Antibiotics and antifungals

Question 1

Bacteria

Answer 1

• single cell microorganisms (cell wall and cell membrane included)
• entire phylogenetic domain
• ~ one third are pathogenic

Question 2

Gram positive bacteria membrane properties

Answer 2

• prominent thick peptidoglycan cell wall

- EG: Staphylococcus Aureus

Question 3

Gram negative bacteria membrane properties

Answer 3

• outer membrane with lipopolysaccharide

- EG: Escherichia Coli

Question 4

Mycolic bacteria membrane properties

Answer 4

• outer mycolic acid layer

- EG: Mycobacterium Tuberculosis

Question 5

Prokaryotic protein synthesis

Answer 5

1) Nucleic Acid synthesis
2) DNA replication
3) RNA synthesis
4) Protein synthesis

Question 6

Nucleic acid synthesis

Answer 6

1) Dihydropteroate (DHOp)
- produced from paraaminobenzoate (PABA)
- converted into dihydrofolate (DHF)
- sulphonamides inhibit DHOp synthase
2) Tetrahydrofolate (THF)
- produced from DHF by DHF reductase
- important in DNA synthesis
- Trimethoprim inhibits DHF reductase

Question 7

DNA replication

Answer 7

1) DNA gyrase
- Toposimerase which releases tension in DNA
- Fluoroquinolones (eg: Ciprofloxacin) inhibits DNA gyrase and topoisomerase IV

Question 8

RNA synthesis

Answer 8

1) RNA polymerase
- produces RNA from DNA template
- differs from eukaryotic RNA polymerase
- rifamycins (eg: Rifampicin) inhibits bacterial RNA polymerase

Question 9

Protein synthesis

Answer 9

1) Ribosomes
- produces protein from RNA templates
- prokaryotic ribosomes (70S) differ from eukaryotic ribosomes (80S)
- ribosomes inhibited by aminoglycosides (eg: Gentamicin), chloramphenicol, macrolides (eg: erythromycin) and tetracylcines

Question 10

Bacterial wall synthesis

Answer 10

1) Peptidoglycan synthesis
2) Peptidoglycan transportation
3) Peptidoglycan incorporation

Question 11

Peptidoglycan synthesis

Answer 11

• pentapeptide is created on N-acetyl muramic acid (NAM)
• N-acetyl glucosamine (NAG) associates with NAM forming peptidoglycan
• Glycopeptides (eg: vancomycin) binds to pentapeptide to prevent peptidoglycan synthesis

Question 12

Peptidoglycan transportation

Answer 12

• peptidoglycan transported across membrane by bactoprenol

- Bacitracin inhibits bactoprenol regeneration preventing peptidoglycan transportation

Question 13

Peptidoglycan incorporation

Answer 13

• peptidoglycan incorporated into cell wall when transpeptidase enzyme cross-links peptidoglycan pentapeptides
• Beta-lactams bind covalently to transpeptidase to inhibit peptidoglycan incorporation into cell wall (eg: carbapenems, cephalosporins, penicillins etc)

Question 14

Cell wall stability

Answer 14

• Lipopeptide (eg: Daptomycin) disrupts gram positive cell walls
• Polymyxins bind to lipopolysaccharides and disrupts gram negative cell membranes

Question 15

Threat of antibiotic resistance

Answer 15

0~70% of bacteria developed resistance

-25,000 yearly death rate in Europe and the US

Question 16

Causes of antibiotic resistance

Answer 16

• Unnecessary prescription (~50% of antibiotic prescriptions are not required)
• Livestock farming (~30% of UK antibiotic use in livestock farming)
• Lack of regulation (over the counter availability in Russia, China and India)
• Lack of development (very few antibiotics in recent years)

Question 17

Production of destruction enzymes (resistance mechanism)

Answer 17

-Beta-lactamases hydrolyse C-N bond of the drug’s beta-lactam ring

Examples:

• Penicillins G and V=gram positive bacteria (beta-lactamase non-resistant)
• Flucloxacillin and Temocillin=beta-lactamase resistant
• Amoxicillin (broad spectrum antibiotic)=gram negative bacteria activity and often co-adminstered with Clavulanic acid

Question 18

Additional targets (resistance mechanism)

Answer 18

-bacteria produce another target that is unaffected by drug (different DHF reductase enzyme)

Example:
-E. Coli produces a different DHF reductase enzyme to make them resistant to trimethoprim

Question 19

Alteration in target enzymes (resistance mechanism)

Answer 19

• alteration/mutation to enzyme targeted by drug
• enzyme still effective but drug now ineffective

Example:
-Staphylococcus Aureus=mutations in ParC region of topoisomerase IV confers quinolone resistance

Question 20

Hyperproduction (resistance mechanism)

Answer 20

-bacteria significantly increase DHF reductase levels (overproduction)

Example:
-E. Coli=produces additional DHF reductase enzymes making trimethoprim less effective

Question 21

Alterations in drug permeation (resistance mechanism)

Answer 21

-reductions in aquaporins (decreased drug influx) and increased efflux systems

Example:
-important in gram negative bacteria

Question 22

Fungal infection classification

Answer 22

• Classified in terms of tissue/organs
  1) Superficial (outermost skin layers)
  2) Dermatophyte (skin, hair or nails)
  3) Subcutaneous (innermost skin layers)
  4) Systemic (primarily respiratory tract)

Question 23

Antifungals

Answer 23

-15 anti-fungal drugs licensed in UK

Two most common categories:

• Azoles
• Polyenes

Question 24

Azoles

Answer 24

-inhibits cytochrome P450-dependent enzymes involved in membrane sterol synthesis (ergosterol synthesis)

Example:
-Fluconazole (oral admin)=treats candidiasis and systemic infections

Question 25

Polyenes

Answer 25

-interacts with cell membrane sterols (ergosterols) forming membrane channels/pores

Example:
-Amphotericin (I.V. admin)=treats systemic infections