Lecture 13: Antimicrobials

Question 1

1) Name 3 types of antimicrobial compounds
2) Define antimicrobial resistance and briefly explain its history
3) Why is antibiotic and antimicrobial resistance a concern?

Answer 1

1) Antiseptics, disinfectants, and antibiotics
2) Adaptation to chemical defenses; was taken for granted (we stopped researching antibiotics until resistance became a huge problem)
3) Bacterial resistance to antibiotics and disinfectants could undermine major health advances (e.g. elective surgeries)

Question 2

1) What is the safety net of modern medicine?
2) Is resistance to disinfectants and antiseptics well understood?

Answer 2

1) Antimicrobial compounds
2) No; it’s poorly understood (but does occur)

Question 3

1) Define bactericidal
2) Define bacteriostatic
3) When is the difference between bactericidal and bacteriostatic clinically important?

Answer 3

1) Bactericidal: antimicrobial compounds that kill bacteria
2) Bacteriostatic: antimicrobial compounds that stop or slow the growth of bacteria
3) Bacteriostatic should not be used for immunocompromised individuals since they don’t have a good enough immune system to ‘finish the job’

Question 4

Define disinfectants, antiseptics, and antibiotics in terms of how they’re administered

Answer 4

Disinfectants: compounds applied to inanimate objects
Antiseptics: compounds applied to skin
Antibiotics: compounds that can be injected or ingested

Question 5

1) Define antiseptics and disinfectants; are most bactericidal or bacteriostatic?
2) Are antiseptics and disinfectants broad or narrow? Can they be used internally?
3) Do they target one aspect of a microbe or multiple?

Answer 5

1) Chemicals that kill or inhibit the growth of bacteria and other microorganisms; most are bactericidal
2) Very broad in coverage; generally too toxic for internal use in humans
3) Tend to attack multiple targets in microbes

Question 6

Name the 5 categories of antiseptics and disinfectants (from previous chapter) and list what part of the bacteria they target

Answer 6

1) Halides: chlorine (household bleach) and iodine are strong oxidants that inactivate many bacterial proteins
2) Hydrogen peroxides: inactivation of proteins
3) Quaternary ammonium compounds: disruption of cell membranes
4) Alcohols (ethanol, isopropanol): denature proteins
5) Phenols: denature proteins, disrupt cell membranes

Question 7

List the 3 types of antibiotics that are cell wall synthesis inhibitors and give examples of each

Answer 7

1) B-lactam: Penicillins, cephalosporins, carbapenems, and monobactams
2) Glycopeptides: Vancomycin
3) Polypeptides: Bacitracin and polymyxins

Question 8

1) What is the most widely used class of all antibiotics?
2) What are the 4 types it includes?
3) What is the main toxicity problem of this class? What is used as an alternative?

Answer 8

1) B-lactam
2) Penicillins, cephalosporins, carbapenems, and monobactams
3) An allergic reaction to penicillins and cephalosporins; monobactams are used as an alternative

Question 9

1) What do B-lactam antibiotics inhibit?
2) How do they do this?
3) What is the net result and what does this lead to?

Answer 9

1) β-lactam antibiotics inhibit the last steps in peptidoglycan synthesis
2) β-lactam antibiotics bind to and inhibit the carboxypeptidase and transpeptidase; these proteins are referred to as penicillin-binding proteins (PBPs)
3) Net result: Autolysins are activated and degrade cell wall; leads to osmotic lysis

Question 10

1) What do autolysins normally do?
2) Would B-lactam antibiotics be effective without autolysins?

Answer 10

1) Autolysins normally function in the turnover of PG in the bacterial cell and are normally kept in check by the bacterium
2) Without the autolysins, they would not be effective

Question 11

1) Are B-lactam antibiotics bactericidal or bacteriostatic?
2) When do B-lactam antibiotics work? When should they not be given?

Answer 11

1) Bactericidal
2) Only work on rapidly growing bacteria actively making new cell wall, therefore β-lactam antibiotics SHOULD NOT be given with antibiotics which slow down/inhibit protein synthesis (e. g. tetracycline and aminoglycosides)

Question 12

1) Name 2 resistance mechanisms in B-lactam antibiotics
2) Describe and give an example of how it has been combated

Answer 12

1) Mutation of PBPs
2) Production of β-lactamases (enzymes that hydrolyze the β-lactam ring in β-lactam antibiotics)
3) Selected β-lactam antibiotics have been combined with β-lactamase inhibitors
-Example: Combination of clavulanic acid with amoxicillin (brand name is Augmentin)

Question 13

Why might antibiotics be biochemically modified? (2 reasons)
Give an example

Answer 13

To combat drug resistance and improve pharmacokinetic properties antibiotics (Example: Cephalosporins)

Question 14

The biochemical modification of antibiotics led to the creation of 4 antibiotic classes; what are they?

