Bacteria As Causes Of Disease

Question 1

Commensal definition

Answer 1

Micro-organism which can be cultured from the host but is not disease causing - asymptomatic carriage

Question 2

Infection definition

Answer 2

Micro-organism can be cultured from the host and is causing disease

Question 3

Pathogen definition

Answer 3

Any type of micro-organism that causes disease

Question 4

Virulence definition

Answer 4

The degree to which a given micro-organism is pathogenic

Question 5

Opportunistic pathogen/infection definition

Answer 5

Either term is used to refer to micro-organisms that only cause disease when the host defence is compromised.

Question 6

Which patients might get opportunistic infection?

Answer 6

Those on immunosuppressed drugs
- Transplant patients
- Rheumatoid
- IBD
Immunocompromised patients
- HIV/AIDS
- Elderly
- Pregnant
- Premature babies
- Genetic immunodeficiency (IgA deficiency)
- Chemotherapy
Breakdown of host defence:
- Skin - IV drug users, eczema
- Lugs - cystic fibrosis, TB, COPD

Question 7

Types of disease transmission

Answer 7

Direct contact
Air
Indirect contact
Food
Insect
Rabid animal

Question 8

How to grow bacteria

Answer 8

Solid media = agar plate, agar slope
Liquid media = test tubes
- can be selective or non-selective

Question 9

Types of agar

Answer 9

Blood
Chocolate
Mac-conkey
Xylose lysine deoxychocolate agar (XLD agar)

Question 10

What is blood agar used for

Answer 10

Streptococcus + others

Question 11

What is chocolate agar used for

Answer 11

Fastidious Neisseria

Question 12

What is Mac-Conkey used for

Answer 12

For lactose status
- Lactose fermenting = PINK
- Non-lactose fermenting = COLOURLESS = Enterobacteria

Question 13

What is XLD used for

Answer 13

Selective growth medium for salmonella and shingella species from clinical samples + from food. The shingella and salmonella both ferment the lactose + XLD and go red but then salmonella also turns black dot due to hydrogen sulphide production.

Question 14

Difference between gram positive and gram negative bacteria

Answer 14

The peptidoglycan layer soaks up the crystal violet. If its thin then it will be decoloured by washing with alcohol (gram negative) = PINK
If its thick then the gram stain cannot be washed away + will remain (gram positive) = PURPLE

Question 15

Gram staining process (ComeInAndStain)

Answer 15

1. Add primary stain e.g. CRYTAL VIOLET to heat fixed bacteria
2. Add IODINE which binds to crystal violet and helps fix it to cell wall
3. Decolourise with ALCOHOL
4. Counterstain with SAFRANIN

Question 16

Example of bacteria which doesn’t stain with gram stain

Answer 16

Acid fast bacilli
e.g. TB - Ziel Neelson Stain = stain red on blue bacteria

Question 17

Structure of gram positive bacteria (In to Out)

Answer 17

Plasma membrane
Periplasmic space
Thick peptidoglycan layer

Question 18

Characteristics of gram positive bacteria

Answer 18

Prefer dry and dusty environment = great skin colonisers + can spread by breathing in shed skin scales

Question 19

Structure of gram negative bacteria (In to Out)

Answer 19

Plasma membrane
Periplasmic space
Thin peptidoglycan layer
Periplasmic space
Outer membrane (lipopolysaccharide + proteins)
= think as extra protective layer again stain lie cling film

Question 20

Characteristics of gram negative bacteria

Answer 20

Prefer wet and damp environment = majority prefer to colonise on mucous membranes