Lecture 25 - Molecular Epidemiology of HCAI

Question 1

What are we detecting when we do PFGE?

Answer 1

SNPs within the bacterial population

Create a phylogeny

Can determine relatedness and thus common source

Question 2

How do we differentiate bacterial in the population?

Answer 2

Sequencing

• compare whole genomes
• SNPs

Question 3

Describe E. faecium resistance

Answer 3

Intrinsic:

• cephalosporins
• aminoglycasides

Acquired:

• glycopeptide antibiotics
• VRE

Question 4

How does E. faecium get resistance to vancomycin?

Answer 4

Transposon Tn1549
with the VanB locus

Changes the peptidoglycan so that vancomycin can no longer bind

Question 5

Describe the transmission models of VRE

Answer 5

Model A:
- VRE spread from patient to patient in hospital

Alternative model:

• people have gut anaerobe commensals with vanB locus, as well as VSE
• in hospital, treatment with vancomycin selects for VRE

Question 6

What were the surprising findings about our gut commensal?

Answer 6

50% of gut commensals have vanB

Question 7

What was the French experiment with the mice?

Answer 7

• Mice with gut commensals with vanB and VSE
• Mice treated with vancomycin
• Tn1549 transferred to VSE
• VRE now selected for

Question 8

Is VSE commonly found in the gut microflora?

Answer 8

Yes

Question 9

Describe the process of genome sequencing

Answer 9

1. Bacteria grown up
2. DNA extracted
3. Sequenced
4. Analysed

Question 10

Describe the sequencing stage

Answer 10

Machines:

• illumina
• ion torrent

‘Reads’ read against reference genomes

Question 11

In the study, there were 61 …

Answer 11

invasive E. faecium isolated studied

Question 12

What is the phylogeny on slide 26?

Answer 12

Relatedness of the 61 isolates

• core genomes of 61 were sequenced
• many SNPs analysed
• relatedness gave us the tree

Question 13

What is the phylogeny on slide 28?

What does it tell us?

Answer 13

Analysis of the sequence of the Tn1549
(resistance transposon)

• two different types of the transposon
• some other bacterial species have this transposon

Question 14

What are the different parameters to do with the transposon that can be analysed?

Answer 14

• sequence of transposon
• orientation of transposon
• position of insertion
• coupling sequence

Question 15

What would be observed when all the information about the transposon is overlayed on the core genome phylogeny if model A were correct?

Why?

Answer 15

Clonal spread

Closely related bacteria have similar transposon features

The transposon has been passed on from a common ancestor

Question 16

What would be observed when all the information about the transposon is overlayed on the core genome phylogeny if the alternative model were correct?

Why?

Answer 16

De Novo formation

Closely related bacteria have differe transposon features

Because the transposon has arisen de novo, not been passed on from common ancestors

Question 17

What conclusions have been made from this new study?

Answer 17

VRE arises by de novo evolution
(not clonal)

Gut anaerobes are potential sources of Tn1549

Question 18

What is the transposon that conferrs vancomycin resistance?

Answer 18

Tn1549

Question 19

What is vanB?

Answer 19

This is a single locus (gene) on the Tn1549 transposon

Question 20

What are the implications of the study for infection control?

Answer 20

• patients being treated with vancomycin are potential sources of VRE
• identification and isolation of patients with VSE also required
• need antibiotic control policies
• universal precautions still warranted