Microbiology & Infectious Diseases

Question 1

What is a virus?
Briefly describe its structure

Answer 1

An infectious, obligate intracellular parasite
Comprised of genetic material (DNA OR RNA) surrounded by a protein coat and/or a membrane

Question 2

Big difference between Virus and Bacteria?

Answer 2

Virus IS dependent on host cell (bacteria is not)

Bacteria has cell wall, organelles, DNA AND RNA and are alive
Viruses have none of these things and are not alive

Question 3

When virus is outside the cell, what is it called?

Answer 3

Virion?

Question 4

How do viruses replicate?
Describe the 6 stages

Answer 4

1. Attachment to specific receptor on host cell
2. Cell entry - only central viral core (carrying the nucleic acids and some proteins) enters host cell
3. Interaction with host cells - uses cell materials (e.g. enzymes, amino acids) for their replication
4. Replication may localise in nucleus, cytoplasm or both
5. Assembly - occurs in nucleus, cytoplasm or cell membrane
6. Release - bursting open of cells OR leaking from cell over period of time

Question 5

How do viruses cause disease?
Give examples

Answer 5

Damage by :
- direct destruction of host cells e.g. HIV
- modification of host cell structure or function e.g. rotavirus
- over-reactivity of host host as response to infection e.g. hepatitis
- cell proliferation and cell immortalisation e.g. HPV
- evasion of extra AND intracellular host defences

Question 6

What is a pathogen?

Answer 6

Organism that causes or is capable of causing disease

Question 7

What is a commensal?

Answer 7

Organisms which colonises the host BUT causes no disease in normal circumstances

Question 8

What is an opportunist pathogen?

Answer 8

Microbe that only causes disease if host defences are compromised

Question 9

Define virulence / pathogenicity

Answer 9

Ability to cause disease once established

Question 10

What is asymptomatic carriage?

Answer 10

When a pathogen is carried harmlessly at a tissue site where it causes no disease

Question 11

What do gram negative bacteria have that gram positive don’t?

Answer 11

DOUBLE CELL MEMBRANE
2 membranes separated by lipoprotein, periplasmic space and petidoglycan

Question 12

State and describe the types of bacterial toxins

Answer 12

ENDOTOXIN - part of outer membrane of bacteria

EXOTOXIN - proteins secreted by gram pos and neg bacteria

Question 13

What does the coagulase test distinguish?

Answer 13

Distinguishes S.aureus from other staph
- will be COAGULASE POSITIVE

Question 14

Types of gene mutation

Answer 14

Base sub
Deletion
Transfer

Question 15

Types of gene transfer

Answer 15

Transformation e.g. plasmid
Transfunction e.g. via phage
Conjugation e.g. via sex pilus

Question 16

Describe how to gram stain

Answer 16

1. Apply primary stain (crystal violet) to heat fixed bacteria
2. Add IODIDE - binds to crystal violet and fixes it to cell wall
3. DECOLOURISE with ethanol or acetone
4. COUNTERSTAIN with safranin (pink)

Question 17

What does gamma haemolysis imply?

Answer 17

NO haemolysis

Question 18

Why does alpha haemolysis occur?
How does the agar appear?

Answer 18

BC production of hydrogen peroxide oxidising haemoglobin
GREEN

Question 19

Why does beta haemolysis occur?

Answer 19

Bc lysis of RBCs by haemolysis such as Streptolysin O produced by S. pyogenes

Question 20

What does an oxidase test test for?

Answer 20

If micro-organisms contains a cytochrome oxidase
implies organism able to use O2 as terminal e- acceptor

Question 21

Gram pos bacteria stain what colour?
Give an example

Answer 21

PURPLE
Staph. Aureus - coagulase positive

Question 22

What are the groups of Streptococci classification?

Answer 22

1. Haemolysis
2. Lancefield typing
3. Biochemical properties

Question 23

What is lancefield grouping?

