30. Antibiotics and anti-fungals

Question 1

Briefly compare gram positive, gram negative and mycolic bacteria

Answer 1

• G+ - thick peptidoglycan cell wall
• G- - thin peptidoglycan cell wall between inner membrane and outer lipopolysaccharide membrane
• Mycolic bacteria - like G+, with outer mycolic acid layer

Question 2

Describe the production of tetrahydrofolate (THF) in prokaryotic nucleic acid synthesis?

Answer 2

Paraaminobenzoate (PABA) => dihydropteroate (DHOp) [DHOp synthase]
=> dihydrofolate (DHF) => tetrahydrofolate (THF) [DHF reductase]

Question 3

What enzyme releases tension from the prokaryotic DNA to allow DNA replication?

Answer 3

DNA gyrase

Question 4

Which enzyme produces RNA from prokaryotic DNA template?

Answer 4

RNA polymerase (different from eukaryotic RNA polymerase)

Question 5

How do Sulphonamides affect bacteria?

Answer 5

• Inhibit DHOp synthase
• Prevent DHOp production

(most bacteria are resistant)

Question 6

How does Trimethoprim affect bacteria?

Answer 6

• Inhibits DHF reductase

* Prevents THF production

Question 7

How do Fluoroquinolones affect bacteria?

Answer 7

• Inhibit DNA gyrase + topoisomerase IV

* Prevent unwinding of DNA so it can’t be replicated

Question 8

How do Rifamycins affect bacteria?

Answer 8

• Inhibit bacterial RNA polymerase
• Reduce subsequent production of proteins required for cell survival

(useful in mycobacterium)

Question 9

Which antibiotics inhibit bacterial ribosomes (preventing protein production)

Answer 9

• Aminoglycosides
• Chloramphenicol
• Macrolides
• Tetracyclines

Question 10

Is it generally harder to target G+ or G- bacteria?

Answer 10

G+ as they have a more prominent cell wall

Question 11

How is the peptidoglycan wall synthesised?

Answer 11

• Pentapeptide created on N-acetyl muramic acid (NAM)
• N-acetyl glucosamine (NAG) and a pentapeptide associates with NAM, forming peptidoglycan
• This is transported from inside the cell to the periplasm (across the cell membrane) by a bactoprenol molecule
• It is incorporated into the cell wall
• Transpeptidase cross-links the pentapeptides

Question 12

Which antibiotics interfere with the bacterial cell wall and cell membrane?

Answer 12

• Glycopeptides
• Bactitracin
• Beta-lactams
• Lipopeptides
• Polymyxins

Question 13

How do glycopeptides e.g. vancomycin, inhibit the bacterial cell wall?

Answer 13

Bind the pentapeptide, preventing synthesis

Question 14

How does bacitracin inhibit the bacterial cell wall?

Answer 14

Inhibits bactoprenol regeneration, preventing transportation

Question 15

How do beta-lactams inhibit the bacterial cell wall

Answer 15

• Bind covalently to transpeptidase
• This inhibits peptidyglycan incorporation into a cell wall

(carbapenems, cephalosporins, penicillins)

Question 16

Why are antibiotics targeting cell membranes used more?

Answer 16

Due to antibiotic resistance

Question 17

What do lipopeptides and polymyxins do to bacteria?

Answer 17

• Lipopeptides - disrupt gram-positive cell membranes

* Polymyxins - bind to LPS and disrupt gram-negative cell membranes

Question 18

What percentage of bacteria have developed resistance?

Answer 18

Around 70%

Question 19

What are the causes of antibiotic resistance?

Answer 19

• Unnecessary and inappropriate prescription (50% not required)
• Livestock farming
• Lack of regulation (OTC in Russia etc.)
• Lack of development (few new antibiotics)

Question 20

What resistance is there to beta-lactams?

Answer 20

• Beta-lactamase
• Hydrolyses the C-N bond of the beta-lactam ring of the antibiotics

• Can also have molecules around the beta-lactam ring, shielding it
- steric hindrance

Question 21

Is penicillin G (IV) and V (oral) more effective for G+ or G-?

Answer 21

G+

Question 22

Which penicillins are beta-lactamase resistant?

Answer 22

Flucloxacillin and temocillin

Question 23

Is Amoxicillin beta-lactamase resistant?

Answer 23

Broad-spectrum, but not resistant

Co-administered with clavulanic acid (confers beta-lactamase resistance)

Question 24

How can E. coli become resistant to trimethoprim?

Answer 24

• Produces different DHF reductase enzyme, a version not affected by the antibiotic
• Can produce more of the enzyme too for better evasion

Question 25

How doe S. Aureus become resistant to quinolones?

Answer 25

Mutations in the ParC region of topoisomerase IV

Question 26

How can bacteria become resistant through alterations in drug permeation?

Answer 26

• Reductions in aquaporins (antibiotics enter using these)
• Increased efflux systems

(Used in gram negative bacteria)

Question 27

What are the 5 bacterial resistance mechanisms?

Answer 27

• Beta lactamase
• Production of additional targets (e.g. different version of an enzyme produced that is unaffected by AB)
• Alteration in the target enzyme (same enzyme)
• Hyper-production
• Alteration in drug permeation

Question 28

What does the fungal cell membrane contain that is a common drug target?

Answer 28

Ergosterol

Question 29

What are the 4 different types of fungal infections?

Answer 29

1) Superficial
2) Dermatophyte (skin, hair or nails)
3) Subcutaneous
4) Systemic (primarily respiratory tract)

Question 30

What are the 2 most common categories of anti-fungals?

Answer 30

Azoles e.g. fluconazole

Polyenes e.g. amphotericin

Question 31

How do Azoles work?

Answer 31

Inhibit ergosterol production
• Inhibit CYP450-dependent enzymes involved in membrane sterol synthesis
• Important in ergosterol production, which is incorporated into the cell wall

Question 32

What is fluconazole (oral) used for?

Answer 32

Candidiasis and systemic infections

Question 33

How do polyenes (amphotericin) work?

Answer 33

• Create pores in the cell membranes of fungal cells
• Binds to ergosterol
• Cell becomes permeable and bursts

(makes pores in all cells, so there are bad side effects)

Question 34

What is amphotericin (I-V) used to treat?

Answer 34

Systemic infections