M: Bacterial Pathogenesis 1+2 - Week 2

Question 1

Define Normal Microbiota

Answer 1

Diverse group of microbes in and on the body

- found on skin and mucous membranes (incl. the eye)

Question 2

An example of a virulence determinant are ‘siderophores’. What are siderophores? And what do they do?

Answer 2

small, high-affinity iron-chelating compounds secreted by microorganisms (such as bacteria and fungi)
– they serve to transport iron across cell membranes

Question 3

How do specific bacterial factors assist virulence? List 6 ways they help virulence

Answer 3

1. assist colonisation
2. assist growth
3. protect from immune response (e.g. capsule)
4. cause damage (e.g. endotoxin, exotoxin)
5. allow spread (e.g. IgA protease)
6. transduction

Question 4

Explain transduction

Answer 4

involves the transfer of genetic material by infection with a bacteriophage.

• as a bacteriophage infected donor cell breaks down, the bacteriophage takes up some of the DNA of the organism and that goes on to infect another bacterial cell
• so it shares this with the DNA of another bacterium

Question 5

Explain transformation

Answer 5

(alongside transduction, is) Another process that can transfer DNA b/w bacteria
- free DNA strands enter the recipient bacteria cell and undergo homologous recombination to share the DNA

Question 6

Explain conjugation

Answer 6

is where a bacterial cell may contain some extra DNA in the shape of a circle called a ‘plasmid’

• the bacteria here use a sex pillus to join or “conjugate” another bacterium (of any species/genus)
• then the plasmid is shared between the 2 bacteria

Question 7

List the 4 methods of bacterial transfer of DNA

Answer 7

1. Transformation
2. Transduction
3. Conjugation
4. Transposition

Question 8

Why might transduction be considered safer for bacteria than transformation?

Answer 8

Transduced DNA is always protected within the environment because it is contained within the bacteriophage (whereas in transformation it’s free dna)

Question 9

Koch’s postulates describe the initial criteria used for proving that a particular microorganism caused a specific disease.

List all 4 of them.

Answer 9

1. the microorganism must be present in every case of the disease
2. the microorganism must be grown in pure culture from the host
3. inoculation of the microorganism into a susceptible env. host should produce the same disease
4. The microorganism must be able to be recovered from the infected experimental host

Question 10

What are the 3 limitations of Koch’s postulates?

Answer 10

1. normal flora is ineligible (b/c “microorganism can’t be present in healthy people”)
2. fastidious bacteria cause problems (b/c “microorganism needs to grow in pure culture”)
3. no animal models for human disease

Question 11

In 1998 Falkow modified koch’s postulates to create the “Molecular Koch’s postulates”

List the 4 new criteria used

Answer 11

1. Phenotype should be assoc. with pathogenic members of a genus/species
2. Gene responsible for symptoms should be found in all pathogenic strains but not in no-pathogenic strains of that genus/species
3. Inactivation of the gene assoc. with suspected virulence trait should lead to loss of virulence
4. Replacement of the mutated gene should result in a return of virulence

Question 12

In what ways might normal microbiota protect the host against disease? (5)

Answer 12

• prevent colonization
• produce pathogen growth inhibitors (basis of antibiotics)
• fatty acids
• bacteriocins
• deplete nutrients

Question 13

How does the outcome of colonization differ between pathogens and normal microbiota?

Answer 13

Normal microbiota: live mutualistically with the host (role in human nutrition and metabolism)

Pathogens: cause disease

Question 14

T/F: normal microbiota can become pathogenic if moved

Answer 14

True

Question 15

Where do pathogens come from? (2)

Answer 15

1. Endogenous - other sites on the host (normal microbiota)

2. Exogenous - other people (their normal microbiota or pathogens). Or animals (zoonoses)

Question 16

What are the 2 most common normal microbiota found in the eye?

Answer 16

• staph. epidermidis (60-70%)

- staph. aureus (38%)

Question 17

Who has more Strep. pneumoniae in their eyes: children or adults? What does this mean?

Answer 17

Children. This means that children are at greater risk of infection with strep. pnuemoniae

Question 18

Name the 4 steps of infectious disease that can be asymptomatic

Answer 18

1. colonization
2. invasion
3. replication
4. evasion of innate immunity

Question 19

Name the 3 steps of infectious disease that are symptomatic

Answer 19

1. damage host
2. dissemination
3. immunity

Question 20

Where do pathogens bind to in ocular infections? (2)

Answer 20

1. mucosal epithelium (sparse microflora, so less competititon) and/or
2. ocular biomaterials (contact lenses, IOL lenses, sutures, etc.)

Question 21

What 2 factors does the ability to colonise a particular site depend on?

Answer 21

1. the presence of appropriate adhesion receptors

2. a suitable micro-environment for the organism’s survival (e.g. nutrients)

Question 22

What does epithelial colonisation depend on?

Answer 22

interactions b/w ligands (adhesins) on the microorganism surface, and specific receptors on the epithelial cell surface

Question 23

What feature of epithelial cells results in receptors being differentially expressed on different surfaces?

Answer 23

the epithelial cells are often polarized

Question 24

Bacterial adhesion can be fimbrial or non-fimbrial. List the 2 roles of fimbriae and describe their structure

Answer 24

Fimbriae (pili) serve 2 functions - adhesion and conjugation

structure: short, relatively rigid, has a hollow core, has a ligand for binding (adhesin) which is on the tip of the fimbriae

Question 25

How does non-fimbrial adhesion work?

Answer 25

microorganism adheres using molecules on their cell wall, utilizing non-fimbrial adhesins as ligands for ocular cell surface receptors

Question 26

Does psudeomonas aeuroginosa exhibit fimbrial or non-fimbrial adhesion?

Answer 26

Could be either. Depends on the strain.

Question 27

Provide an example of non-fimbrial adhesion

Answer 27

Staphylococcus aureus and streptococci use cell wall teichoic acids as adhesins. Receptors (for the teichoic acid) on ocular surfaces include fibronectin and collagen

Question 28

Some bacteria produce a glycocalyx external to their cell wall to contribute to (non-fimbrial) adhesion. Describe the 2 forms of glycocalyx

Answer 28

1. Capsule (typically high MW polysaccharides)
   - adheres firmly to cell wall; also aids evasion of innate immune system
2. Slime layer - loose unorganized layer that can be easily washed off: protects against dehydration and loss of nutrients

Question 29

Glycocalyxes are also involved in biofilm formation. What are Biofilms?

Answer 29

complex structures of bacteria held together by polysaccharides

Question 30

Explain transposition

Answer 30

Donor cell with transposon-containing plasmid. Plasmid transfer occurs via conjugation. Transposon Transposes.