Answer 14

1) Narrow-spectrum (first-generation)
2) Expanded-spectrum (second-generation)
3) Broad-spectrum (third-generation)
4) Extended-spectrum (fourth-generation)

Question 15

1) What are glycopeptide antibiotics able to do in gram-positive bacteria?
2) What are they unable to do in gram-negative bacteria? Give an example
3) What do glycopeptide antibiotics bind to and when?
4) Why are they medically important? Give an example

Answer 15

1) Inhibit peptidoglycan synthesis in growing gram-positive bacteria
2) Unable to penetrate the outer membrane of gram-negative bacteria (ex: vancomycin)
3) Glycopeptide antibiotics bind to the D-Ala-D-Ala portion of the pentapeptide during peptidoglycan synthesis (of the cell wall)
4) They’re the last drugs effective against some gram-positive pathogens (e.g. S. aureus and Enterococcus species that are resistance to β-lactams)

Question 16

1) Bacitracin is a mixture of what? What are these isolated from?
2) What is bacitracin used in?
3) What bacteria are resistant to it; why?
4) What is its mode of action?

Answer 16

1) Mixture of polypeptides isolated from Bacillus licheniformis
2) In topically applied products
3) Gram-negative organisms are resistant; cannot pass through outer membrane
4) Inhibits cell wall synthesis by interfering with recycling of bactoprenol (undecaprenol) (the lipid carrier required for peptidoglycan synthesis)

Question 17

What is bactoprenol? (undecaprenol) What interferes with it?

Answer 17

The lipid carrier required for peptidoglycan synthesis; bacitracin interferes with it

Question 18

1) What are polymyxins derived from?
2) How are they similar to detergents?
3) What do they interact with, and in what bacteria?

Answer 18

1) Derived from Bacillus polymyxa
2) Insert into bacterial membranes like detergents
3) Interact with LPS in the outer membranes of Gram-negative bacteria

Question 19

1) What is the end result of polymyxins?
2) What bacteria are resistant and why?
3) What are they generally used in?

Answer 19

1) Eventually disrupt the phospholipid bilayer leading to osmotic lysis
2) Gram-positives do not have an outer membrane, they are resistant
3) Topically applied products

Question 20

What is neosporin made of and why?

Answer 20

Bacitracins and polymyxins to target both gram negative and gram positives

Question 21

Describe cationic antimicrobial peptide resistance

Answer 21

Cationic antimicrobial peptides are present everywhere; attracted to the negatively charged surfaces of gram-negative cells

Question 22

1) What does vancomycin bind to?
2) What do B-lactams prevent?
3) What does bacitracin prevent?

Answer 22

1) Vancomycin: binds to D-ala-D-ala
2) B-lactams: prevent transpeptidation
3) Bacitracin: prevents recycling

Question 23

What 3 categories of antibiotics inhibit protein synthesis? Give examples

Answer 23

1) Aminoglycosides: Tobramycin and gentamicin
2) Tetracyclines
3) Macrolides: erythromycin, azithromycin and clarithromycin

Question 24

1) What do aminoglycosides target?
2) What subunit do they bind to?
3) Are they bactericidal or bacteriostatic? What is essential?
4) Give 2 examples of common aminoglycosides

Answer 24

1) Target the bacterial ribosome
2) The 30S subunit of the bacterial ribosome
3) Aminoglycosides are bactericidal: protein synthesis is essential
4) Gentamicin and tobramycin

Question 25

1) What type of bacteria are aminoglycosides more effective against? Why?
2) How could you overcome this problem with the bacteria it’s less effective against?
3) What are its side effects?

Answer 25

1) More effective against Gram-negative than Gram-positive bacteria cell wall prohibits uptake
2) Use a certain cell wall inhibitor
3) Prolonged use can lead to hearing loss and/or impairment of kidney function

Question 26

1) What do tetracyclines bind to?
2) Are they bactericidal or bacteriostatic?
3) Why are they not used for young children?
4) What has it been overprescribed for before? What has this led to?

Answer 26

1) The 30S subunit of the bacterial ribosome
2) Bacteriostatic
3) Tetracyclines are absorbed into bone and stain teeth of young children
4) Still very low toxicity, but has been overprescribed in the past for:
-Acne
-Feed additive to promote growth of livestock
-Now resistance is widespread

Question 27

1) What do macrolides bind to?
2) Are they bactericidal or bacteriostatic? Do they work on all bacteria?
3) Give an example of a model macrolide
4) Give 2 examples of newer macrolides
5) Which macrolide is also called Z-pack?

Answer 27

1) Inhibit protein synthesis by binding the 50S subunit at a site that includes a looped segment of 23S rRNA
2) Bacteriostatic for MOST bacteria
3) Model macrolide: Erythromycin
4) Newer macrolides: azithromycin and clarithromycin
5) Z-pack or Zithromax (azithromycin)

Question 28

Which nucleic acid synthesis inhibitor group is one of the most widely used classes of antibiotics?

