Pathogens and the Host

Question 1

What are signs and symptoms of clinical infection?

Answer 1

inflammation
pain
pyrexia
tachycardia
rigors
increased white cell count
Increased C reactive protein (CRP)

Question 2

What is a pathogen?

Answer 2

organism that can cause disease

Question 3

What is a commensal?

Answer 3

A commensal is an organism which is part of normal flora e.g. E. coli in the gut, Staph aureus in the nose, axilla

Question 4

How do we know if organism is a pathogen or commensal?

Answer 4

koch’s postulates:
organism must be found in all cases of the disease
able to be cultured outside the body for several generations
should reproduce the disease on inoculation

sterile vs non-sterile sites

knowledge of
normal flora for site
organism’s pathogenicity
clinical context

Question 5

What is pathogenicity?

Answer 5

The capacity of a micro-organism to cause an infection

Question 6

What are requirements for pathogenicity?

Answer 6

infectivity- ability to become established

virulence- ability to cause harmful effects once established

Question 7

How is infectivity achieved?

Answer 7

Attachment
E. coli-
P-fimbriae
Receptor on uroepithelial cells

Acid resistance
Helicobacter pylori-
Urease
Makes ammonia from urea

Question 8

How is virulence achieved?

Answer 8

Conferred by virulence factors
Genetically determined microbialcomponents-
Invasiveness
Toxin production
Evasion of immune system

specific to strains not species (staphaureus does not share same pathogenicity as other staphylococci)

Question 9

How is invasiveness achieved in streptococcus pyogenes (Group A streptococci)?

Answer 9

connective tissue breakdown
-hyaluronidase
-collagenase

fibrinolysis
-streptokinase enzyme

Question 9

Describe exotoxins?

Answer 9

released extracellularly by the micro-organism

Question 9

Describe endotoxins?

Answer 9

structurally part of the gram negative cell wall

Question 10

Describe enterotoxins?

Answer 10

exotoxins which act on the GI tract

Question 11

Describe the exotoxin: Tetanus?

Answer 11

Clostridium tetani (obligate anaerobe)

Infection of dirty wounds

Toxin production:
Binds to nerve synapses
Inhibits release of inhibitory neurotransmitters

Death caused by respiratory paralysis

Treated by debridement, antibiotics and antitoxin

Question 12

Describe the enterotoxin: Cholera?

Answer 12

Vibrio cholerae-
Colonises small intestine

Enterotoxin production:
Increases cAMP levels
Inhibits uptake of Na+ and Cl- ions
Stimulates secretion of Cl- and HCO3- ions
Passive (massive) outflow of H2O

Causes death by dehydration

Treated by rehydration

Question 13

Describe super antigens?

Answer 13

Certain exotoxins of Strep pyogenes and Staph aureus
Able to stimulate division of T cells in the absence of specific antigen
Overwhelming cytokine production causes “toxic shock”

Question 14

How is endotoxin structured?

Answer 14

Component of the Gram-negative bacterial cell wall:

Lipopolysaccharide-
Lipid A -THE NASTY PART
Oligosaccharide core
Specific polysaccharide chain

Question 15

What is meaning of MRSA and MSSA staphylococcus aureus?

Answer 15

MRSA- methicillin resistant staph aureus
MSSA- methicillin sensitive staph aureus

Question 16

Why is staphylococcus aureus commonly penicillin resistant?

Answer 16

due to beta lactamase

Question 17

Describe clostridium perfringens? (gram positive bacilli)

Answer 17

Found in soil and normal commensal in human and animal gut/faeces

in contaminated food, can cause food poisoning (enterotoxin-producing strains)

in serious wounds (wartime) can cause “gas gangrene”

Question 18

What are three characteristic ways by which virus causes pathogenesis in the host?

