Antibiotic Resistance

Question 1

What is selection pressure?

Answer 1

Factors that contribute to selection which variations will provide the individual with an increase chance of surviving over others –> organisms with certain phenotypes have an advantage

e.g. predation, disease and competition

Question 2

How can selection pressure lead to antibiotic resistance?

Answer 2

Bacteria are constantly subject to spontaneous gene mutations.

Mutations that confer a survival advantage favour the growth and propagation of the mutant strain.

If the organism is growing in the presence of a sub-inhibitory concentration of antibiotics the development of a resistance gene will confer a survival advantage.

The resistant mutant will have a survival advantage and out-compete other strains.

A resistant strain is born.

Question 3

Define a sub-inhibitory conc of antibiotics?

Answer 3

Not completely producing inhibition (as of bacterial growth)

Question 4

How can the transmission of resistant organisms lead to antibiotic resistance?

Answer 4

Resistant bacteria gain access to the human microbiome directly or via environmental sources (fomites).

Resistant genes can be transferred between bacteria of different species (this commonly happens in the large bowel).

The result is a human microbiome that contains a mixture of sensitive and resistant bacteria.

Question 5

What are fomites?

Answer 5

Objects which are likely to carry infection, such as clothes or utensils

Question 6

If a microbiome that contains a mixture of sensitive and resistant bacteria is exposed to antibiotics, what happens?

Answer 6

Sensitive bacterial strains will die out and strains carrying resistance genes will become the dominant strains.

Subsequent endogenous infection is more likely to be caused by antibiotic-resistant organisms.

Question 7

What is an endogenous infection?

Answer 7

A disease arising from an infectious agent already present in the body but previously asymptomatic.

Question 8

Gut and antibiotic resistance explained:

Answer 8

The gut contains very large numbers of bacteria. Changes in the composition of the gut flora, due in particular to antibiotics, can lead to the selection of highly resistant bacteria and Candida species.

These resistant organisms may remain for months in the gut of the carrier without causing any symptoms or migrate to cause infections (e.g. UTIs). These infections then are antibiotic-resistant.

Especially: E. coli

Question 9

What are the 2 distinct ways in which antibiotic resistance can arise in a patient?

Answer 9

1. Exposure to antibiotics (and spontaneous mutation)

2. Transmission of resistant organisms

Question 10

What methods are used to prevent antibiotic resistance?

Answer 10

1. Reducing antibiotic exposure to the minimum safe level - antibiotic stewardship
2. Minimising transmission through infection prevention & control (IPC)

Question 11

If antibiotic exposure is ceased, the resistant organisms will often be out-competed and replaced with sensitive organisms.

Why is this?
What is the benefit of this?

Answer 11

Resistance mutations usually affect vital bacterial cell functions e.g. cell wall construction, DNA synthesis etc

For this reason antibiotic resistant organisms are often less fit than strains that do not carry resistance genes

If antibiotic exposure is ceased, the bacteria will eventually be susceptible to antibiotics again.

Question 12

What are the main 2 ways in which antibiotic resistance can be identified?

Answer 12

1. Antimicrobial sensitivity testing

2. Detection of antimicrobial resistance genes

Question 13

What are antimicrobial resistance genes?

Answer 13

Genes that are known to encode resistance mechanisms

Question 14

How does antimicrobial sensitivity testing work?

Answer 14

1. Try to grow the organism in the presence of an antibiotic
2. If it grows in the presence of a high concentration (high MIC) it is “resistant”
3. If it is killed at a low concentration (low MIC) it is “sensitive”

Therefore, the lower the MIC the more sensitive is the organism to antibiotics (killed more easily).

Question 15

What is MIC?

Answer 15

The minimum inhibitory concentration (MIC) –> the lowest concentration of a drug which prevents visible growth of bacteria

Question 16

The MIC value must then be compared with a breakpoint value.

What is a breakpoint value?

Answer 16

The breakpoint is the highest plasma concentration of the drug that can safely be achieved in the patient and defines whether an organism is susceptible or resistant to the drug.

Question 17

If the organism MIC is LOWER than the breakpoint MIC, what does this mean?

Answer 17

Then the organism is said to be sensitive

Question 18

If the organism MIC is higher than the breakpoint MIC, what does this mean?

Answer 18

Then the organism is said to be resistant

Question 19

How is disk sensitivity testing (solid media) carried out?

Answer 19

1. Add organism to agar plate
2. Add different antibiotics
3. Incubate
4. Growth around some antibiotics, no growth around others (zone of inhibition)

Question 20

What is the zone of inhibition?

