Introduction - The Nature of Infection

Question 1

List different types of micro-organisms/infectious agents in order of decreasing size

Answer 1

Parasites
Fungi
Bacteria
Viruses
Prion Proteins

Question 2

Describe the naming of organisms

Answer 2

Organisms have 2 names (in italics):
GENUS name - 1st letter is UPPER CASE

SPECIES name - 1st letter is LOWER case

Question 3

What are prion proteins? Describe what they do and how they can be removed

Answer 3

Rare infectious agents that have no DNA/RNA; cause an incurable brain infection (brain matter lost, appears spongy)

Cannot be reliably removed by sterilisation/disinfection so surgical instruments are destroyed after use on such patients

Responsible prion disease like “mad cow disease” (AKA CJD - Creutzfeldt-Jakob Disease)

Question 4

What are viruses? Describe their structure

Answer 4

Very small, obligate, intracellular parasites that contain either RNA or DNA

Can only be seen using an electron microscope

They have a protein coat (capsomere) and/or a lipid envelope

Question 5

What are the methods used to diagnose a virus?

Answer 5

Cell cultures - no common now
Serology - antigen or antibody detection in blood
PCR - to detect the DNA or RNA. Important in giving real-time results as faster than cell culturing

Question 6

Describe bacterial structure and contents

Answer 6

Can be seen with a light microscope

Contain both DNA and RNA and have a single double-stranded chromosome

They can have plasmids (extra pieces of circular DNA that often code for antibiotic resistance)

Have a cell membrane and cell wall and/or a capsule

The flagellae allow movement of the cell

The pili (fimbriae) allow adhesion of cell to surfaces

Question 7

What are the different bacterial shapes?

Answer 7

Coccus (round)
Bacillus (rod-shaped)
Spirochaetes (spirals)

Question 8

What is Gram Stain?

Answer 8

Stains the bacterial cell wall

Gram positive - purple (bacteria have a thicker peptidoglycan wall)

Gram negative - pink (thin bacterial cell wall so do not hold onto stain well)

Both can result from coccus or bacillus bacterial cells

Question 9

Importance of gram stain

Answer 9

Allows guessing of the most likely organism

Also, many antibiotics act on cell wall so this is important in determining what antibiotic to use

Question 10

Gram stain limitations

Answer 10

Not all organism stain well:
Mycobacterium tuberculosis has a waxy coat that does not take up stain
Treponema pallidum (spirochaete organism; causes syphilis (STD)

Other staining method must therefore be used for these infections

Question 11

What are commensal bacteria and give examples of regions where they are found

Answer 11

Normal bacterial flora that usually not harmful, if in the correct region

Upper respiratory tract
Lower small intestine and large bowel
Vagina
Skin - resident and transient flora (hand-washing effectively rids the transient flora)

Damp areas (armpits, groin) have rich flora

Question 12

What is a pathogen? What is an opportunistic pathogen

Answer 12

A harmful organism

An organism that will probably only cause infection in an immunocompromised individual (e.g: due to immunosuppressants)

Question 13

What is meant by pathogenicity?

Answer 13

Ability of a microorganism to cause disease

Question 14

What is meant by virulence of a pathogen?

Answer 14

The degree of pathogenicity of an organism (how easily it causes disease)

Question 15

How do bacteria replicate and what do they require?

Answer 15

Via binary fission. Require:
Food (any organic material)
Correct temp (37 C for most human pathogens)
Correct pH (7.4 for most human pathogens)

Question 16

How does binary fission occur?

Answer 16

Bacteria undergo nucleic acid replication
2 sets of chromosomes go to opposite sides of cell
Cell wall forms, dividing cell into 2
Bacteria double

Question 17

What are the phases of bacterial growth?

Answer 17

Lag phase (1st part of curve)
Log phase (exponential growth - nutrients available)
Stationary phase (rate of death = rate of regeneration; nutrients not abundant)
Death phase

Question 18

What kinds of atmosphere do bacteria grow in?

