Antibiotic Resistance

Question 1

Define resistance.

Answer 1

a previously susceptible organism is no longer inhibited by an antibiotic at levels that can be safely achieved clinically (causes MIC to rise)

Question 2

Define intermediate resistance.

Answer 2

Resistance which can be treated with an increase from the standard dose.

Question 3

Define therapeutic index.

Answer 3

Difference between dose necessary for treatment, and minimum dose causing harm.

Question 4

Define intrinsic resistance.

Answer 4

All strains of a species are resistant (regardless of dose of antibiotic)

Question 5

Define minimum inhibitor concentration (MIC) + minimum bactericidal concentration (MBC).

Answer 5

Lowest concentration of an antibiotic that completely inhibits the growth of a bacterium.
Lowest concentration of an antibiotic that completely kills a bacterium.

Question 6

What is the MBC of bacteriostatic drugs ?

Answer 6

They do not have one (they do not kill the bacteria)

Question 7

Describe a method to determine the MIC and MBC of a drug.

Answer 7

TUBE METHOD

• Antibiotic added in increasing concentration in medium inoculated with bacterial cells.
• MIC is first clear tube
• Then, transfer content of each clear tube including and following the MIC tube to fresh medium containing no antibiotic.
• First clear tube at that point is the MBC

Question 8

Define antibiotic breakpoint. What factors might change the antibiotic breakpoint.

Answer 8

• Concentration chosen for laboratory tests which will differentiate sensitive from resistant populations of bacteria (makes it easier for labs, allows them to test only one concentration rather than having to determine the MIC)
• The breakpoint of a given antibiotic will be different for different species because they will have different intrinsic resistance/susceptibility.

Question 9

What are the factors which determine the breakpoint of an antibiotic.

Answer 9

• The distribution of MICs of the target bacteria
• Achievable therapeutic concentration in tissue (i.e. because if the breakpoint is at a toxic dose, then breakpoint needs to be reduced)
• Maximum achievable concentration

Question 10

Identify organisms with intrinsic resistance, and identify the classes of antibiotics each is resistant to and the reason they are resistant.

Answer 10

Streptococci are naturally resistant to aminoglycosides (because their MIC is higher than the toxic concentration)

Pseudomonas spp. normally resistant to beta-lactams (because intrinsic beta lactamases, although these can be overcome by design)

Mycoplasma spp. are all resistant to beta lactams (because no peptidoglycan in cell wall, which beta lactams act against)

Enterobacteriacae are all resistant to metronidazole (because they are facultative anaerobes, so normally grow in aerobic conditions)

Question 11

Define acquired resistance.

Answer 11

Previously susceptible species or stain develops an increased MIC which takes it beyond therapeutic range.

Question 12

Identify the main mechanisms of resistance.

Answer 12

1) Enzymatic inactivation by destruction
2) Enzymatic inactivation by addition
3) Efflux
4) Alternative pathway
5) Altered target

Question 13

Identify an enzyme responsible for the process of enzymatic inactivation by destruction. Identify a class of antibiotics affected by this.

Answer 13

Beta lacatamases
Expanded spectrum beta lactamases

Beta lactams (including cephalosporins) are effected by this.

Question 14

Identify a class of antibiotics affected by the process of enzymatic inactivation by addition.

Answer 14

Aminoglycosides

Question 15

Describe the process of efflux. Identify an antibiotic affected by the process by efflux.

Answer 15

“Efflux pumps are transport proteins involved in the extrusion of toxic substrates (including virtually all classes of clinically relevant antibiotics) from within cells into the external environment.” Mechanism of resistance which may have a role in organisms gaining higher levels of resistance.

Affects tetracyclines, quinolones, macrolides.

Question 16

Describe the process of alternative pathway. Identify an antibiotic affected by this mechanism of resistance.

Answer 16

Some bacteria may bypass the route acted upon by the antibiotic and result in the same product via another pathway.

Meticillin (with MRSA)

Question 17

Describe the process of altering target. Identify an antibiotic affected by this mechanism of resistance.

Answer 17

Alterations in the binding site of the target molecule result in an inability for the antibiotic to bind.

Rifampicin, fluoroquinolones, sulphonamides are all affected.

