Chapter 34: Attacking the enemy: Antimicrobial agents and chemotherapy

Question 1

What kind of antimicrobial is neomycin and why is it only used topically?

Answer 1

aminoglycoside; too toxic for parenteral use

Question 2

What kind of antimicrobial is streptomycin , what is its use restricted to and why?

Answer 2

aminoglycoside, restricted to treatment of tuberculosis due to issues with toxicity

Question 3

Augmentin is the most widely used comination drug. What is its mechanism of action?

Answer 3

2 parts amoxicillin, 1 part potassium clavulanate. Clavulanic acid, a product of Streptomyces clavuligerus, inhibits the most common beta-lactamases and allows amoxicillin to inhibit cells producing these enzymes.

Question 4

What is the basic mechanism of cycloserine?

Answer 4

cell wall synthesis inhibitor; Inhibits reactions involved in incorporation of alanine into cell wall precursor

Question 5

What is the basic mechanism of glycopeptides?

Answer 5

cell wall synthesis inhibitor; bind to terminal D-ala-D-ala residues; prevent incorporation of subunit into growing peptidoglycan

Question 6

What is the basic mechanism of bacitracin?

Answer 6

cell wall synthesis inhibitor; prevents dephosphorylation of phospholipid carrier, which prevents regeneration of carrier necessary for synthesis to continue

Question 7

What is the basic mechanism of beta-lactams?

Answer 7

cell wall synthesis inhibitor; bind to and inhibit penicillin binding protein enzymes

Question 8

What kind of structural change produces vancomycin resistance?

Answer 8

changing terminal Ala to lactate so vancomycin cannot bind anymore

Question 9

What is the basic mechanism of chloramphenicol?

Answer 9

bacteriostatic; binds to 50S ribosomal subunit, blocks peptidyl transferase thereby preventing peptide bond synthesis

Question 10

What do macrolides, lincosamides, and streptogramins have in common and what is their mechanism of action?

Answer 10

share overlapping binding sites on ribosomes.
Macrolides (bacteriostatic), large structures, 14-, 15- or 16-membered rings.
Macrolides bind to the 23 S ribosomal RNA (rRNA) in the 50 S subunit of the ribosome and block the translocationstep in protein synthesis, thereby preventing the release of transfer RNA after peptide bond formation

Question 11

What is the clinical use, route of administration, and mechanism of the macrocyclic drug fidaxomycin?

Answer 11

Fidaxomicin is orally administered bactericidalcompound targeting Clostridium difficile without major disturbance of the intestinal microbiota. The drug acts by interfering at the earliest stage of protein synthesis (mRNA transcription) inhibiting bacterial RNA polymerase

Question 12

What is the most important and clinically used drug in the lincosamides class, and what is its mechanism?

Answer 12

Lincosamides (bacteriostatic)

Clindamycin most important and clinically used drug in this class. Binds 50S ribosome and blocks the release of transfer RNA after peptide bond formation. (inhibits protein synthesis)

Question 13

Describe the spectrum of activity of erythromycin

Answer 13

Clindamycin has a spectrum of activity similar to erythromycin. Clindamycin is much more active than macrolides against anaerobes, both Gram-positive and Gram-negative. However,C. difficileis often resistant. Clindamycin is not active against aerobic Gram-negative bacteria because of poor penetration of the outer membrane.

Question 14

What is the mechanism of action of streptogramins?

Answer 14

Streptogramins (bacteriostatic/bactericidal)

Streptogramin A and B: Bacteriostatic alone and bactericidal together. Binds 23S rRNA in 50S ribosome and inhibits protein synthesis.

Question 15

What do macrolides, lincosamides, and streptogramins have in common?

Answer 15

Macrolides, lincosamides and streptogramins

These three groups of antibacterial agents share overlapping binding sites on ribosomes, and resistance to macrolides confers resistance to the other two groups.

Question 16

What type of bacteria is the combination of streptogramins A and B active against?

Answer 16

Gram-positive cocci, including multidrug-resistant isolates

Question 17

What type of bacteria are oxazolidinones active against, and what is their mechanism of action?

Answer 17

Active against a wide range of Gram-positive bacteria, including multi-resistant strains.
Inhibits initiation of protein synthesis by targeting 23 S ribosomal RNA in the 50 S subunit prevents formation of a functional 70 S complex.

Question 18

Describe the structure, mechanism and bacterial target of fusidic acid.

Answer 18

Fusidic acid is a steroid-like compound that acts by inhibiting protein synthesis by preventing the turnover of elongation factor G (EF-G) from the ribosome.

Fusidic Acidis a bacteriostatic antibiotic and used as a topical medication to treat skin infections.

Fusidic acidis effective primarily on gram-positive bacteria.

Question 19

List the stages at which nucleic acid synthesis is inhibited, and listed examples of these categories.

Answer 19

• inhibitors of DNA replication (DNA gyrase and topoisomerase): quinolones
• inhibitors of RNA polymerase: rifampicin
• antimetabolites inhibiting precursor synthesis: sulphonamides, trimethoprim

Question 20

What structural feature gives sulphonamides their inhibitory ability, and which sulphonamide is important in treating Mycobacterium leprae?

Answer 20

The sulphonamide ring structure is very similar to the structure of para-aminobenzoic acid (PABA), the substrate of dihydropteroate synthetase enzyme, the first step in THF synthesis. Remember THFA is required for the synthesis of purines and pyrimidines and therefore nucleic acid synthesis.
Dapsone is important in treatment of Mycobacterium leprae

Question 21

What structural feature gives trimethoprim its inhibitory ability?