Answer 20

The Zone of inhibition is a circular area around the spot of the antibiotic in which the bacteria colonies do not grow.

Question 21

What is the zone of inhibition then compared with?

Answer 21

Zone diameter breakpoint values

Question 22

If the zone of inhibition is larger than the zone diameter breakpoint, what does this mean?

Answer 22

Organism is sensitive

Question 23

If the zone of inhibition is smaller than the zone diameter breakpoint, what does this mean?

Answer 23

Organism is resistant

Question 24

How is the detection of antibiotic resistance genes carried out?

Answer 24

Nucleic acid amplification tests –> Mainly the polymerase chain reaction (PCR)

Question 25

How is MRSA gene testing done?

Answer 25

Nose/skin swabs. Detect a gene that produces a pencillin binding protein (PBP2a). If this gene is present, then the mutation is present. Then the organism will be MRSA.

Question 26

How is the presence of Carbapenemase producing Enterobacterales (CPE) done?

Answer 26

Rectal/faecal swabs

Question 27

What are Enterobacterales?

Answer 27

Enteric gram-negative bacilli that live naturally and harmlessly in people’s guts,

Question 28

What are Carbapenemase producing Enterobacterales (CPE)?

Answer 28

Bacteria that are likely to be resistant to most antibiotics. Enterobacterales: Enteric gram-negative bacilli that live naturally and harmlessly in people’s guts, Carbapenems: a class of broad spectrum β-lactam antibiotics which are highly effective against most Gram-negative infections and are important in the treatment of infections resistant to most other antibiotics. Carbapenemase: an enzyme that some bacteria produce, which make them resistant to important antibiotics, including carbapenems.

Question 29

What are the advantages and disadvantages of antibiotic resistance genes testing?

Answer 29

Advs: Sensitive and fast Disadvs: Presence of a gene does not always correlate with resistance

Question 30

What are the 2 broad categories of resistance mechanisms?

Answer 30

1. Innate | 2. Acquired

Question 31

What is innate antibiotic resistance?

Answer 31

A fundamental property of the bacterium/antibiotic combination that usually relates to permeability/entry of the antibiotic into the cell. E.g. Glycopeptides and daptomycin don't get into Gram-negatives

Question 32

What are glycopeptides and daptomycin unable to treat?

Answer 32

Gram-negative bacteria negatives due to their inability to cross the outer membrane

Question 33

What are glycopeptides and daptomycin used to treat?

Answer 33

Gram-positives

Question 34

What is Aztreonam not effective against?

Answer 34

Gram-positive bacteria or anaerobes

Question 35

What is Aztreonam effective against?

Answer 35

Gram-negative aerobic bacteria (e.g. Neisseria, Pseudomonas, Haemophilus influenzae)

Question 36

What are aminoglycosides not effective against? Why?

Answer 36

1. Not effective against anaerobes as their uptake across bacterial cell membranes depends on energy derived from aerobic metabolism 2. Not effective against streptococci

Question 37

What is acquired antibiotic resistance?

Answer 37

A gene that encodes an antibiotic resistance mechanism has been acquired by an organism: - New mutation - Horizontal transfer

Question 38

What are the most commonly acquired resistance mechanisms?

Answer 38

Usually an antibiotic-modifying enzyme or an alteration of the antibiotic target site (e.g. cell wall)

Question 39

What are the 5 main methods of antibiotic resistance?

Answer 39

1. Absent target 2. Decreased permeability 3. Target modification 4. Enzymatic degradation 5. Drug efflux

Question 40

Explain when there is an 'absent target' during antibiotic resistance

Answer 40

Target of drug is not present: - Antibacterial agents against fungi - Antiviral agents against bacteria

Question 41

What is 'decreased permeability' regarding antibiotic resistance?

Answer 41

This is an example of innate resistance. Example: Vancomycin/Gram-negative bacilli - Gram-negatives have an outer membrane that is impermeable to vancomycin. Example: Gentamicin/anaerobic organisms: - Uptake of aminoglycosides requires an O2 dependent active transport mechanism

Question 42

What is Vancomycin not effective against?

Answer 42

Gram-negatives

Question 43

What is 'target modification' regarding antibiotic resistance?

Answer 43

Alteration in the target sites of antibiotics.

Question 44

What has MRSA conferred resistance to? How?

Answer 44

Resistance to all beta-lactam antibiotics, including methicillin, flucloxacillin, cephalosporins, and carbapenems. By target modification: Altered penicillin-binding protein does not bind β-lactams

Question 45

What is VRE?

Answer 45

Vancomycin-resistant enterococci (Enterococci are bacteria that live in the GI tract of most people without causing illness.). Vancomycin is an antibiotic used to treat infections caused by enterococci. When enterococci become resistant to vancomycin, they are called VRE.

