S1: Principles of Infection I

Question 1

Name some microbes

Answer 1

Virues
Bacteria
Fungi
Protozoa
Helminthes
Ectoparasites

Question 2

What are saprophytes?

Answer 2

Harmless microbes that live on inanimate material and derive their nutrition from environmental sources

Question 3

What are symbionts?

Answer 3

Microbes that require a human/animal host to survive and multiple –> It means ‘living together’

Note it does not distinguish between harmful and beneficial relationships

Question 4

What is commensalism?

Answer 4

It is a relationship in which the symbiont benefits from the relationship (e.g. shelter, food) but the host is neither helped or harmed

Question 5

What is mutualism?

Answer 5

Where both the host and symbiont benefit from each other

Question 6

What are commensals sometimes called?

Answer 6

Normal flora

Question 7

Difference between colonisation and infection

Answer 7

• Commensals (normal flora) are said to colonise the host, no harm is done to the host
• Infection implies harm to the host. Usually the host will manifest an inflammatory response to a pathogen but not a colonizer at a normally non-sterile part of the body.

Question 8

Give examples of situations where commensals can act a pathogen

Answer 8

• If hosts defences are weakened (immunocompromised) infection may occur –> This is called opportunistic infection
  e. g. HIV patient infected by an organism with low virulence
• Also some organisms are commensals at one site but pathogens at another
  e. g. staphylococcus aureas is commensal in the nose but in a wound is a pathogen
• If an organism gains entry to a normally sterile site

Question 9

What is a pathogen?

Answer 9

A microbe that causes disease (initiates infection) with only small number via natural routes

Question 10

What does virulence mean?

Answer 10

It is a term used to describe the degree of pathogenicity

Question 11

Name some body sites that are normally sterile and do not have normal flora

Answer 11

• Lower Respiratory Tract (LRT)
  
  • Blood
  • Bone, joint and subcutaneous connective tissue
  • Female upper genital tract
  • Urinary tract
  • CNS including CSF and eye
• Other viscera e.g. liver, spleen, pancreas

Question 12

What is a strict pathogen?

Answer 12

Organisms that always cause disease

Highly pathogenic microbes are said to be virulent

Question 13

What are the 2 main factors that determine if infection with result

Answer 13

1. The properties of the microbe
   - adhesins
   - toxins
   - capsule
2. Defensive mechanisms of the host
   - natural barriers
   - defensive cells
   - complement
   - immune response

If balance is in favour of microbe, then infection will occur

If balance is in favour of the host, then infection will result

Question 14

What are the 6 steps that need to be completed before infection is established?

Answer 14

1) Survival of microbe and spread to host
2) Adherence of microbe to host and entry into host
3) Multiplication
4) Evasion of host’s defences
5) Damage to the host
6. Shedding of the microbe and spread to environment or another host

Question 15

Explain step 1a of establishing an infection: survival of the microbe

Answer 15

Microbes need to be able to survive in the enviroment they live in

• Physical environment: some produce spores that survive heat and dryness, others have a waxy coat
• Competition with others: There may also be competition with other microbes so they release bacteriocins to kill these off

Question 16

Explain step 1b of establishing an infection: routes of spread

Answer 16

Airborne/droplets - Aerosol

GI Tract/Faecal-Oral Route

Skin

Through inanimate objects (called fomites)

Other epithelial surfaces

Question 17

Explain step 2 of establishing an infection: Adherence and entry into host

Answer 17

This is done by various mechanisms including attachment to tissue via fimbriae (pili), adhesins and other molecules present on the surface of some of these microbes

Question 18

Explain step 3 of establishing an infection: Multiplication

Answer 18

During the process of multiplication, many microbes produce tissue destroying enzymes which allow invasion of surrounding tissues as well as deeper sites.
The toxins are capable of local or systemic actions

Question 19

Explain step 4 of establishing an infection: Evasion of host’s defences

Answer 19

The hosts defence mechanisms soon mobilises its resources to contain the infection and microbes use a variety of methods to evade the hosts response. As mentioned earlier a successful pathogen will tip the balance in their favour in comparison to hosts and evade the hosts defences.

Question 20

Explain step 5 of establishing an infection: Damage to host

Answer 20

This may result from actions of the microbe or sometimes from the host’s own immune response

Question 21

Explain step 6 of establishing an infection: Shedding infection

Answer 21

In order to perpetuate the infection, the microbe must find a new host.

Question 22

Describe our natural barriers of infection

Answer 22

• Major natural physical barriers are the skin and mucous membrane. Skin covers the largest surface of the host and is impermeable to most infectious agents. When the continuity of intact skin is broken e.g. by cuts, lacerations, injection and animal and insect bites, microbes may be implanted into those sites from the environment. An infection may then be established.

The chemical barriers are generally the constituents of Innate Immunity

Question 23

Name the characteristics of innate immunity

Answer 23

a) Immunity is non-specific i.e. are active against a variety of infectious agents.
b) Non-specific immunity can not be enhanced by repeat exposure.
c) Microbicidal substances e.g. lysozyme in tears, gastric acid and antibacterial substances in urine.
d) Acute phase proteins, interferons and complement are chemical substances and are all associated with mild to moderate inhibitory effect against various microbes.
e) Phagocytic cells e.g. Mononuclear phagocytes-monocytes and macrophages, and natural killer (NK) cells.

Question 24

List some local and systemic symptoms of infection

Answer 24

Local symptoms (inflammation): Redness, swelling, warmth, pain, loss of function, pus (=pyogenic infection)

Systemic symptoms = Fever, rigors, chills, tachycardia, tachypnoea

Question 25

What is acute infection?

Answer 25

Infection established and disease progresses rapidly with major local and systemic manifestations. E.g. pyogenic skin lesion or toxin disease.

Question 26

What is chronic infection?

Answer 26

Usually slower onset, but may still have major local and systemic symptoms, a chronic inflammatory response results when the host does not succumb immediately to the infection but cannot clear it e.g. TB

Question 27

What is asymptomatic infection?

Answer 27

Infection with a pathogenic microbe and the inflammatory response is mild or none at all and damage to the host is mild or none at all. No symptoms present.