

describe type 1 exotoxins

Answer 25

they act at the level of the host cell membrane, interfering with cell signalling by inappropriately activating host cell membrane receptors

they target receptors such as Rho and Ras proteins, guanylyl cyclase and adenylyl cyclase

Question 26

what type of toxin is the E.coli stable heat toxin

Answer 26

type 1 - membrane acting toxin

Question 27

how does E.coli stable heat toxin act as a type 1 exotoxin to disrupt normal function and health?

Answer 27

mechanism:
- E.coli stable heat toxin binds to guanylyl cyclase coupled receptor, affecting cGMP levels
- this affects protein kinase G2 activity which activates CFTR - a cystic fibrosis transmembrane regulator
- this affects ion channel exchanges, affecting electrolyte balances in the gut and dysregulating water absorption = leads to watery diarrhoea

• changed cGMP levels also inhibits PDE3 which increase cAMP production, activates PKA
• PKA blocks Na-H proton exchanger activity, inhibiting Na+ absorption into the gut lumen so more water follows osmotically = leads to diarrhoea

affects normal health by inducing diarrhoea

Question 28

describe type 2 exotoxins

Answer 28

act at the level of the host cell membrane, damage it by:
- inserting channels into the host cell membranes
- causing enzymatic damage

Question 29

list the two types of type 2 exotoxins

Answer 29

1. receptor mediated
2. receptor independent

Question 30

what are the two types of type 2 exotoxins? how do they cause damage? examples of exotoxins?

Answer 30

1. receptor mediated
   - pore formation
   - e.g. beta sheet toxins, alpha toxins like diphtheria toxin
2. receptor independent
   - enzymatic activity
   - e.g. alpha PSMs

Question 31

how do type 2 membrane damaging receptor-mediated toxins cause damage? why are some cells more damaged than others?

Answer 31

they bind to a specific receptor on a cell membrane, form a pore that isn’t regulated

contents of the cell leave through the unregulated pore

some cells are more damaged by a receptor-mediated toxin as they have the specific receptor the toxin needs to bind to for the pore to form

Question 32

how do type 2 membrane damaging receptor-independent toxins cause damage?

Answer 32

exotoxin attaches to membranes and destroys cell through enzymatic activity

Question 33

describe type 3 exotoxins

Answer 33

intracellular toxins - AB toxins, two components

released into extracellular space, taken up by host cells via specific receptors present on the cell surface and act inside the cell

Question 34

describe the structure and role of AB toxins

Answer 34

AB toxins have two components:

1. component A
   - enzymatic and toxigenic; acts within cell through enzymatic activity
   - interferes with essential intrac. processes causing cell death and damage
2. component B
   - for receptor binding and translocation/ internalisation into host cell
   - toxin binds to specific receptors on host cell surface, internalised through receptor-mediated endocytosis into an endosome

Question 35

what other mechanism do type 3 intracellular toxins use to cause disease?

Answer 35

they use secretion systems and factors - through complex multi-structured proteins, bacteria inject toxins directly into host cells

type III secretion factors/ systems allow fast delivery of toxins into host cells

Question 36

describe the mechanism through which S. aureus toxins alter/affect T-cell activity, involving superantigens and inflammatory cytokines - what effect does this have?

Answer 36

gram-positive exotoxins - e.g. superantigens released by S. aureus - can bypass the specific peptide recognition and binding step of T-cells to APC cells presenting an antigen peptide via the MHC complex

superantigens lock MHC presenting the antigen and T-cell together through non-specific binding - triggers polyclonal T-cell activation of non-specific T cells

activated T cells produce high levels of inflammatory cytokines - e.g. IL-2, 6, interferon alpha and gamma, TNF-1

cytokines induce an inflammatory immune response regardless of antigen specificity = body acts like it’s under attack, induces inflammation and promoted bacterial colonisation

macrophages subsequently deactivated, more interferon-gamma and cytokines produced = more inflammation

S. aureus toxins can induce toxic shock syndrome this way from over activating the immune system

Question 37

what is a toxoid?

Answer 37

inactivated toxin - through using formaldehyde or glutaraldehyde

Question 38

how are toxoids used in treatment of bacterial infections? examples?

Answer 38

toxoid = inactivated toxins using formaldehyde or glutaraldehyde

they retain their antigenic properties and structure, lose their toxic effects and makes them highly immunogenic

used in vaccines for toxin-mediated diseases by inducing an immune response, the body produces antibodies against the infection

e.g. tetanus and diphtheria vaccines

Question 39

what is an antibody?

Answer 39

proteins produced by the immune system in response to foreign substances/ antigens like toxins

Question 40

how do antibodies work? how are they used in the treatment of disease? examples?

Answer 40

antibodies bind to specific disease antigens - neutralise their effects and mark them for destruction by immune cells

antibodies can also block toxin activity by preventing them from binding to their receptors on target cells

vaccines stimulate antibody production against a specific toxin/antigen = provides immunity

e.g. diphtheria toxin from horse antibodies, tetanus antitoxin from pooled and purified human immunoglobulins

Question 41

what are monoclonal antibodies?

Answer 41

labatory produced antibodies that target specific antigens/toxins

used for therapeutic purposes – e.g. neutralising toxins, blocking their effects, targeting toxin-producing cells