Answer 23

A method of grouping catalyse neg, coagulase neg bacteria
Based on bacterial carbohydrate cell surface antigens

Question 24

How do gram-pos bacteria spread?

Answer 24

Aerosols, surface-to-surface, colonisation of prostheses

Question 25

Virulence factors of Gram Neg bacteria?

Answer 25

1. Colonisation factors - adhesins, invasins 2. Toxins (effectors) - usually secreted proteins - damage, subversion

Question 26

Name the 4 major groups of Gram-Neg bacteria Describe their shape

Answer 26

1. Proteobacteria - ALL rod-shaped 2. Bacteroids - rodshaped 3. Chlamydia - round pleimorphic 4. Spirochaetes - spiral/helical

Question 27

Why can is be difficult to culture pathogens in the lab?

Answer 27

Bc of dependency on host

Question 28

3 Types of Worms

Answer 28

1. Nematodes (roundworms) 2. Trematodes (flatworms) 3. Cestodes (tapeworms)

Question 29

What type of nematodes can you get?

Answer 29

Intestinal Larva migrans Tissue (filaria)

Question 30

What type of trematodes can you get?

Answer 30

Blood Liver Lung Intestinal

Question 31

What type of cestodes can you get?

Answer 31

Non-invasive Invasive

Question 32

What is the pre-patent period?

Answer 32

Interval between infection and appearance of eggs in the stool

Question 33

What is a protozoa?

Answer 33

Single-celled eukaryotic organism with a definitive nucleus

Question 34

Name medically important mycobacteria

Answer 34

M. TB M. leprae - leprosy M. avium - disseminated infection in AIDS M.kansasii - chronic lung infection M.marinum - fish tank granuloma M.ulcerans - buruli ulcer Rapidly growing mycobacteria

Question 35

What is a Mycobacteria?

Answer 35

**Aerobic**, non-spore forming, non-motile bacillus Slow reproduction Slow response to treatment Slow growing ∴ hard to culture

Question 36

Describe the cell wall of Mycobacteria

Answer 36

Contains high molecular weight lipids Weakly gram-pos (or colourless) Survive inside macrophages, even in low pH environments

Question 37

What is acid fast bacilli used for?

Answer 37

To identify organisms with wax-like, thick cell walls E.G. MYCOBACTERIA (which are resistance to gram stain)

Question 38

Name some direct lab methods to diagnose bacterial infections

Answer 38

Gram stain Acid fast stains Wet film KOH (fungi) India ink

Question 39

Name some culture lab methods to diagnose bacterial infections

Answer 39

Solid media Liquid media Blood culture

Question 40

How do fungi move?

Answer 40

By growth or through generation of spores - carried thru air/water

Question 41

Describe fungi

Answer 41

Eukaryotic Chitinous cell wall Heterotrophic

Question 42

What is a dimorphic fungi?

Answer 42

Fungus that exists as both yeast and mould - switches between the 2 when conditions suit

Question 43

What are yeasts?

Answer 43

Small single celled organisms that divide by budding

Question 44

What do moulds form?

Answer 44

Multicellular hyphae and spores

Question 45

What does selective toxicity in treating fungal disease rely on?

Answer 45

Target does not exist in humans Target is signif different to human analogues Drug is conc in organism cell (respect to humans) ↑ permeability to compound Human cells being 'rescued' from toxicity by alternative metabolic pathways

Question 46

Why can fungi be hard to treat?

Answer 46

Relatively few classes of effective antifungals

Question 47

Why can the total worm burden in a human not increase (unless constant re-exposure)?

Answer 47

Bc adult worms can't reproduce in the body (without a period of development outside) ∴ if worm can't get out, it will die Can produce larvae/eggs but once grown, will die

Question 48

Define invasiveness

Answer 48

Capacity to penetrate mucosal surfaces to reach normally sterile sites

Question 49

5 Major groups of protozoa

Answer 49

Flagellates Amoebae Sporozoans Ciliates Microsporidia