Lecture 4 - Bacteria and Disease

Question 1

What is the role of bacteria in flatulence?

Answer 1

Flatulence
* Healthy people fart 10-20 times/day.
○ CO2 odourless and not flammable
* Some members of the gut can fix N2 (but limited)
○ N2 - odourless and not flammable
* No guilds in the gut use the H2S and CH4
○ H2S smells of rotten eggs and flammable
○ No purple/green Sulphur bacteria (need light)
○ No Methanotrophs (need oxygen)
Other odorous gases
* Bacteria produce Volatile Organic Compounds (VOC)
* Alkenes, alcohols, ketones, benzenoids, pyrazines, sulphides, and terpenes.
* You can identify bacterial genera by their smell on agar plates.
* Bacteria VOCs contribute to our distinctive odour
○ Our odour can change with age, diet, environment etc.
○ Odour can also change if we have a disease or disorder
§ Non-microbial e.g. cancer and diabetes
Or Microbial

Question 2

Define Disease, infection and pathogenicity.

Answer 2

Disease - A pathological condition of a part, organ, or system of an organism resulting from various causes, such as infection, genetic defect, or environmental stress, and characterised by an identifiable group of signs or symptoms.

Infection - The invasion of a host’s bodily tissues by disease - causing organisms (pathogens), their multiplication, and the reaction of host tissues to these organisms and the toxins they produce

Pathogenicity - the ability to cause disease.

Question 3

What is virulence and Infective dose?

Answer 3

Virulence - Infective Dose (ID)
The number of bacterial cells required to establish an infection (not L50)
The lower the ID, the more virulent the pathogen

Virulence factors
* Offensive
○ Toxins and tissue degrading enzymes
○ Invasins
○ Adhesins
* Non specific
○ Siderophores
○ Extracellular enzymes
* Defensive
○ Capsules
○ IgA proteases
○ Superoxide dismutase

Virulence factors can be carried on the chromosome such as the capsule and endotoxin. They do produce a metabolic burden on the cell and therefore non virulent mutants grow faster.
Virulence factors can also be carried on plasmids e.g. toxins, enzymes and adhesins. Having a low copy number can reduce the burden on the cell.

Question 4

What is the difference between primary and opportunistic pathogens?

Answer 4

Primary pathogens are the microorganisms that cause diseases in healthy individuals

Opportunistic pathogens are the microorganisms that are ordinarily in contact with the host and cause disease when the hosts resistance is low.

Very few microbes are always pathogenic. There are many microbes that are opportunistic however most are never pathogenic.

Question 5

How do bacteria invade the host?

Answer 5

The pathogen must:
* Have a natural reservoir
* Be successfully transmitted to a host
* Penetrate the host’s natural barriers
The pathogen must then:
* Colonise a sterile surface in the body
* Over-colonise their existing (compromised) surface
* Be able to colonise a pre-existing natural biofilm

Non-human reservoirs
* Environment
○ Oil water
○ Fomites - bedding, surgical instruments
* Animals - “zoonosis”
Human reservoir
* Reservoirs showing no symptoms “Carriers”
Reservoirs showing symptoms - Cough and sneezing

Question 6

How are bacteria tranmitted?

Answer 6

Aerosol
Direct contact
Vector-borne

Question 7

Name some physical and chemical barriers that prevent infection?

Answer 7

The Physical and chemical barriers to prevent an infection
Tears
Sweat
Skin
Stomach acid

Question 8

What occours after bacterial entry

Answer 8

First interaction often occurs on a mucosal surface
* All body cavities open to the environment such as the respiratory tract, gastrointestinal tract, etc.
Mucosal surfaces are subject to flushing mechanisms
* Saliva in the mouth and mucus moved by cilia
To overcome flushing mechanisms pathogens must attach to host cell surfaces and form a biofilm.

The skin is a natural barrier until damaged which can lead to opportunistic infections by normal flora then incoming pathogens.

Question 9

Describe the features of the normal human flora.

Answer 9

• > 1014 bacteria live on and within our body
  • Most are attached to surfaces of the body (“biofilms”)
  • Different abiotic conditions
  • Acidity, aerobic, anaerobic etc
  • Initial attachment is difficult but once attached, difficult to remove
  • Pathogens have difficulty colonising a pre-existing biofilm unless:
  • normal flora is compromised (wounded)
    the bacterium has a number of virulence factors