Week 12 - Antimicrobial Resistance

Question 1

Innate Antimicrobial Resistance

Answer 1

Already encoded in genome
Natural and inherited
Predictable once know particular bacterial group

Question 2

Mechanisms of Drug Resistance

Answer 2

Alter the target site
Bypass mechanisms of resistance
Destroy the antimicrobial agent
Modify the antibacterial agent
Reduce intracellular levels of antibiotics

Question 3

Microbial Biofilms

Answer 3

Can stop penetration of antimicrobials
Biofilms responsible for numerous infections that are difficult to manage
Can be mixed or pure

Question 4

What are biofilms made up of?

Answer 4

Polysaccharide matrix (75-95%)
Bacterial cells (5-25%)

Question 5

Acquired Antimicrobial Resistance

Answer 5

Results from altered cellular physiology and structure caused by changes in microorganisms usual genetic makeup

• successful genetic mutation
• acquisition of genes from other organism
• combination of mutation and gene transfer

Question 6

Origins and Evolution of Antibiotic Resistance Genes

Answer 6

Bacterium can become resistant by 2 methods:
- Mutation
- Acquisition of a new gene
Emergence of new genes
Spread of old genes to new hosts

Question 7

Spread of Antibiotic Resistance Genes

Answer 7

Bacteria transsfer resistance genes among themselves by 3 mechanisms:

• Transformation
• Transduction
• Conjugation

Question 8

Transformation

Answer 8

Incorporation of naked DNA into recipient cell
Recipient cell capable of taking up naked DNA said to be competent
Only occurs naturally in about 40 species

Question 9

Transduction

Answer 9

DNA transferred as part of a virus
Two ways:
- generalised
- specialised

Question 10

Conjugation

Answer 10

Promotes horizontal transfer of genetic material from a donor cell to a recipient cell via cell-to-cell contact
Genes from conjugation usually borne on conjugative plasmids and occasionally on conjugative transposons

Question 11

IS elements

Answer 11

Way microorganisms get information
Elements of DNA that can move around and regulate themselves
Don’t care about homology (can line up in genome wherever they like)
Smallest one made up of transposase bounded by inverted repeats (IR)
Transposase - binds to transposon and catalyses movement by cut and paste mechanism
IR 15-25 bp at each end of element and unique to the IS
Transposase makes staggered cut in target DNA molecule
IS inserts into cut and is replicated

Question 12

Transposons

Answer 12

Similar to IS elements but include gene not required for transposition
Encode additional proteins that offer selective advantage to their host

Question 13

How is antibacterial resistance emerging?

Answer 13

Selective pressures
Over prescription
Incorrect following of directions

Question 14

How can microorganisms alter the target site?

Answer 14

Mutate 30S ribosomal unit so streptomycin can’t bind
Mutations in B unit of RNA pol reduces affinity of rifampicin
Producing altered transpeptidases reduces affinity for B-lactam antibiotics
Producing altered 50S ribosomal subunit that still functions but antibiotic can no longer bind

Question 15

How can microorganisms bypass mechanisms of resistance?

Answer 15

Instead of using traditional pathway to make folic acid use alternate enzymes so that it can bypass sulphonamides

Question 16

How can a microorganism destroy the antibacterial agent?

Answer 16

Penicillinases or B-lactamases destroy the B-lactam ring
- Klebsiella sp. produce B-lactamases that destroy ampicillin before drug can reach target
Amidases remove the R-group from penicillins

Question 17

How can a microorganism modify the antibacterial agent?

Answer 17

Chloramphenicol Acetyltransferase
- attaches an acyl group chloramphenicol preventing it from binding to ribosomes
- lock and key doesn’t work
IMPORTANT!!!

Question 18

How can a microorganism reduce intracellular levels of antibiotics?

Answer 18

Pumping the drug out of the cell as soon as it comes in
Some cells naturally have resistance to some antimicrobial agents
- penicillin can’t pass through outer membrane of GN cells
Some cells can alter permeability of cell membrane to antibiotic
- Pseudomonas aeruginosa alters size or conc of porins in outer layer
- providing resistance to B-lactamases

Question 19

Efflux Pumps

Answer 19

Allows cell to remove drugs

Question 20

Multidrug Resistance Transporters

Answer 20

Get drugs and pump out other harmful components such as heavy metals and compounds
2 classes:
- secondary transporters driven by proton (H+ ions) or sodium motive force
- ATP-binding cassette primary transporters which use hydrolysis of ATP to fuel transport

Question 21

Generalised Transduction

Answer 21

Phage infects bacterial cell
Hijacks replication machinery making lots of copies of the phage
During packaging, accidentally picks up bacterial DNA instead
This makes a transducing phage
This phage doesnt have genetic code for phage, has code for bacteria
Puts bacterial DNA into new cell
Now doesnt code for hijacking replication machinery, might code for antibacterial resistance gene

Question 22

Pseudomonas aeruginosa Resistance Characteristics

Answer 22

Produces B-lactamases
Impermeable outer membrane - alters size or concentration of porins in the outer membrane
Active efflux pumps
Forms biofilms