Lecture 9. Mode of Action

Question 1

What are biofilms?

Answer 1

An assembly of microbial cells associated with a surface, and enclosed in an extracellular matrix made principally of polysaccharides

Question 2

What are biofilm-associated organisms fundamentally different from?

Answer 2

Populations of suspended (planktonic) cells

Question 3

What does the formation of biofilms allow bacteria to become?

Answer 3

Tolerant to antibiotics/antimicrobials, and generates resistance

Question 4

What does slow diffusion lead to?

Answer 4

Tolerance

Question 5

What do sublethal concentrations of antimicrobials in the biofilm lead to?

Answer 5

Selection of antimicrobial resistance (AMR)

Question 6

What is the antibiotic resistance mechanism in Gram-negative bacteria?

Answer 6

Intrinsic - have a double-membrane structure that makes the cellular envelope relatively impermeable

Question 7

What is the antibiotic resistance mechanism in Gram-positive bacteria?

Answer 7

Acquired - Alterations to envelope structure, such as porin loss reduce permeability to antibiotics leading to acquired antibiotic resistance

Question 8

What are two examples of non-specific antibiotic resistance mechanisms?

Answer 8

Porins and efflux pumps

Question 9

What do efflux pumps do?

Answer 9

Decrease the concentration of antibiotic within the cell

Question 10

What antibiotics target cell wall synthesis?

Answer 10

Penicillins
Cephalosporins
Carbapenems
Monobactams

Question 11

What antibiotics target nucleic acid synthesis?

Answer 11

Quinolones

Question 12

What antibiotics target the 30S subunit in the ribosome?

Answer 12

Amicoglycosides
Tetracyclines

Question 13

What antibiotics target the 50S subunit in the ribosome?

Answer 13

Macrolides
Chloramphenicol

Question 14

What is a β-lactam antibiotic?

Answer 14

A member of a large group of broad-spectrum agents related to β-lactam

Question 15

What do most β-lactam antibiotics target?

Answer 15

Penicillin-binding proteins

Question 16

What are penicillin-binding proteins?

Answer 16

Penicillin-binding proteins (PBPs) are DD-transpeptidases that make cross-links (peptide bonds) between D-amino acid residues in sugar-lined pentapeptides in the bacterial cell walls
Essential for bacterial cell wall synthesis

Question 17

How do β-lactam antibiotics work?

Answer 17

The reactive β-lactam ring binds the active site of the transpeptidase, permanently inactivating D-alanine the PBP: cell wall cross-linking ceases

Question 18

What are the four ways resistance to β-lactam antibiotics can occur?

Answer 18

1. Mutation of PBP, lowering the affinity for penicillins, etc
2. Down-regulation of porins in Gram-negative bacteria
3. Acquisition of β-lactamase- ESBL (extended spectrum β-lactamase)
4. Up-regulation of efflux pumps

Question 19

What are quinolone antibiotics?

Answer 19

A member of a large group of broad-spectrum bacteriocidals that share a bicyclic core structure related to the compound 4-quinolone

Question 20

What do quinolone antibiotics target?

Answer 20

Topoisomerase

Question 21

What do type II topoisomerases convert?

Answer 21

Positive supercoils (over-wound) into negative supercoils (under-wound)

Question 22

What do topoisomerases regulate?

Answer 22

Supercoiling in DNA synthesis and RNA synthesis

Question 23

What do gyrase and topoisomerase IV both consist of?

Answer 23

Two A subunits and two B subuints

Question 24

What does the B subunit do?

Answer 24

Carries out the nicking of DNA

Question 25

What does the A subunit do?

Answer 25

Introduces negative supercoils, and then reseals and ligates the strands

Question 26

How do ciprofloxacin (and other quinolone antibiotics) work?

Answer 26

Quinolones act by converting their targets, type II topoisomerases (gyrase and topoisomerase IV), into toxic enzymes that fragment the bacterial chromosome These double stranded cuts are hard to repair Ultimately they cause bacterial cell death

Question 27

What are the four ways resistance to quinolones can occur?

Answer 27

1. Mutation in gyrase and topo IV weaken quinolone-enzyme interactions 2. Plasmid – encoded Qnr proteins decrease topoisomerase–DNA binding 3. A plasmid - encoded enzyme acetylates ciprofloxacin decreasing its effectiveness 4. Plasmid-encoded efflux pumps decrease the concentration of quinolones in the cell

Question 28

What are aminoglycoside antibiotics?

Answer 28

Contains amino-sugar structures

Question 29

What do aminoglycoside antibiotics inhibit?

Answer 29

Protein synthesis

Question 30

What do aminoglycoside antibiotics display bactericidal activity against?

Answer 30

Gram-negative aerobes but generally not against anaerobic Gram-negative bacteria

Question 31

How do aminoglycoside antibiotics work?

Answer 31

Aminoglycosides inhibit protein synthesis by high affinity binding to the A-site on the 16S ribosomal RNA of the 30S ribosome

Question 32

How does binding to the A site cause problems?

Answer 32

Aminoglycoside binding to the 30S subunit of the ribosome causes codon misreading. This leads to misincorporation of amino acids into elongating peptides – error prone protein synthesis

Question 33

What can modify aminoglycosides?

Answer 33

Acetyltransferases