Answer 21

Trimethoprim resembles the aminohydroxypyrimidine moiety of folic acid and in this way antagonizes the enzyme dihydrofolate reductase. DHFR normally catalyzes the conversion of dihydrofolic acid to tetrahydrofolic acid. Remember THFA is required for the synthesis of purines and pyrimidines and therefore nucleic acid synthesis.

Question 22

How does the structure of daptomycin affect its function?

Answer 22

Chemical structure of the cyclic lipopeptide, daptomycin, consists of a 13-member amino acid cyclic lipopeptide with a lipophilic tail, which attacks the bacterial cell membrane, causing depolarization and a potassium iron efflux.

Question 23

How can the antibiotic susceptibility of an organism be tested?

Answer 23

The antibiotic susceptibility of an organism can be tested by the application of filter paper impregnated with antibiotic onto a lawn of the organisms seeded on an agar plate. After overnight incubation the organism grows and the antibiotics diffuse to produce a zone of inhibition that indicates the degree of susceptibility. Diameter of zone of inhibition is measured. Each antibiotic has a threshold diameter for the zone of inhibition to render the bacteria sensitive to it

Question 24

How can minimum inhibitory concentration be measured?

Answer 24

24 hour incubation of growth medium + antibiotic in decreasing concentrations of antibiotic in test tubes, check turbidity. Measures BACTERIOSTATIC concentration

Question 25

How can minimum bactericidal concetration be measured?

Answer 25

48 hour culture. Subculture from MIC test tubes and incubate another 24 hours or plate growth medium + decreasing concentrations of antibiotic onto drug-free agar and incubate for 48 hours.

Question 26

How can synergy of two antibacterials be observed on an cultured agar plate? Give an example of two antimicrobials that would exhibit this.

Answer 26

Synergy can be recognized by the fact that the zones of inhibition become continuous between the two disks.Ex: sulphonamide and trimethoprim

Question 27

How can antagonism of two antibacterials be observed on an cultured agar plate? Give an example of two antimicrobials that would exhibit this.

Answer 27

Nitrofurantoin is capable of antagonizing the activity of nalidixic acid. When the disks are placed far apart, nalidixic acid (quinolone) inhibits the test organism, but when placed close together this inhibition is antagonized by the presence of nitrofurantoin (inhibitor of cytoplasmic membrane function), shown by the foreshortening of the zone of inhibition

Question 28

List and describe the combination of antimicrobials that neosporin contains.

Answer 28

Neomycin—aminoglycoside—protein synthesis inhibitor (gram-, some g+)

Polymyxin B—bind to LPS and phospholipids: cell membrane inhibitor (gram-)

Bacitracin—cell wall inhibitor (gram+)

Question 29

Why do Mycobacterium tuberculosis and other mycobacterial infection need prolonged treatment?

Answer 29

have a waxy outer layer that makes them difficult to penetrate with antibiotics
• have an intracellular location, that makes it difficult for antibiotics to get to them
• grow and multiply extremely slowly
• are common and increasing in the wake of the AIDS epidemic in resource-poor countries, where the cost of drug treatment can be prohibitive.

Question 30

List three anti-tuberculosis agents that were discussed, and their modes of action.

Answer 30

Isoniazid: Its bactericidal activity results from inhibition of mycolic acid synthesis

Ethambutol: acts by inhibiting the polymerization of arabinoglycan, a critical constituent of the mycobacteria cell wall.

Pyrazinamide: appears to target mycolic acid synthesis.

Question 31

Describe a typical anti-tuerculosis treatment regimen

Answer 31

Where susceptibility is uncertain, an initial course of isoniazid, ethambutol, rifampicin, and pyrazinamide is typically followed for 2 months followed by two drug (e.g. isoniazid and rifampicin) continuation for an additional 18 weeks. If the strain is susceptible to both isoniazid and rifampicin (RNA polymerase inhibitor, blocks mRNA synthesis), ethambutol is then discontinued.

Question 32

Which drugs are used in the treatment of Mycobacterium leprae, and how long does the treatment last?

Answer 32

-Dapsone: a sulphonamide blocking folic acid synthesis
-clofazimine: affects DNA replication, and immune response
-rifampicin: inhibitor of RNA polymerase block mRNA synthesis
treatment lasts 6 months to 1 year

Question 33

What are the desired properties of a new antimicrobial agent?

Answer 33

•Selectivity for microbial rather than mammalian targets
•Cidal activity (antibacterial and antifungal agents)
•Slow emergence of resistance
•Narrow spectrum of activity*
*Narrow-spectrum drugs cause less disturbance to the microbiota and may contribute less to emergence of antibiotic resistance, whereas broad-spectrum compounds are more useful for empiric therapy and treatment of polymicrobial infections.

Question 34

What are the desired pharmacological activities of a new antimicrobial agent?

Answer 34

• Non-toxic to the host
• Long plasma half-life (once-a-day dosing)
• Good tissue distribution including CSF (cerebrospinal fluid)
• Low plasma–protein binding
• Oral and parenteral dosing forms
• No interference with other drugs

Question 35

What are the steps for rational design an an antimicrobial agent?

Answer 35

• Select an appropriate target.
• Identify a chemical lead (i.e. a new molecule with inhibitory activity on the target).
• Modify the lead compound to enhance potency.
• Evaluate in vitro activity.
• Evaluate in vivo activity and toxicity.
• Test in clinical trials and develop.

Question 36

What are the three ways of classifying an antibacterial agent?

Answer 36

1. according to whether they are bactericidal or bacteriostatic
2. by target site
3. by chemical structure

Question 37

What are the five main target sites for antibacterial action?

Answer 37

• cell wall synthesis
• protein synthesis
• nucleic acid synthesis
• metabolic pathways
• cell membrane function

Question 38

What are the main mechanisms of resistance that antibacterials develop?

Answer 38

1. altered target
2. altered uptake
3. drug inactivation