M1: How bacteria cause disease and infective spread

Question 1

What is a commensal organism

Answer 1

One which colonises the healthy host and doesn’t cause disease

Question 2

What is an opportunistic pathogen

Answer 2

One which causes disease when the opertunitry arises due to disturbance in the homeostasis between host and commensal organisms e.g. in

• immunosuppression
• when they get to sites they don’t normally live in
• antibiotics and other therapies
• dietary imbalances

Question 3

How does the normal flora protect against pathogenic invasion

Answer 3

It competes with the pathogen for colonisation sites and so acts as a barrier against invasion by undesirable exogenous organisms

Question 4

What is virulence

Answer 4

The quantitative ability of a pathogenic micro-organism to cause disease - virulence of microbe depend on microbial and host factors

Question 5

List the microbial factors influencing virulence

Answer 5

1. transmissibility
2. infectivity
3. invasiveness/toxicity
4. ability to evade host defences

Question 6

List the host factors influencing virulence

Answer 6

1. age
2. genetic factors
3. general host defences and local host defences
4. immunodeficiency

Question 7

Outline the exogenous sources of infection

Answer 7

• person to person contact
• zoonosis (animals to humans)
• environmental e.g. water and soil

Question 8

Outline the endogenous sources of infection

Answer 8

Normal flora can invade causing an endogenous infection = opportunistic pathogens

Question 9

What is an endemic

Answer 9

An infection that is always present in the population at more or less constant levels

Question 10

What is an epidemic

Answer 10

An infection that is at higher than normal levels within the population which is usually much higher than the endemic level

Question 11

What is an outbreak

Answer 11

A localised increase in the disease incidence

Question 12

What is a pandemic

Answer 12

An epidemic spreading between continents

Question 13

What are the two modes of transmissions for infections

Answer 13

1. Horizontal = transmission of organism between individuals
2. Vertical = transmission from mother to offspring in utero/around birth

Question 14

How do congenital infections occur

Answer 14

Via mothers bloodstream and crossing into placenta or can be acquired shortly before or during delivery

Question 15

What are the modes of transmission

Answer 15

• respiratory spread
• faecal-oral spread
• via the skin or mucous membranes
• blood
• fomites (inanimate object carrying transmit microorganisms)
• zoonoses
• sexual transmission

Question 16

How do skin to skin transmissions of infection occur

Answer 16

The skin is usually a good barrier to infection, but transfer can occur via direct contact or via fomites (shared towels); this usually occurs through abrasions in the epidermis

Question 17

What is cellulitis

Answer 17

A spreading infection of the soft tissues commonly caused by streptococcus progenies getting into small abrasions in the skin and it presents as red, swollen, painful and hot (erythema, oedema = cardinal signs of inflammation)

Question 18

Give an example of infections that can be spread via saliva

Answer 18

Herpes simplex

Question 19

What are nosocomial infections and give the most common types of nosocomial infections

Answer 19

Those that are hospital acquired

• UTIs
• wound/skin and soft tissue infections
• respiratory tract infections

Question 20

How can the spread of infection within hospitals be limited

Answer 20

1. Isolating patients with known resistant organism
2. Strict adherence to sensible infection control policies
3. Hand washing between patients

Question 21

Which factors determine how easily an infection can spread

Answer 21

• ability to survive
• ability to find alternative host
• shedding capacity
• infectivity
• virulence
• ability to evade immune response
• damaging the host : exotoxins
• producing enzymes

Question 22

How does neisseria gonorrhoea attach and invade host tissues

Answer 22

Adheres to genital mucosa by fimbriae

Question 23

How does giardiasis laamblia attach and invade host tissues

Answer 23

Attaches to jejunal mucosa by special sucking disk

Question 24

How does influenza virus attach and invade host tissues

Answer 24

By its hemagglutinin antigen

Question 25

How do respiratory bacteria partake in immune evasion

Answer 25

They secrete an IgA protease which degrades the hosts immunoglobulin

Question 26

How do streptococcus pyogenes evade host immune defences

Answer 26

Express protein A which binds host immunoglobulin preventing opsonisation and complement activation

Question 27

How does streptococcus pneumoniae evade host immune defences

Answer 27

Has a polysaccharide capsule which inhibits uptake by polymorphs

Question 28

How are infections diagnosed

Answer 28

1. History
2. Clinical examination
3. Laboratory investigations
4. Culture; pathogen detection

Question 29

What do serological diagnosis look for

Answer 29

A four-fold rise in antibody titre in acute infection or the presence of IgM; this is useful in the diagnosis of viral infections or when it is hard to grow the presumptive bacteria