Question 46

How has VRE conferred resistance?

Answer 46

Altered peptide sequence in Gram-positive peptidoglycan reduces binding of vancomycin 1000-fold

Question 47

What is 'enzymatic degradation' regarding antibiotic resistance?

Answer 47

Many bacteria code for enzymes that can modify/degrade antibiotics: Example: Beta-lactamases inactivate certain penicillins and cephalosporins

Question 48

What are beta-lactamases?

Answer 48

Enzymes produced by bacteria that provide multi- resistance to β-lactam antibiotics such as penicillins, cephalosporins, cephamycins, and carbapenems

Question 49

What is 'drug efflux' regarding antibiotic resistance?

Answer 49

Active transport mechanism of bacteria that results in antibmicrobial agent being removed from the target cell.

Question 50

Drug efflux is a key mechanism of resistance in what bacteria?

Answer 50

Gram-negative bacteria

Question 51

What are many resistance mechanisms encoded by?

Answer 51

Single genes e.g. antibiotic-modifying enzymes, altered antibiotic targets

Question 52

Resistance genes can be encoded in plasmids. What is a plasmid?

Answer 52

Circular DNA sequences transmitted within and between species, mainly by conjugation.

Question 53

Spread of resistance genes can either be by horizontal or vertical transfer. What is horizontal transfer?

Answer 53

- Enabled by transposons and integrons: DNA sequences designed to be transferred from plasmid to plasmid and/or from plasmid to chromosome - Often contain cassettes with multiple resistance genes i.e. movement of resistance genes from one species to another

Question 54

What is vertical transfer?

Answer 54

Chromosomal or plasmid-borne resistance genes transferred to daughter cells on bacterial cell-division i.e. vertical transfer of all genetic material to progeny

Question 55

Chromosomal vs plasmid mediated resistance?

Answer 55

Plasmid-mediated permits intraspecies and interspecies transfer to occur Chromosomal resistance can only be passed on to progeny.

Question 56

What is a gene cassette?

Answer 56

A gene cassette is a type of mobile genetic element that contains a gene and a recombination site

Question 57

Examples of antibiotic resistant bacteriaL

Answer 57

Meticillin-resistant Staphylococcus aureus (MRSA) Vancomycin/glycopeptide-resistant enterococci (VRE/GRE) Extended-spectrum β-lactamase-producing Enterobacterales (ESBL) Carbapenemase-producing Enterobacterales (CPE) Multi-drug resistant tuburculosis (MDR-TB) Extremely-drug resistant tuberculosis (XDR-TB)

Question 58

What is empiric therapy?

Answer 58

Best guess therapy used at the early stages of an infection

Question 59

What is the risk of antibiotic resistance in empiric therapy?

Answer 59

Risk of under-treatment if ‘traditional’ antibiotic is used Use of excessively broad-spectrum treatment if risk of resistance is taken into account

Question 60

What are the implications of antibiotic resistance in targeted therapy?

Answer 60

Requires the use of alternatives which may be: 1) expensive 2) toxic 3) 'last line'

Question 61

What are last lie antibiotics?

Answer 61

The last treatment options for patients infected with bacteria resistant to other available antibiotics e.g. carbapenems for multi-resistant Enterobacterales

Question 62

Antibiotic resistance can be monitored via 'local surveillance'. What is this?

Answer 62

Local (hospital) surveillance for individual cases eg MRSA, CPE etc to trigger immediate IPC interventions. Detects trends trends in resistance eg gradual increases in prevalence of resistant organisms.

Question 63

Where does resistance tend to be highest in Europe?

Answer 63

In the South

Question 64

The Indian subcontinent is a worldwide resistance ‘hotspot' for which antibiotic?

Answer 64

Carbapenems

Question 65

A septic patient has a bacteraemic urinary tract infection caused by E. coli. The minimum inhibitory concentration (MIC) of antibiotic A for the E. coli is 0.0125 mg/L and the MIC of antibiotic B is 1.0 mg/L. Both the MICs are below their respective breakpoint MICs for E. coli. Which antibiotic should the patient be treated with?

Answer 65

There is insufficient information available to guide an antibiotic choice. The relationship between MIC and clinical response depends on the concentration of antibiotic that is available within the patient, so a lower MIC does not necessarily indicate a higher likelihood of clinical response. The E. coli is sensitive to both antibiotics so the infection might respond to either. However, the ultimate choice of antibiotic depends on a combination of antibiotic sensitivity results and patient factors such as the allergies, interacting drugs and potential contraindications.