Lecture 5: Biological agents as causes of disease

Question 1

__ of all humans deaths are caused by pathogens

Answer 1

1/3

Question 2

pathogens are

Answer 2

infectious viruses, bacteria, protozoans or fungi

Many scientist would include worms and prions on this list

Question 3

The average human contains __ human cells and __ microbial cells

Answer 3

10^13
10^14

These flora are usually distinct from pathogens

Question 4

Obligate pathogens

Answer 4

can only survive in host - usually very specific to host species

Question 5

Facultative pathogens

Answer 5

present in the environment (reservoir) waiting for host

Question 6

Opportunistic pathogens

Answer 6

normally benign but cause disease in compromised host

Question 7

why do pathogens make us sick?

Answer 7

The symptoms of the disease usually help spread the pathogen (eg diarrhoea) or because the pathogen kills cells in order to replicate. In some cases the symptoms appear to have no advantage or are part of the host’s response.

Question 8

Bacteria :

Answer 8

cause cholera, food poisoning, syphilis and gonorrhea

Question 9

Eucaryotes:

Answer 9

cause malaria, thrush and athlete’s foot

Question 10

Viruses:

Answer 10

cause AIDS, small pox and the common cold

Question 11

Two closely related species can be pathogenic and harmless:

Answer 11

the species often differ by only a few genes called virulence genes.

Question 12

Fungal pathogens often show

EXAMPLES

Answer 12

DIMORPHISM.

-Histoplasma capsulate:
The soil form grows as a mould, but in a warm body it switches to the yeast morphology.

Candida albicans is consumed by macrophages.

After the macrophages engulf the yeast-like the fungus responds by rapidly growing a “germ tube”.

This projection eventually pierces the macrophage from the inside, killing the attacking macrophage.

Question 13

Protozoa parasites often have

Answer 13

more than one host
MALARIA:
Plasmodium falciparum

Gametes (haploid cells) are made in human blood - These fuse to form zygotes in the gut of the Anopheles mosquito

Question 14

Biting insects are a great way to

Answer 14

get into a host - called vectors.

Viruses that cause dengue and yellow fever can also replicate in insect cells

Question 15

much of our epithelia are densely populated by

Answer 15

bacterial and fungal flora which serves as a barrier to infection

Question 16

epithelial cells are fit together very tightly and held together by

Answer 16

tight junctions, these asc like a gasket to make sue that pathogens can’t squeeze between cells

Question 17

Often epithelia secret…

Answer 17

mucous that is continuously flushed from the lower lung, small intestine and bladder.

Question 18

breaks in the epithelia are quickly recognised by

Answer 18

WBC

Question 19

Some bacteria have P pili and use sticky proteins called _____ to anchor them to the epithelia.

Answer 19

ADHESINS

Adhesins have “receptors” (protein targets) on the host cell that normally serve other function (eg we can’t just get rid of the receptors to block infection).

Question 20

Why breach the cell membrane?

Answer 20

To inject toxins or to replicate inside of the cell

Question 21

Toxins are often used to kill host cells for two reasons:

Answer 21

• killing host cells provides nutrients

- killing white blood cells helps pathogens evade the immune system.

Question 22

Many bacteria have Type ___

Answer 22

lll (3) secretion systems that act like syringes

Question 23

E.Coli assembles a ___ to overcomes barriers

Answer 23

an actin pedestal to hold them in place.

Tie alone is sufficient to cause this massive rearrangement of the host cell’s morphology. (forms pinch round it almost)

Question 24

bacteria___ to enter cells

Answer 24

bacteria hide in cells.

• Pathogens can evade the immune system by hiding in a cell.
• Legionnaire’s disease is caused by a bacterium that normally infects amoebae, Legionella pneumophila.

If inhaled, the bacteria is phagocytosed by macrophages. However, unlike most bacteria, L. pneumophila replicates inside the macrophage.

Question 25

Bacteria also invade nonphagocytic cells by two methods:

Answer 25

Zipper mechanisms
-like velcro

Trigger mechanism:
-active rearrangement of the cell membrane

Question 26

Listeria entering the cell

Answer 26

• L. monocytogenes secretes hemolysin, a protein that breaks down the membrane of the phagosome.
• Once in the cytoplasm, the effects of hemolysin are stopped by the PEST amino acid sequence which targets the protein for destruction (PEST = proline/glutamic acid/serine/threonine).
• L. Monocytogenes then begins to replicate within the host cell.

Question 27

Listeria entering the next cell

Answer 27

L. monocytogenes then assemble actin tails that push them into the neighbouring cell.

ActA is a bacterial protein that is localized to the tail end and nucleates actin polymerization at this end of the cell.

Question 28

Antibiotics stop bacterial growth by

Answer 28

disrupting cellular processes

Question 29

antibiotics: cell membrane

Answer 29

polymyxins

Question 30

antibiotics: cell wall synthesis

Answer 30

• Vancomycin
• penicillins
• cephalosporins

Question 31

antibiotics: folic acid biosynthesis

Answer 31

• trimethoprim

- sulfonamides

Question 32

antibiotics:

DNA gyrase

Answer 32

quinolones

Question 33

antibiotics: RNA polymerase

Answer 33

rifampin

Question 34

Antibiotic: Protein synthesis, 30S inhibitors

Answer 34

• tetracycline

- streptomycin

Question 35

antibiotic: protein synthesis, 50S inhibitor

Answer 35

• erythromycin

- chloramphenicol

Question 36

antibiotics: Addition of different side chains at the R and x positions have led to

Answer 36

new generations of cephalosporins that have improved activities. Most drugs are continually being tinkered with in this fashion.

Question 37

pathogens modify the behaviour of the host to suit their needs:

Answer 37

• Yersinia pestis blocks the flea’s digestive tract causing it to starve - and bite more.
• lyssavirus sp. cause rabies - biting spreads the disease.
• Toxoplasma gondii can only complete its life cycle in cats - but it gets from cat to cat travelling by rat. Infected rats loose their fear of cats and are even attracted to cat smells!
• Cordyceps sinensis infects ants and then causes them to climb up branches and clamp on then die. The hosts body then serves as a source of nutrients while the fungus grows. Finally, the fruiting body of the fungus emerges from the insect and spreads more spores.