Principles of infection 1

Question 1

Define saprophyte

Answer 1

Harmless microbes that live on inanimate material and derive their nutrition from these environmental sources. DON’T require a human or animal host to survive.

Question 2

Define commensal

Answer 2

Microbes that reside on the surfaces or mucosa of the body without harming the hosts health.

These are sometimes referred to as ‘normal flora’

Question 3

Opportunistic infection

Answer 3

When the commensals have a pathogenic effect on their host due to the host’s immune system and defence being compromised

Question 4

Define pathogen

Answer 4

A microbe that causes harm to its host and initiates infection, usually resulting in an inflammatory response from the host

Question 5

Describe the first stage of infection

Answer 5

• Survival of microbe and spread to host

Microbes must be able to survive in environment they live in. Some use host environments, others water, soil, air, dust. Some produce spores which can survive extreme heat ad drought and survive for many years.

Others have a waxy coat

Question 6

Describe the second stage of infection

Answer 6

Adherence of microbe to host cell and entry into the host

Microbes must adhere to host tissue

This can be by attachment to host tissues via fimbriae (pili), and adhesins

Question 7

What are fimbrae (pili) ?

Answer 7

Hair- like protein appendages present on the cell wall of bacteria, allowing attachment to other cells and surfaces

Question 8

What are adhesins?

Answer 8

Cell surface components of bacteria that enable adherence to other cells and surfaces

Question 9

Describe the third stage of infection

Answer 9

Multiplication

Once microbes are internalised, either by active processes (viral) or passive (bacterial0, they multiply locally to produce a focus of infection

During multiplication, many microbes produce variety of tissue-destroying enzymes which allow invasion of surrounding tissues as well as deeper sites

Number of bacteria also produce toxins + tissue destroying enzymes

Following multiplication, infection may remain localised e.g. abscess OR spread to contiguous tissues e.g. cellulitis, septicaemia via blood.

Question 10

Describe the 4th stage of infection

Answer 10

Evasion of host defences

As multiplication process continues, host defence mechanism mobilises resources in attempt to contain process

Question 11

Describe the fifth stage of infection

Answer 11

Damage to the host

This can be a result of the actions of the microbe or the host’s own immune response

Question 12

Describe the 6th stage of infection

Answer 12

Shedding of the microbe and spreading to the environment or another host

If host defence succumbs, pathogenic microbe overwhelms the host, causing disease. In very severe infections, this can mean death, so the microbe would need to find a new host. The microbe, hence, must exit the host, and this can be via airborne such as sneezing, coughing, via GI tract, epithelial surfaces such as genetical tract, bodily fluids onto fomites and blood-sucking insects

Question 13

What are the routes/modes of transmission of infection?

Answer 13

• Airborne/droplet spread - usually occurs in respiratory tract infections, e.g. mycobacterium tuberculosis, influenza A
• Via GI tract - faeco-oral route
• Through skin or bloodstream- Mosquito bites lead to transmission of plasmodium falciparum (malaria), staphylococcus aureus via intravenous lines
• Fomites- these are objects likely to carry infection- these could be toys contaminated with staphylococcus aureus or RSV

Question 14

What are acute infections?

Answer 14

Rapid onset, brief period of symptoms (can be mild, moderate, severe or asymtomatic) and resolve within days

Question 15

What are chronic infections?

Answer 15

Persist for weeks, months, maybe years or entire life. For example, tuberculosis and HIV are chronic infections

Question 16

What determines whether an infection is acute or chronic?

Answer 16

The properties of the microbe and/or the host determine whether an infection is acute or chronic

Some acute infections can become chronic if a person is immunocompromised and their defence isn’t strong enough.

Some infections can become chronic in every host, as the microbe has properties that allow it to elude the host’s defences and immune system

Question 17

What are asymptomatic infections?

Answer 17

When patients do not experience symptoms but remain infectious. Covid-19 caused hosts that were asymptomatic.

Virulent microbes almost always cause symptomatic infections, but many range in severity of symptoms.