Bacterial Pathogens And Disease 1

Question 1

Define pathogen

Answer 1

A microorganism capable of causing disease.

Question 2

Define pathogenicity

Answer 2

The ability of a infectious agent to cause disease

Question 3

Define virulence

Answer 3

The quantities ability of a 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

Question 5

What are the 4 virulence mechanisms?

Answer 5

Adherence factors

Biofilms

Invasion of host cells and tissues

Toxins- endotoxins and exotoxins

Question 6

What are exotoxins?

Answer 6

Heterogeneous group of proteins produced and secreted by living bacterial cells

Produced by both gram negative and gram positive bacteria

Cause disease symptoms in host during disease

Act via a variety of diverse mechanisms

Question 7

What is the advantage of bacteria having exotoxins?

Answer 7

• Evade immune response
• Enable biofilm formation
• Enable attachment to host cells
• Escape from phagosomes

All allowing for colonisation, niche establishment and carriage - evolutionary advantage

Question 8

Do toxins cause disease?

Answer 8

No, but may help transmission

Question 9

How does the case of straphylococcus aureus express the important of exotoxins?

Answer 9

Haemolytic toxins cause cells to lyse by forming pores and have a Importance in symptoms of S aureus

Phenol soluble Modular’s PSM - aggregate the lipid bilayer of host cells (lysis)

Question 10

Describe the function of the different toxins associated with the movement of staphylococcus aureus?

Answer 10

Alpha toxin - initial attachment

Beta toxin - accumulation

E DNA - secondary structure formation

PSMs - detachment

Question 11

How are exotoxins classified?

Answer 11

Classification can be by the toxins activity
- membrane acting toxins - type 1
- membrane damaging toxins - type 2
- intracellular toxins - type 3

Question 12

Where are toxins expressed in the bacterial genome?

Answer 12

By extrachronsomal genes

• Plasmids
• Lysogenic bacteriophage

Question 13

What is the problem with classifying exotoxins?

Answer 13

Many toxins may have more than one type of activity

As mechanisms better understood this classification tends to break down

Question 14

How to membrane acting toxins TYPE 1 work?

Answer 14

Act:
- act from without the cell.

Interfere:
- interfere with host cell signalling by inappropriate activation of host cell receptors

Target:
- target receptors include:
. guanylyl cyclase (increase intracellular cGMP)
. adenyl cyclase, (increase intracellular cAMP)
. Rho proteins
. Ras proteins

Question 15

Give a example of membrane acting toxin?

Answer 15

E coli table heat toxin

Question 16

How do membrane damaging toxins TYPE 2 work?

Answer 16

Interact with the receptor and form a pore

1. Insert channels into host cell membrane (beta sheet toxins and alpha helix toxins)
2. Enzymatical damage

Can be receptor mediated or receptor independent

Question 17

How do intracellular toxins work?

Answer 17

Have 2 components (A and B)

Component B has the capacity to interact with a specific receptor - gets internalised by receptor mediated endocytosis

Component A is a toxigenic (enzymatic) that interfere with cellular processes

AB5 is heat sensitive while the rest are heat resistant

Question 18

Give examples of component A of intracellular toxins

Answer 18

ADP - ribosyl transferase

Glucosyltransferase

Deamidase

Protease

Adenylcyclase

Question 19

What are the 2 mechanisms by which exotoxins can induce a inflammatory response?

Answer 19

• Superatigen
• via activation of the different inflammasome leading to release IL1 and IL18

Question 20

How can exotoxin’s cause inflammation by superantigen?

Answer 20

Non specific bridging of the MHC class 2 and class T cell receptor leading to cytokine production.

Question 21

How can toxins be inn-activated?

Answer 21

inactivated using formaldehyde or glutaraldehyde → toxoids

Question 22

What are toxoids?

Answer 22

Toxoids are inactive proteins but still highly immunogenic – form the basis for vaccines.

Question 23

How are toxin mediated diseases treated?

Answer 23

Affected by administering preformed antibodies to the toxin

Eg.
. Diphtheria antitoxin – horse antibodies.
• Tetanus – pooled human immunoglobulin.

Question 24

What is clostridium difficile?

Answer 24

• gram-positive bacillus.
• anaerobic.
• spore-forming.
• toxin-producing.
• can be carried asymptomatically in the gut. • 3 toxins.

Question 25

What is the epidemiology of C. difficile?

Answer 25

. Common hospital acquired infection worldwide. • Spread by ingestion of spores Risk factors - antibiotic use, - age, - antacids - prolonged hospital stay.

Question 26

How can antibiotics contribute to the spread of C. difficile?

