Module 6 Section 2 (Antibiotics)

Question 1

How can a bacterial infection be stopped?

Answer 1

A bacterial infection can be stopped by inhibiting the growth and reproduction of bacteria (bacteriostatic effects) or by directly killing the bacteria (bactericidal effects).

Question 2

What occurs as a result of inappropriate and over use of antibiotics?

Answer 2

Inappropriate and over use has resulted in a new problem, resistant strains of organisms.

Question 3

Discuss the structure of bacterial cells.

Answer 3

Bacterial cells have a rigid cell wall, which surrounds the cytoplasmic membrane.
The cell wall contains a peptidoglycan layer, which is a complex, cross-linked polymer of polysaccharides and polypeptides.
- These cross-links give the cell wall its structural rigidity, and are responsible for maintaining the cell’s shape and integrity, preventing cell lysis from high osmotic pressure.

Question 4

Bacteria are classified as either gram-positive or gram-negative. What does that entail?

Answer 4

Gram-positive bacteria have a thick peptidoglycan layer in their cell wall.
- These bacteria are able to retain the colour of the crystal violet dye in the Gram stain, hence the name.

Gram-negative bacteria have a much thinner peptidoglycan layer in their cell wall and an outer membrane.
- These bacteria do not retain the crystal violet dye in the Gram stain.

Question 5

Most antibiotics will target one of three key bacterial processes. What are those processes?

Answer 5

1) Cell wall and cell membrane synthesis.
2) Protein synthesis.
3) Nucleic acid metabolism.

Question 6

Antibiotics target the function of the bacterial cell wall and cell membrane in order to disrupt the structural integrity of the cell. How is this done?

Answer 6

• Breaking down the already formed cell wall

- Inhibiting the production of components necessary to build the cell wall

Question 7

What is penicillin?

Answer 7

Penicillin was the first antibiotic introduced into use. Since then, many other antibiotics have been derived from this original structure. This group of antibiotics are called the Penicillins.

Question 8

What are cephalosporins?

Answer 8

Cephalosporins are a group of antibiotics closely related to the Penicillin.

Question 9

What are the mechanisms of action for penicillin?

Answer 9

Penicillin is closely related to (D)-alanyl-(D)-alanine, a chemical component necessary for the formation of new bacterial cell walls.

Under normal conditions, (D)-alanyl-(D)-alanine binds to the enzyme, transpeptidase, to form cross-links between peptidoglycan chains.

• In the presence of Penicillin, transpeptidase will bind to Penicillin instead, inhibiting the formation of the cross-links and thus, the formation of the cell wall.
• The resulting cells are formed without cell walls - these cells are known as protoplasts and are fragile and can readily burst.

Question 10

Discuss the therapeutic uses for penicillin.

Answer 10

Penicillins are currently the drugs of choice for a large number of infectious diseases.

• Upper respiratory tract infections
• Urinary tract infections
• Pneumococcal infections and pneumonia
• Streptococcal pharyngitis (Strep throat)
• Syphilis

Question 11

Discuss the adverse effects for penicillin.

Answer 11

The most common adverse effect to penicillin is an allergic reaction.

• If an individual is allergic to one penicillin preparation, they will be allergic to all penicillin preparations.
• A variety of studies suggest that 1 to 10% of the population is allergic to penicillin.
• The most common manifestations of penicillin allergy include rash, diarrhea, fever, face and tongue swelling, and an eruption of itchy hives (urticaria).
• In rare cases, individuals may experience anaphylactic shock.

Resistance to penicillins is becoming a problem, with certain bacteria being able to produce penicillinase, which is an enzyme capable of inactivating penicillin.

Question 12

What are the mechanisms of action for cephalosporins?

Answer 12

Cephalosporins are closely related to the penicillins, and they have a similar mechanism of action.

Cephalosporins are selective inhibitors of transpeptidase, so they also cause the formation of cell walls without crosslinks.

Cephalosporins are relatively resistant to penicillinase.

Question 13

What are the therapeutic uses for cephalosporins?

Answer 13

Third generation cephalosporins are used for:

• serious infections caused by E. coli, among others.
• all forms of gonorrhea and for severe forms of lyme disease.
• treatment of meningitis. Not all antibiotics distribute to the CNS, but some third-generation cephalosporins do, making them useful for treating meningitis.

Question 14

What are the adverse effects for cephalosporins?

Answer 14

The most common adverse effects include fever and skin rashes.

Renal toxicity may occur in rare cases.

If a patient is allergic to penicillin, they are also allergic to the cephalosporins.

Question 15

What is the general mechanism of action of cell wall synthesis inhibitors? Why don’t these antibiotics target human cells?

Answer 15

Antibiotics that target the cell wall do so by antagonizing the enzyme transpeptidase.

• This enzyme is responsible for creating the cross-links between peptidoglycan layers in the cell wall.
• In the absence of these cross-links, the cell wall is unstable and the cell can easily burst.