Question 18

What is the mechanism of action of fluoroquinolones. How can their activity be reduced ?

Answer 18

Binding and inhibiting DNA gyrase to inhibit DNA supercoiling.

Reduced binding to DNA by DNA gyrase (due to mutations), result in resistance to quinolones.

Question 19

In general, how does resistance arise ? Give examples.

Answer 19

Resistance arises through mutations (everytime DNA being transcribed or dividing, possibility for mutation to be created) in critical chromosomal genes (which are involved with the target of the antibiotic).

For instance rifampicin, DNA gyrases.

Question 20

Describe the evolution of resistance in quinolones (including the genes involved).

Answer 20

Two genes involved: gyrA and parC
Point mutations in these genes result in changes in the affinity of the proteins for DNA
Mutation in one gene encodes low level resistance where as in both it encodes high level resistance.

Question 21

Identify the most active fluoroquinolone against 1st and 2nd step mutants.

Answer 21

Gemifloxacin

Question 22

Identify the main mechanisms of resistance transmission. Give an example of antibiotic affected by each.

Answer 22

1. Transformation (e.g. penicillin)
2. Conjugation (e.g. B-lactamases)
3. Transposons (e.g. erythromicin)

Question 23

Describe the process of transformation for transmission of resistance. Identify an antibiotic and pathogen for which this applies.

Answer 23

Bacterium takes up naked DNA from its environment into its own nucleus (through homologous recombination, forming mosaic gene).
If the naked DNA has a resistance gene mutation, the organism becomes resistant.

Neisseria gonorrhea and penicillin

Question 24

Describe the process of conjugation for transmission of resistance.

Answer 24

♣ During conjugation, one bacterium serves as the donor of the genetic material, and the other serves as the recipient.
♣ The donor bacterium transfers genetic material to the recipient bacterium in the form of a plasmid (a small, accessory circular piece of DNA).
♣ This genetic material transferred can encode virulence, metabolic functions, and resistance determinants
♣ Rapid

Question 25

Describe the process of transposons for the transmission of resistance.

Answer 25

• Transposons (and potentially integrons) are small segments of DNA which encode their own transmission
• Many bacteria have these mobile genetic elements
• Allows genome plasticity (i.e. alterable nature of prokaryotic genomes that enables the fluid exchange of DNA from one microorganism to another and allows prokaryotes to adapt their genomes rapidly so they can survive changes in environmental conditions)
• May collect resistant determinants

Question 26

Define the term superbug.

Answer 26

Organism that has gained resistance to a critical antibiotic (e.g. MRSA, with penicillin) or organism that has gained resistance to multiple antibiotics.

Question 27

Identify examples of superbugs, and the antibiotic they are resistant to.

Answer 27

♣ MRSA - Meticilin
♣ GISA - Glycopeptide Resistant Staph Aureus
♣ VRE - Vancomycin Resistance Enterococcus
♣ ESBLs - Expanded Spectrum Beta-Lactamases (gene encoding extended spectrum beta lactamase, helping make bacteria resistant, not a bacteria itself )

Question 28

To what extent are most bacteria susceptible to Carbapenams ?

Answer 28

Most bacteria ARE susceptible to Carbapenams BUT new resistant stains (especially Enterobacteriaceae) containing following enzymes: Klebsiella Pneumoniae carbapenamase (KPC) or New Delhi metallo-betalactamase (NDM) or Oxa-48 carbapenemase.

Question 29

Why do we worry about resistance in some organisms and not others?

Answer 29

1) When the consequences of infection are severe and the organism is common (e.g. MRSA and ESBL Gram negatives)
2) When the organism is naturally resistant to many antibiotics (E. faecium, and Ps. aeruoginosa)
3) When the infection is in a site where it is difficult to get high concentrations of antibiotic (pneumococcal meningitis)

Question 30

How do we address the problem of antibiotic resistance.

Answer 30

• Optimise the treatment of bacterial disease
• Better diagnosis
• Focused treatment
• Appropriate length of courses

Question 31

What is the treatment for resistant bugs ?

Answer 31

• Culture + susceptibility testing
• Use most bactericidal drug available
• Consider use of combinations