Answer 18

Aerobic
Microaerophilic
Anaerobic

Question 19

What are aerobic bacteria?

Answer 19

Grow in the presence of oxygen

Many will also grow anaerobically but not as well

Question 20

What are microaerophilic bacteria?

Answer 20

Grown in an atmosphere with reduced oxygen concentration with enriched with CO2

Question 21

What are anaerobic bacteria?

Answer 21

Only “strict” anaerobes

Grow in an atmosphere of no oxygen

Question 22

What toxins do bacteria produce?

Answer 22

These are enzymes

Exotoxin (mainly gram positive bacteria) - produced inside cell and exported from it

Endotoxin (mainly gram negative bacteria) - part of the outer membrane of the cell wall and usually only released for membrane when cell dies

Question 23

What are some effects of bacterial toxins?

Answer 23

Interact with immune cells causing release of cytokines

Damage red and white blood cells, leading to “leaky” small blood vessels; causes decreased BP and affects blood clotting.
Leads to sepsis and septic shock

Question 24

What are spores?

Answer 24

Inactive, hardy forms of bacteria that cannot replicate but can survive adverse conditions for many years (they are thick coats containing DNA)

Produced by some gram positive bacilli

Question 25

Steps of spore formation

Answer 25

Chromosome condensation and septum formation Calcium dipicolinate accumulates in the core of the developing spore

Question 26

How are bacteria classified?

Answer 26

Gram positive/negative Atmosphere in which they grow Shape (bacilli/cocci) Other: biochemical tests, coagulase, types of haemolysis on blood

Question 27

Bacteria diagnostic methods

Answer 27

``` Microscopy Culture Detection of antigen Detection of antibodies in blood PCR NAATS - (Nucleic Acid Amplification Tests) detection of organism's DNA/RNA ```

Question 28

Describe steps in bacterial cell culture

Answer 28

Culture samples on agar plates to allow bacterial growth Use specialised tests to help identify organism, like biochemical reaction Provide info on which antibiotics will be most effective: Sensitive (S) - can be used to treat infection Resistant (R) - cannot be used to treat infection Gradually replaced by PCR

Question 29

Length of time for bacteriology tests

Answer 29

Microscopy - same day PCR - same day (sometimes batched) Agar culturing and antibiotic sensitivity tests - 48 hrs (24 to grow organism and 24 for sensitivity) Serology - same day (often batched) TB culture - 4-12 weeks (can be detected using PCR faster)

Question 30

Give examples of fungi and what they do

Answer 30

Moulds - complex structures that produce spores (spread in air currents) and hyphae (filaments that invade organic tissue). Do not stain with gram stain. Most common cause of fungal infection is Aspergillus spp Yeast - single cell reproduce by BUDDING - not binary fission (mother cell gives rise to several budding daughter cells). Most common cause of fungal infection is Candida spp. Stain as large gram +ve oval structures

Question 31

Why are antibiotics not used to kill fungi?

Answer 31

Cell wall different from bacteria. Special anti-fungal drugs used

Question 32

Describe parasites and their structure

Answer 32

Complex interaction with the host - "life cycle" Structures are varied, from single-celled organisms (protozoa) to more complex organisms

Question 33

Give an example of a parasite with a simple life cycle and a complex life cycle

Answer 33

Giardidis - human host only Shistosoma - worm whose life cycle involves humans and snails

Question 34

What is sterilisation?

Answer 34

Destruction and removal of 99.9% of microorganisms and spores (may not inactivate prion proteins)

Question 35

What are the methods of sterilisation?

Answer 35

Steam and pressure (autoclaves) Dry heat in oven at 160C for over an hour Exposure to ethylene oxide gas - explosive so must be done carefully Gamma irradiation

Question 36

What is disinfection? What are powerful and mild disinfectants used for?

Answer 36

Removal/destruction of PATHOGENIC microorganisms to make item safe Powerful - medical equipment that does not need to be sterile Milder - hands, patient's skin (antispectics)

Question 37

Methods of disinfection?

Answer 37

Hot Water | Chemical: bleach and handwash