Theme 3 Lecture 18: Antibiotic resistance

Question 1

What are the two distinct ways in which antibiotic resistance can arise in a patient?

Answer 1

1. exposure to antibiotics/ selection pressure

2. transmission of resistant organisms

Question 2

How does selection pressure cause antibiotic resistance?

Answer 2

• mutations in bacteria that confer a survival advantage favour the growth of the mutant strain
• if the organism is growing in the presence of antibiotics the development of a resistant gene will confer a survival advantage
• the resistant mutant will have survival advantage and out-compete other strains

Question 3

How does transmission of resistant organisms cause antibiotic resistance?

Answer 3

• resistant bacteria gain access to the human microbiome directly or via environmental sources (fomites)
• resistant genes can be transferred between bacteria of different species
• the result is a human microbiome that contains a mixture of sensitive and resistant bacteria
• if the microbiome is exposed to antibiotics, sensitive bacterial strains will die out and the strains carrying resistant genes become dominant

Question 4

Name two methods used to prevent antibiotic resistance

Answer 4

1. antibiotic stewardship - reducing antibiotic exposure to the minimum safe level
2. minimising transmission through infection, prevention and control (IPC)

Question 5

What are the two ways we can identify antibiotic resistance?

Answer 5

1. antimicrobial sensitivity testing

2. detection of antimicrobial resistance genes

Question 6

What is MIC?

Answer 6

-Minimum inhibitory concentration

Question 7

How does antibiotic sensitivity testing work?

Answer 7

• try to grow the organism in the presence of an antibiotic
• if it grows in the presence of high conc (high MIC) it is resistant
• if it is killed at a low conc (low MIC) it is sensitive

Question 8

What is the relationship between MIC and sensitivity of an organism?

Answer 8

the lower the MIC, the more sensitive the organism

Question 9

Explain microtitre plate susceptibility testing

Answer 9

1. Put antibiotics onto wells on microtitre plate
2. Double dilutions from the left to right side of plate (so you are decreasing the concentration of the antibiotic)
3. Add micro organism to wells and incubate overnight
4. Size of growth determines resistance (pale orange = original colour of organism, dark orange = growth)
5. Further to the left that the organism grows, the more resistant it is – as this means its been able to grow at a high concentration
6. Read MIC
7. Compare MIC with “breakpoint”
8. If the organism MIC is lower than the breakpoint MIC then the organism is sensitive, and if it is higher then the organism is resistant

Question 10

Explain disc sensitivity testing

Answer 10

1. Add organism to agar plate
2. Add antibiotics in filter paper discs
3. Incubate and grow over night
4. We will then have growth on some parts of the plate and not others (dark orange is growth)
5. Zone of inhibition = zone around antibiotic where organism hasn’t growth
6. Compare zone of inhibition with breakpoint zone of inhibition
7. If the zone is larger than breakpoint then the organism is sensitive, if it is smaller then the organism is resistant

Question 11

Where would we take a sample when looking for MRSA in a patient?

Answer 11

Nose/skin

Question 12

Where would we take a sample when looking for carbapenemase producing enterobacterales (CPE) in a patient?

Answer 12

Rectal/faecal

Question 13

What method do we use to detect the presence of antibiotic resistance genes?

Answer 13

nucleic acid amplification tests - mainly PCR

Question 14

What does ‘innate resistance mechanisms’ mean? Give examples

Answer 14

• fundamental property of the bacterium/ antibiotic combination
• usually relates to permeability/ entry of the antibiotic into the cell
  e. g glycopeptides cannot enter gram -ve organisms, and aztreonam cannot enter gram +ve

Question 15

What does ‘acquired resistance mechanisms’ mean?

Answer 15

• acquisition of a gene that encodes an antibiotic resistance mechanism
• new mutation
• horizontal transfer
• usually an antibiotic-modifying enzyme or target alteration

Question 16

Name 5 types of resistant mechanisms

Answer 16

1. Absent target e.g antibacterial agents used for fungal infections - target won’t be present
2. Decreased permeability - e.g gram -ves have an outer membrane that is impermeable to vancomycin
3. Target modification
   e. g in MRSA, there is altered PBPs so they don’t bind to B-lactams
4. Enzymatic degradation
   e. g B-lactamases
5. Drug efflux - pumping drugs out of cells

Question 17

How is horizontal transfer relevant in antibiotic resistance?

Answer 17

• resistance genes are encoded in plasmids

- DNA sequences designed to be transferred from plasmid to plasmid and/or from plasmid to chromosome

Question 18

How is vertical transfer relevant in antibiotic resistance?

Answer 18

• resistance genes transferred to daughter cells on bacterial cell-division
• so daughter cells also have resistance genes
• binary fission

Question 19

What is the consequence of antibiotic resistance?

Answer 19

bacterial infections become resistant to antibiotics traditionally used to treat them

Question 20

Give examples of antibiotic resistant strains

Answer 20

• MRSA (Meticillin-resistant staphylococcus aureus)
• VRE (vancomycin resistant enterococci)
• carbapenemase-producing enterobacterales (CPE)
• Multi-drug resistant tuberculosis (MDR-TB)

Question 21

What is empiric therapy when prescribing antibiotics?

Answer 21

clinical “educated” guess where some information is missing regarding the pathogen as the diagnosis of infection is rarely certain

Question 22

What factors need to be considered when choosing which antibiotic to use?

Answer 22

• MIC
• antibiotic sensitivity results
• patient factors such as allergies, interacting drugs and potential contraindications