Antimicrobial resistance Flashcards
Give 2 examples where a disease has become resistant to an antimicrobial
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
Where does antimicrobial resistance come from? (4)
Health care
Community
food/farms
the world
What are the 2 major forms of AM resistance?
INTRINSIC RESISTANCE
ACQUIRED RESISTANCE
What is intrinsic resistance? (basic definition)
Inherited or natural resistance
e.g. chlamydia do not have peptidoglycan, so are not susceptible to penicillins
What is acquired resistance? (basic definition)
Developed through alteration of the microbial genome
Outline the features of intrinsic resistance (3)
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)
Outline acquired resistance (3)
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)
What are the 2 main routes to acquired AM resistance?
Alteration of microbial genome can be through:
- Vertical evolution: mutation and natural selection
- Horizontal evolution: transfer of genes from one microbe to another (not directly in microbial genome itself)
What is the mechanism for transfer of genes in acquired resistance? (2)
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
Name the 3 ways resistance evolves within a strain
Mutations
Gene duplication/amplification
Transfer of genes
Outline the 3 ways resistance can evolve in a strain via mutations
Spontaneous point mutations
Mistakes in DNA repair
Transposon insertion
Outline the 2 ways resistance can evolve in a strain via gene duplication/amplification
Homologous recombination
Other forms of recombination event (e.g. transposons)
Outline the 3 ways resistance can evolve in a strain via gene transfer
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)
Does genetic material transfer only happen between the same strains/species?
No, it can happen between different strains/species
What is Minimum inhibitory concentration (MIC)?
The smallest concentration of antibiotic that inhibits the growth of organism
How do you test for AM susceptibility?
Liquid media (dilution)
Solid media (diffusion) - disk diffusion (Kirby-Bauer), E-tests
Briefly describe AM susceptibility tests with dilution in liquid broth (3)
Tubes containing increasing antibiotic concentrations
Incubation during 18hr at 37C
Tedious
Briefly outline the Kirby-Bauer disc testing for AM susceptibility (3)
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)
Name the 6 mechanisms of resistance
Antimicrobial exclusion Enzymatic degradation of drug Modification of drug target Target bypass Enhanced production of target Efflux mechanisms
Describe antimicrobial exclusion (2)
Preventing antimicrobial from entering microbe (e.g. outer membrane of gram neg bacteria)
Outer membrane acts as barrier to extracellular compounds
Describe enzymatic degradation (2 + 1 example)
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)
Where do you find B-Lactamases in gram-negative bacteria?
Periplasmic space, between outer and inner membrane
Concentrated at strategic location
Where and when would you find B-Lactamases in gram positive bacteria?
They are induced - produced in response to the drug. This is economical for bacteria
They are extracellular - long-range action, dilution of effectiveness
How could you overcome the issue of B-lactamases in enzyme degradation with regards to AM resistance?
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.)