Human cells do not have cell walls and are therefore unaffected by these antibiotics. Thus, penicillins and cephalosporins are selectively toxic to bacteri

Question 16

The second antibiotic target is protein synthesis. Antibiotics in this class prevent the functioning and growth of bacterial cells by inhibiting translation, and therefore, protein synthesis. How is this done?

Answer 16

Protein synthesis occurs in the ribosome (a complex molecule made of ribosomal DNA).

The bacterial ribosome consists of two subunits - a large subunit (50S) and a small subunit (30S).

Amino acids are brought to the ribosome by transfer RNA (tRNA) and added to the growing polypeptide chain via a peptide bond.

Protein synthesis inhibitors disrupt this process through a variety of mechanisms.

Question 17

What are the two protein synthesis inhibitors?

Answer 17

• Macrolides

- Antifolate drugs

Question 18

What are the mechanisms of action for macrolides?

Answer 18

The macrolides are a group of antibiotics that bind to the 50S subunit of the bacterial ribosome.

• Upon binding, macrolides bind to and antagonize the enzyme responsible for forming peptide bonds between adjacent amino acids in the polypeptide chain.
• Thus, amino acids can’t be added to the growing polypeptide chain and protein synthesis is inhibited.

This antibiotic is active against several bacterial infections caused by Gram-positive microorganisms.

Question 19

What are the therapeutic uses for macrolides?

Answer 19

Macrolides are the drug of choice for treating pertussis and first-line therapy for mycobacterial infections.

When an individual is allergic to penicillin, erythromycin is an effective alternative in treating streptococcal and staphylococcal infections.

Question 20

What are the adverse effects for macrolides?

Answer 20

Common adverse effects include anorexia, nausea, vomiting, and diarrhea.
- Diarrhea is the most common cause of discontinuing treatment with erythromycin.

It is important to note that some macrolides (e.g. erythromycin, clarithromycin) strongly inhibit CPY3A4 and cause significant drug interactions.

Question 21

What are the mechanisms of action for antifolate drugs? IMPORTANT

Answer 21

The second class of protein synthesis inhibitors are antifolate drugs, or folate inhibitors.

These drugs inhibit the formation of tetrahydrofolic acid (THF), which is essential for the bacteria to synthesize DNA and protein.

Pteridine + para-aminobenzoic acid (PABA) —dihydropteroate synthetase—> dihydropteroic acid ——> dihydrofolic acid —dihydrofolate reductase—> tetrahydrofolic acid

Question 22

What are the 2 antibiotics in the antifolate drug class?

Answer 22

1) Sulfamethoxazole: member of the sulfonamide group, competitively inhibits para-aminobenzoic acid (PABA) incorporation into dihydropteroic acid.
- Susceptible microorganisms must synthesize tetrahydrofolic acid from PABA, however, mammalian cells use preformed tetrahydrofolic acid from their surroundings.
- Thus, sulfonamides are selectively toxic to microorganisms.

2) Trimethoprim: it inhibits dihydrofolic acid reductase, thus inhibiting tetrahydrofolic acid formation.
- While humans do possess this enzyme, trimethoprim is selectively toxic to bacteria because it has greater inhibitory actions on the bacterial enzyme than on the human enzyme.
- By inhibiting sequential steps in the metabolic pathway of folate, a synergistic antibacterial effect is produced.
- As such, the drug cotrimoxazole was developed, which consists of sulfamethoxazole and trimethoprim.

Question 23

What are the therapeutic uses for antifolate drugs?

Answer 23

Cotrimoxazole is very useful in the treatment of recurrent bacterial infections of the urinary tract, and in the treatment of mild acute exacerbations of chronic bronchitis.
- It is also an important antimicrobial agent used in the treatment of Pneumocystis carinii infection in HIV-positive individuals.

Question 24

What are the adverse effects for antifolate drugs?

Answer 24

Cotrimoxazole causes up to three times as many dermatological reactions (e.g. urticaria) as sulfonamides alone.

Mild and transient jaundice has also been reported.

Individuals that are folate deficient may be more likely to experience adverse effects, as the margin between toxicity for bacteria and that for humans may be relatively narrow.

Question 25

The third and final antibiotic target is the one that targets DNA synthesis (nucleic acid inhibitors). The major class of antibiotics that have this function are the fluoroquinolone. What are the 2 important enymes that fluoroquinolones interfere with?

Answer 25

DNA gyrase: DNA gyrase is responsible for introducing negative supercoils into the DNA to combat excessive positive supercoiling that can occur during DNA replication.

Topoisomerase IV: Topoisomerase IV acts by separating interlinked daughter DNA molecules that are the product of DNA replication.

Question 26

What are the therapeutic uses of fluoroquinolones?

