Topic 6.1 Bacteria and Disease

Question 1

Pathogenic bacteria

Answer 1

• Some bacteria are pathogenic.
• This means they re capable of causing disease in other organisms.

Question 2

Tissue invasion

Answer 2

By growing and dividing in tissues, they can disrupt their normal function.

Question 3

Toxin production

Answer 3

Bacteria release chemical toxins which can cause cell damage.

Question 4

Endotoxins

Answer 4

• Toxins that are restricted to the bacterium cell wall.
• They are not freely released into the extracellular space or tissues.
• Cause cell damage around the site of infection.

Question 5

The tissue Escheria coli invades

Answer 5

• The non-phagocytic eukaryotic cells.
• Eg. epithelial and endothelial cells.

Question 6

Symptoms of Escheria coli

Answer 6

-Diarrhoea
-Stomach cramps
-Fatigue
-Nausea and vomiting

Question 7

Exotoxins

Answer 7

• Soluble proteins.
• Released by bacteria and are free to move round the body.
• Very diverse.
• They can:
  -act as enzymes to break down cells
  -act as enzyme inhibitors to prevent normal enzyme functions.

Question 8

The name of the bacterium that causes tuberculosis

Answer 8

Mycobacterium tuberculosis

Question 9

How is TB spread?

Answer 9

Droplets from coughs/sneezes

Question 10

Which people are vulnerable to TB infections?

Answer 10

-Those with HIV/AIDS
-Young/elderly
-Pregnant women
-Those with compromised immune systems

Question 11

Streak plating

Answer 11

Used to identify and isolate individual colonies.

Question 12

Generation time

Answer 12

The time between divisions.
(Can be as short as 20 minutes or as long as hours/days).

Question 13

Generation time using logs

Answer 13

2^x = y
log(2)y = x (x is the number of divisions)

Question 14

Exponential growth rate

Answer 14

Question 16

Measuring bacteria growth

Answer 16

Measuring the number of cells present at various time intervals.

Question 17

How to get a viable cell count

Answer 17

• Haemocytometer are special microscopic slides with two viewing chambers.
• They’re used to count cells.

Question 18

Turbidity

Answer 18

• How opaque (or cloudy) a solution is.
• The cloudier the solution, the higher the turbidity and the higher number

Question 19

How can bacterial resistance be controlled?

Answer 19

-Controlling prescriptions
-Controlling use in agriculture
-Hospital hygiene measures
-Antibiotic guardians
-Isolation of patients
-Mass testing
-Infection monitoring

Question 20

Growth curve of bacteria

Answer 20

Lag phase: bacteria adapting to new environment.
Log phase: rate is close to or at maximum.
Stationary phase: total rate is 0; cells made is equal to cells dying.
Death phase: death rate is increasing and growth is close to 0 (due to buildup of toxic waste products & lack of nutrients).

Question 21

List the basic aseptic techniques

Answer 21

-Wipe surfaces with antibacterial cleaner
-Set up Bunsen burner nearby. Convection currents prevent microbes from entering culture
-Flame inoculating loop & neck of bottles before use
-Minimise time that vessels containing bacteria are open
-Sterilise all equipment eg. using an autoclave
-Wear protective clothing

Question 22

Difference between a spread plate and streak plate

Answer 22

-Spread plate: distribute microorganisms evenly with a sterile spreader
-Streak plate: aim to obtain single colonies by rotating that plate to build layers of the culture on at least 3 separate streaks

Question 23

How to conduct a cell count?

Answer 23

1) Dilute broth sample with equal volume of trypan blue to stain dead cells blue.
2) Use a calibrated haemocytometer with volume 0.1mm^3. 3) Count the cells in each of the sets of squares and calculate mean.
4) Number of bacterial cells = number counted x10^4 per cm^3.

Question 24

How to conduct a turbidity measurement?

Answer 24

1) Use colorimeter. Measure absorbance or % transmission of samples with known microorganism count.
2) Plot calibration curve: absorbance/ % transmission (y-axis), number of microorganisms (x-axis).
3) Record absorbance/ % transmission of unknown sample. Interpolate graph.

Question 25

Advantages and disadvantages of using a turbidity

Answer 25

+ Quick
+ Can be conducted in the field
- Expensive equipment
- Counts both living & dead cells
-Required calibration curve from unknown samples
- Assumes equal density of cells across culture

Question 27

How does Staphylococcus cause disease?

Answer 27

Secretes soluble proteins called exotoxins eg:
-Barrel-shaped proteins embed in host cell membrane so contents leak
-Protease toxins
-Superantigens trigger 20% of T cells (usual 0.001%) so can cause toxic shock.

Question 28

How does Mycobacterium tuberculosis cause disease?

Answer 28

1) Triggers inflammatory response by infecting phagocytosis in lungs.
2) Infected phagocytes are sealed in waxy-coated tubercles so bacteria remain dormant. First infection has no symptoms.
3) If another factor weakens immune system, bacteria become active and destroy lung tissue.

Question 29

Wha causes antibiotic resistance?

Answer 29

1) Random genetic mutation, often on plasmid, confers resistance eg. antigen shape changes
2) These bacteria have selective advantage in the presence of antibiotics, reproduce & pass allele for resistance to offspring
3) Directional selection results in resistant strain.

Question 30

What causes antigen variability?

Answer 30

1) Random genetic mutation changes DNA base sequence
2) Results in different sequences of codons on mRNA
3) Different primary structure of antigen = H-bonds, ionic bonds & disulphide bridges form in different places in tertiary structure
4) Different shape of antigen.

Question 30

How does antigen variability affect the incidence of disease?

Answer 30

• Memory cells no longer complementary to antigen => individual not immune => can catch the disease more than once/ cannot recognise pathogen eg. HIV.
• Many varieties of a pathogen => difficult to develop vaccine containing all antigen types.

Question 31

Salmonella

Answer 31

• Gram-negative
• Endotoxins

Question 32

Why is streak plating useful?

Answer 32

• Only one colony can grow
• Colonies spread out on agar
• Separating individual bacteria
• So colonies are separated
• So only 1 type of bacteria can be picked up