Microbes and disease

Question 1

List the 5 main types of pathogens

Answer 1

Viruses, Bacteria, Single cell eukaryotes, host derived infections (e.g. prions)

Question 2

Microbial pathogens are overwhelmingly infectious. What does this mean?

Answer 2

The pathogen passes from infection source to host

Infection not necessarily pathological - also required by commensals and symbionts

Question 3

Bacteriophage are infectious agents

Answer 3

Lytic phage populations can only be sustained by the process of infecting bacterial host, replicate at expense of host, are related into environment upon bacterial lysis, and life cycle is complete when another bacterium is infected

Question 4

Bacteriophage fact

Answer 4

They are the most abundant biological entities on the planet (more bacteriophage than there are stars in the universe)

Question 5

Host to host transmission- define

a number of bacterial sexually transmitted diseases fall into this category

Answer 5

Pathogens reside permanently in the host population without an environmental reservoir - always going from host to host (direct transmission)

Question 6

On average, each infection must give rise to another…

Answer 6

Each infection must give rise to another infection

Question 7

Chlamydia

An obligate intracellular pathogen of humans

Answer 7

• An example of a pathogen that resides permanently in the host without an environmental reservoir
• Evidence of genome reduction (1000 genes) - metabolic specialisation to intracellular life
• Evidence of high rates of recombinations among the strains (not subject to Muller’s ratchet)

Question 8

What is horizontal transmission

Answer 8

Passing of infectious agents among different individuals

Question 9

Vertical transmission

Answer 9

Passing of infections from parent to offspring

Question 10

How can vertical transmission occur

Answer 10

Can occur by intercellular, transplacentally (via milk, or during birth e.g. HIV)

Question 11

Host immunity define

Answer 11

When hosts are immune to further infection they are removed from the transmission system

Duration of protection up to life of the host e.g. measles virus

Question 12

Outline everything you know about the measles virus

Answer 12

• Infectious by the respiratory route
• No known reservoir other than humans
• Disease of childhood. - single infection provides lifelong immunity
• Therefore… the virus and disease can only persist in host populations of sufficient size - the virus and its disease becomes extinct in small island populations
• Immunity is long term so immunisation can eradicate the disease - high rates of coverage are needed

Question 13

What are acute infections

Answer 13

• Occur over a short period of time - during which pass to another host
• Vulnerable to excess host death/immunity

Question 14

How to avoid the problem of acute infections?

Answer 14

• Establishment of a chronic infection - pathogen remains in a quiescent state in the host but can be reactivated to become infectious such as chicken pox/ shingles / tuberculosis

Question 15

What are zoonoses and species leaps

Answer 15

Pathogens that have evolved in one host species occasionally infect another host species –recipient species not capable of passing on transmission - dead end hosts e.g. rabies

BUT if transmission among members of the new species occurs then novel infectious cycle emerges e.g. may be Black Death, Avian flu, SARS, ebola

Question 16

Outline everything you know about the rabies virus

Answer 16

Virus found in many mammal species, spread by biting. Virus moves through Nervous system within the host –> intracellular

Changes behaviour when it reaches the brain (aggression - growth of virus in the salivary gland promotes spread

Question 17

Multi host transmission systems

Answer 17

Some pathogens have complex life cycles with multiple species
Takes adv. of one host to infect another - may be to enable infection, generate an environmental reservoir, or promote spread

Question 18

Why are complex life cycles mostly costly

Answer 18

• Survive in different environments (e.g. insects and mammals)
• Evade different immune responses

Question 19

Schistosomiasis outline

bilharzia

Answer 19

• Humans infected from contaminated water
• Infective cercariae swim to the host and penetrate skin
• Migrate in tissues, reside in veins
• Eggs are deposited and move to the intestine and are released to the environment (urine and faeces)
• Eggs hatch into miracida and infect snails
• Growth of sporocysts in the snail results in release of cercariae - completing the cycle

Question 20

How can microbes cause cancer

Answer 20

• Direct/indirect effect of infection : alteration of host cells/ production of toxic products
• e.g. Humanpapillomavirus

Question 21

What are prions

Answer 21

Infectious protein agents - infection is a particular form of the protein - present in mammalian brains
conversion of protein to pathogenic form is autocatalytic
can be transmitted orally and are highly resistant to normal sterilisation techniques
Transmissible encephalopathies (TSEs) include kuru in humans

Question 22

Key points

Answer 22

• Microbial pathogens have evolved throughout the tree of life - representing all classes of microbe.
• Predominantly infectious - spreading from one host to another

Can have diverse effects on their hosts - some are necessary for microbial survival and others are incidental