Answer 18

Cell destruction following virus infection
-Death of T4+ cells by HIV

Virus-induced changes to cellular gene expression
-Cellular transformation by tumour viruses

Immunopathogenic disease
-Influenza A virus.
-Coxsackievirus-induced myocarditis

Question 19

How does virus enter cell?

Answer 19

respiratory tract
alimentary tract
urinogenital tract
arthropod vector
capillary
skin
conjunctiva

Question 20

Describe an acute infection?

Answer 20

so after person gets infected, there is a peak up and down. Virus production is high and the human response dominates and is able to control infection.

Question 21

Describe latent infection?

Answer 21

after acute infection, the virus goes and hides away and keeps dormant for ages. After certain stimulus it comes back and attacks exactly the same as original acute infection attack

Question 22

Describe chronic infection?

Answer 22

after acute infection, the virus load doesn’t go back to basic level (doesn’t disappear) and virus remains above baseline for life.

Question 23

Describe tumour virus infection?

Answer 23

acute infection is mild or very little and virus remains in very low level in background causing very slow changes in cell until becomes malignant .

Question 24

Give examples of acute virus infections?

Answer 24

Influenza A virus- respiratory infection
enterovirus- enteric and neurological infections

Question 25

What is viraemia?

Answer 25

means virus reaches the blood stream

Question 26

How does pathogenicity occur in Influenza A?

Answer 26

Virus infects cells of the respiratory tract

Destruction of respiratory epithelium
-Secondary bacterial infections

Altered cytokine expression leading to fever
-e.g interleukin-1 and interferon
-Avian influenza virus (H5N1) causes a “cytokine storm”

Question 27

Describe antigenic drift?

Answer 27

Minor changes (natural mutations) in the genes of flu viruses, occurs gradually over time to generate antigenic variants

Question 28

Describe antigenic shift?

Answer 28

Abrupt major changes in virus antigenic structure

Question 29

How is the generation of novel influenza brought about?

Answer 29

non human hosts for influenza A viruses play a key role in generating new virus types e.g. avian influenza, swine influenza

Question 30

Give examples of enterovirus infections?

Answer 30

Poliomyelitis (poliovirus)
Aseptic meningitis (many enteroviruses)
Myocarditis (coxsackie B viruses)
Pancreatitis (coxsackie B viruses)
Respiratory infections (many enteroviruses)

Question 31

Mechanisms of enterovirus?

Answer 31

acquired through GI system, goes to gut and from the gut, it goes into the bloodstream by viraemia. Either goes to non neuronal tissue (heart…pancreas) or neuronal tissues (CNS) (that can cause paralysis).
Virus in gut gets excreted in stools

Question 32

Give example of latent virus infection?

Answer 32

herpes simplex virus- cold sores (type 1 ) and genital lesions (type 2)

Question 33

Describe the herpes virus infection?

Answer 33

virus infects the epithelium and cold sore happens
after this episode, virus migrates to nerve ganglia and virus latent in nucleus
then a stimulus occurs and virus gets reactivated and migrates to epithelium.
Leads to virus replication

Question 34

Give examples of virus-induced tumours?

Answer 34

Papillomaviruses (HPV)-cervical carcinoma
Retroviruses- lymphomas and leukemias

Question 35

Describe the retrovirus replication cycle?

Answer 35

Virus attaches to cell by membrane fusion and uncoating process happens.

Membrane fusion releases the viral nucleic acid material on its own in the infected cell.

By a process of reverse transcription- RNA/DNA hybrid forms and then there is the viral DNA formed- double stranded DNA.

This migrates into nucleus and integrates into host nucleic acid material and through viral DNA integration, provirus is formed.

Question 36

Describe Human T-lymphotropic virus-1 (HTLV-1) (virus induced tumours)?

Answer 36

Transmission by blood and mother to child.
Infects T cells
Modifies host cell gene expression using a Transactivating protein (tax)
Variety of diseases including leukaemia (Adult T-cell Lymphoma-Leukaemia)
Directly responsible for the tumour.