Answer 26

Fluoroquinolones can be used for oral or IV therapy of infections caused by a wide variety of gram-positive and gram-negative microorganisms.
- Its efficacy by the oral route is a particularly useful attribute, since it avoids the expense and inconvenience of giving a drug by injection.

Some common therapeutic uses are:

• urinary tract infections (more efficacious than trimethoprim-sulfamethoxazole)
• in the management of community-acquired pneumonia and for upper respiratory tract infections

Question 27

What are the adverse effects of fluoroquinolones?

Answer 27

Common adverse effects involve the GI tract, such as mild nausea, vomiting, and abdominal discomfort.

Mild headache and dizziness may also occur.

Arthralgias and joint pain are occasionally reported.

Question 28

Emergence of resistant strains is a long-standing problem with antibiotics, as micro-organisms mutate to have different properties, and therefore become resistant to antibiotics. What are the 2 major factors associated with the development of resistance to antibiotics?

Answer 28

1) Evolution: antibiotics act as negative selection pressures, preferentially selecting naturally resistant bacteria to survive and thrive.
2) Clinical and environmental factors: over-prescription and improper patient compliance, among others, are factors that are implicated in the growing problem of antibiotic resistance.

Question 29

Microorganisms mutate to have different properties, and therefore can become resistant to antibiotics by a variety of mechanisms. What are those mechanisms?

Answer 29

1) Resistance due to reduced entry of the antibacterial into the microorganism.
2) Resistance due to efflux pumps that pump the drug out of the microorganism before the cell can be injured.
3) Resistance due to destruction of the antibiotic. Microorganisms can develop an enzyme that inactivates the antibiotic.
4) Reduced affinity for the target structure A mutation in the target for the antibiotic will reduce the binding of the drug to its target and be ineffective.
5) Development of alternative pathways to those inhibited by the antibiotic.

Question 30

As a rule, antibiotics should be used as single entities whenever possible to decrease cost, toxicity, and development of resistance. What are some exceptions to this rule?

Answer 30

1) Therapy of a severe infection where the microorganism responsible is not known and theinfection is too dangerous to wait for microbiology testing.
2) Treatment of a mixed bacterial infection where no single antibiotic could eliminate all the different bacteria responsible for the infection.
3) Two antibiotics when given together cause a synergistic effect.
4) Treatment of tuberculosis.

Question 31

What is tuberculosis? What causes it?

Answer 31

Tuberculosis is a serious infectious disease that may cause human death.

Mycobacteria are the organisms that cause tuberculosis.

Question 32

Most antibiotics are effective against rapidly growing microorganisms, however, mycobacteria are resistant to most antibiotics, as they grow more slowly than other organisms. Why does this occur? What can be done to overcome this?

Answer 32

The mycobacterium cell wall is more than 60% lipid, which effectively forms a shield around the mycobacterium, making it impermeable to most antibiotics.

A second layer of defense comes from an abundance of efflux pumps in the cell membrane, which pump out antibiotics.

Combinations of two or more drugs are required to overcome these hurdles. In addition, therapy is long standing, lasting six to nine months.

Question 33

What are the 4 most commonly used drugs for the treatment of TB?

Answer 33

1) Isoniazid: inhibits mycolic acid formation, which is essential for cell wall formation.
2) Rifampin: binds to the bacterial RNA polymerase and inhibits protein synthesis. Potently induces some CYPs, decreasing half life for a number of compounds metabolized by those CYPs.
3) Pyrazinamide: activated by acidic conditions. Disrupts cell membrane synthesis.
4) Ethambutol: inhibits the incorporation of essential components into the cell wall, thus blocking cell wall synthesis.

Question 34

How are the 4 drugs that treat TB combined?

Answer 34

All 4 drugs have different mechanisms of action, and thus drug combinations should result in decreased development of resistance and decreased length of therapy.
- The current standard regimen in adults for drug-susceptible TB is isoniazid, rifampin, and pyrazinamide.

If there is resistance to isoniazid, initial therapy may also include ethambutol.

Antituberculosis drug therapy provides an important example of the consequences of between-patient variability: in patients who are adherent to therapy, subtherapeutic concentrations of isoniazid, rifampin, pyrazinamide, and ethambutol account for more than 90% of therapy failure.
- Such variability could be due to genetic variability, weight, height, age, and comorbid conditions such as liver disease.

Traditionally, the 4 drugs have been considered first-line anti-TB drugs, however, this is shifting.
- New treatment regimens (many of which do not include rifampin and isoniazid) are being tested with the aim of treating TB in less than 6 months.

Question 35

Which one of the organelles listed is the target for the bactericidal action of penicillin?

a) Cell membrane
b) Nuclei
c) Cell wall
d) Protein synthesis

Answer 35

c) Cell wall

Question 36

Cotrimoxazole is a combination of trimethoprim and what other drug?

a) Isoniazid
b) Penicillin
c) Tetracycline
d) Sulfamethoxazole

Answer 36

d) Sulfamethoxazole