Antimicrobial resistance

Question 1

Give 2 examples where a disease has become resistant to an antimicrobial

Answer 1

S pnuemonia: 55% resistant to penicillin
S. Dyentariae: 90% resistant to cotrimoxazole
S Typhi: outbreaks of multi-resistant strains
M Tuberculosis: multi drug resistance
P. falciparum (malaria): chloroquinone resistant
HIV resistant to all marketed agents

Question 2

Where does antimicrobial resistance come from? (4)

Answer 2

Health care
Community
food/farms
the world

Question 3

What are the 2 major forms of AM resistance?

Answer 3

INTRINSIC RESISTANCE

ACQUIRED RESISTANCE

Question 4

What is intrinsic resistance? (basic definition)

Answer 4

Inherited or natural resistance

e.g. chlamydia do not have peptidoglycan, so are not susceptible to penicillins

Question 5

What is acquired resistance? (basic definition)

Answer 5

Developed through alteration of the microbial genome

Question 6

Outline the features of intrinsic resistance (3)

Answer 6

Chromosomic genetic support
Affect almost all species strains
Existed before antibiotic use (enterobacter sp has always been resistant to amoxicillin, before we even started using amox)

Question 7

Outline acquired resistance (3)

Answer 7

Can be on Chromosome, plasmids or transposons (genetic support)
Affects a fraction of strains
Increased with antibiotic use (extended spectrum beta-lactamase producing E Coli)

Question 8

What are the 2 main routes to acquired AM resistance?

Answer 8

Alteration of microbial genome can be through:

1. Vertical evolution: mutation and natural selection
2. Horizontal evolution: transfer of genes from one microbe to another (not directly in microbial genome itself)

Question 9

What is the mechanism for transfer of genes in acquired resistance? (2)

Answer 9

Transposons: small, mobile sequences of DNA that can move/be copied to other regions of the genome - either within the gene or to other genes

Plasmids: circular, mini chromosomes that replicate independently of chromosomal DNA

Question 10

Name the 3 ways resistance evolves within a strain

Answer 10

Mutations
Gene duplication/amplification
Transfer of genes

Question 11

Outline the 3 ways resistance can evolve in a strain via mutations

Answer 11

Spontaneous point mutations
Mistakes in DNA repair
Transposon insertion

Question 12

Outline the 2 ways resistance can evolve in a strain via gene duplication/amplification

Answer 12

Homologous recombination

Other forms of recombination event (e.g. transposons)

Question 13

Outline the 3 ways resistance can evolve in a strain via gene transfer

Answer 13

Transduction - lysogenic bacteriophage infection (transfer of DNA from one bacterium to another via bacteriophages)

Conjugation - pili-mediated sex (transfer of DNA material via sexual pilus and requires cell-to-cell contact)

Transformation - ‘leaky’ bacterial uptake of nuclear material (uptake of short fragments of DNA by naturally transformable bacteria)

Question 14

Does genetic material transfer only happen between the same strains/species?

Answer 14

No, it can happen between different strains/species

Question 15

What is Minimum inhibitory concentration (MIC)?

Answer 15

The smallest concentration of antibiotic that inhibits the growth of organism

Question 16

How do you test for AM susceptibility?

Answer 16

Liquid media (dilution)

Solid media (diffusion) - disk diffusion (Kirby-Bauer), E-tests

Question 17

Briefly describe AM susceptibility tests with dilution in liquid broth (3)

Answer 17

Tubes containing increasing antibiotic concentrations
Incubation during 18hr at 37C
Tedious

Question 18

Briefly outline the Kirby-Bauer disc testing for AM susceptibility (3)

Answer 18

AB-impregnated discs placed on agar plate at interface between test organism and susceptible control organism

Resulting zones of inhibition compared, use of controls

Susceptibility is inferred (standard tables)

Question 19

Name the 6 mechanisms of resistance

Answer 19

Antimicrobial exclusion 
Enzymatic degradation of drug
Modification of drug target 
Target bypass
Enhanced production of target
Efflux mechanisms

Question 20

Describe antimicrobial exclusion (2)

Answer 20

Preventing antimicrobial from entering microbe (e.g. outer membrane of gram neg bacteria)

Outer membrane acts as barrier to extracellular compounds

Question 21

Describe enzymatic degradation (2 + 1 example)

Answer 21

Penicillins work by irreversibly binding to the transpeptidase enzymes, stopping the process of peptidoglycan cross linking

Binding occurs via the B-lactam ring

(e.g. resistance to penicillins is mediated by B-lactamases as it prevents penicillin binding to transpeptidases)

Question 22

Where do you find B-Lactamases in gram-negative bacteria?

Answer 22

Periplasmic space, between outer and inner membrane

Concentrated at strategic location

Question 23

Where and when would you find B-Lactamases in gram positive bacteria?

Answer 23

They are induced - produced in response to the drug. This is economical for bacteria

They are extracellular - long-range action, dilution of effectiveness

Question 24

How could you overcome the issue of B-lactamases in enzyme degradation with regards to AM resistance?

Answer 24

Develop resistant B-lactams - so they are resistant to B-lactamases. This is created through increased steric hindrance. e.g. methicillin.

Inhibit B-lactamases with another drug - no AM activity when it binds to transpeptidases (doesn’t bind well to these), but causes irreversible acylation of B-lactamases when it binds which inactivates it and prevents it binding/degrading B-lactams.
e.g. calvulanic acid (isolated from streptomyces spp.)