Answer 26

. Thought to act by disrupting the microbial ecosystem within the gut. • Antibiotics provide a competitive advantage to spore forming anaerobes over non spore forming anaerobes. • Allows C. difficile colonisation and growth. • All antibiotics have potential for causing disease.

Question 27

What are the cytotoxins of C. difficile?

Answer 27

Cytotoxin A - TcdaA coded by TcdaA gene Cytotosin B - TcdB coded by TbdaB gene Binary toxin C - minor role in disease

Question 28

Describe the process by which TedA/TedB cause down stream effects within the host cell.

Answer 28

1. Toxins binding to specific host cell receptors 2. Toxins internalisation 3. ENDO some acidification 4. Pore formation in the endosome 5. GTD release from the endosome to the host cell cytoplasm 6. Rho GTPase inactivation by 7. Downstream effects within the host cell.

Question 29

What is the symptoms of C. difficile?

Answer 29

Asymptomatic Watery dihoreah Dysentery Pseudomembranous Colitis Toxic Megacolon and Peritonitis

Question 30

How is C. difficile diagnosed in patients?

Answer 30

• Clinical signs and symptoms • Raised white cell count in blood. • Detection of organisms and toxins in stool • Detection of tcdA and tcdb genes – PCR • Colonoscopy – pseudomembranous colitis

Question 31

How is C. difficile detected with its toxin using the 2 stage test?

Answer 31

1. Glutamate dehydrogenase – detects if C. difficile organism present. 2. Toxin enzyme linked immunosorbent assay (ELISA) for TcdA and TcdB toxins.

Question 32

How is C. difficile treated?

Answer 32

Dependent on severity and presence of surgical complications • Ideally removal of offending antibiotic – not always possible • Antibiotics fidaxomicin or metronidazole or vancomycin • Surgery – partial, total colectomy • Recurrent – faecal transplant.

Question 33

What is VTEC disease?

Answer 33

Also known as Shiga-toxin (Stx) producing E. coli (STEC) can cause disease mild to life threatening disease. • Identified usually by growth on sorbitol MacConkey agar (SMac) – does not ferment sorbitol and hence is clear.

Question 34

What is the epidemiology of VTEC disease?

Answer 34

naturally colonizes the gastrointestinal tracts of cattle who are generally asymptomatic. Transmission • Predominantly via consumption of contaminated food and water • Person to person, particularly in child day-care facilities, and from • Animal to person. E.g. in petting zoos, dairy farms, or camp grounds.

Question 35

What is the pathogenesis of the toxin causing VTEC?

Answer 35

Toxin – Shiga like toxin (SLT) = shigatoxin (Stx) = verocytotoxin (VTEC) . Type III exotoxin – AB5 • Enzymatic component A = N-Glycosidase • Bound to 5 B subunits

Question 36

What is the mechanism by which Stx toxin effects cells?

Answer 36

• Bind to receptor globotriaosylceramide Gb3 or globotetraosylceramide (Gb4) on host cell membrane • Bound toxin internalised by receptor mediated endocytosis. • Carried by retrograde trafficking via the Golgi apparatus to the endoplasmic reticulum. • The A subunit is cleaved off by membrane bound proteases • Once in the cytoplasm A1 and A2 disassociate • A1 binds to 28S RNA subunit – blocks protein synthesis.

Question 37

Describe the pathogenesis of STEC

Answer 37

• STEC closely adheres to the epithelial cells of the gut mucosa. • The route by which Stx is transported from the intestine to the kidney and other tissues is debated, possibly polymorphonuclear neutrophils (PMNs) • Bind to glomerular endothelial cells of kidney, cardiovascular and central nervous system. • Very high levels of Gb3 in kidney so kidneys most affected. • Thought that Stx favours inflammation resulting in microvascular thrombosis and inhibition of fibrinolysis.

Question 38

What is STEC disease?

Answer 38

Children under 5 years are at greater risk Can be severe and life threatening Abdominal cramps, watery or bloody diarrhoea - may not be present

Question 39

How is STEC diagnosed?

Answer 39

•Clinical signs and symptoms •Haematological and biochemical evidence. •Stool culture – Growth on SMac •PCR for Stx genes

Question 40

How is STEC treated?

Answer 40

•Supportive including renal dialysis and blood product transfusion •Antibiotics have little to no role

Question 41

What are the symptoms of STEC disease?

Answer 41

• Haemolytic uraemic syndrome • Anaemia • Renal Failure • Thrombocytopaenia • Less common are neurological symptoms • lethargy, • severe headache, • convulsions, • encephalopathy.