Answer 28

Quinolones

Question 29

1) What do quinolones do? Are they bactericidal or static?
2) What was the first quinolone?
3) What new class of quinolones was created and how? Give 2 examples.
4) What bacteria are they active against?

Answer 29

1) Inhibit bacterial DNA replication and are bactericidal
2) The first quinolone was nalidixic acid
3) Modification with a fluorine created the fluoroquinolones: ciprofloxacin, levofloxacin, etc.
4) Both Gram-positive and Gram-negative

Question 30

Quinolones are poorly active against two categories of bacteria; what are they, and is this a good thing or a bad thing? Why?

Answer 30

1) Streptococci and anaerobes
2) A good thing; many anaerobes and streptococci live in our gut microbiomes

Question 31

What are the two classes of antibiotics that inhibit nucleic acid synthesis? Give examples

Answer 31

1) Quinolones: ciprofloxacin, levofloxacin, nalidixic acid
2) Rifampin and Rifabutin

Question 32

1) What do rifampin and rifabutin do?
2) What bacteria are resistant to them and why?
3) What makes rifampin and rifabutin similar?
4) What are they important in the treatment of?

Answer 32

1) Inhibit activity of bacterial RNA polymerase
2) Gram-negatives are intrinsically resistant to rifampin; decreased uptake of hydrophobic antibiotics
3) Both are derivatives of rifamycin B produced by Streptomyces mediterranei
4) Important in treatment of isoniazid resistant Mycobacterium tuberculosis

Question 33

1) What was the first category of antimicrobial drugs? 2) Which specific drug was first?

Answer 33

1) Sulfonamides (sulfa drugs)
2) Prontosil

Question 34

What are the two categories of antimetabolites? Give examples

Answer 34

1) Sulfonamides: prontosil
2) Trimethoprim

Question 35

1) What is one negative thing about sulfonamides?
2) What do they inhibit in bacteria? What is this process needed for?
3) How is this process different from mammals?
4) What are sulfonamides effective against?

Answer 35

1) Sulfa allergies are very common
2) Inhibitor of enzymes in the bacterial pathway for production of tetrahydrofolic acid. Tetrahydrofolic acid is needed for synthesis of nucleic acids and formyl-methionine.
3) Mammals require preformed folic acid; we do not make tetrahydrofolate
4) A broad range of Gram-positive and Gram-negative organisms

Question 36

What was the initial problem with prontosil?

Answer 36

Couldn’t make it active in-vitro; found that the drug is metabolized into two pieces in the body to yield the active compound (bioactivation/ a prodrug)

Question 37

1) What is trimethoprim?
2) What does it inhibit?
3) What is it structurally similar to, and what enzyme does it inhibit?

Answer 37

1. Also an antimetabolite drug
2. Inhibits same pathway as sulfonamides
3. Structurally similar to dihydrofolic acid; acts as a competitive inhibitor of dihydrofolate reductase.

Question 38

Give 5 reasons why combining antibiotics may be a good idea

Answer 38

1) Can improve the outcome of treatment
2) Broadens antibacterial spectrum for empirical therapy; emergency situations
3) Broadens antibacterial spectrum if dealing with polymicrobial infection
4) Reduces the overall chance of the emergence of resistant organisms
5) Synergistic killing

Question 39

Define antibiotic synergism and antibiotic antagonism

Answer 39

1) Antibiotic synergism: combination of two antibiotics that have enhanced the overall bactericidal activity compared to each compound alone
2) Antibiotic antagonism: combination of antibiotics in which the activity of one antibiotic interferes with the activity of another

Question 40

Define vaccines

Answer 40

Nontoxic antigens that are injected, ingested, or inhaled to induce a specific defense response without having to go through the disease process

Question 41

Give 5 reasons why there are not more vaccines

Answer 41

1) Financial: lack of interest from big pharma, lack of funding by governments
2) Legal: fear of lawsuits
3) Political: some governments simply do not care
4) Public is unaware of the benefits of vaccination; out of sight, out of mind
5) We listen to Hollywood, rather than the experts

Question 42

Define herd effect

Answer 42

Protection of unvaccinated people in a population where most people are vaccinated due to lessened risk of disease transmission. Herd immunity will be lost if people refuse vaccination, and an outbreak will occur.

Question 43

What is the 4th leading cause of death behind heart disease, strokes, and cancer?

Answer 43

Infectious disease

Question 44

Give 2 examples of nosocomial or ‘hospital acquired’ infections

Answer 44

1) Methicillin resistant S. aureus
2) Vancomycin resistant Gram-negatives

Question 45

Why are the estimated costs of treating bacterial infections rising?

Answer 45

Due to antibiotic resistance ($2.2 billion spent a year)

Question 46

What are the 4 resistance mechanisms bacteria have against antibiotics?

Answer 46

1) Restrict access of the antibiotic target
2) Inactivation of antibiotic either by hydrolysis or modification
3) Modification of antibiotic target
4) Failure to activate the antibiotic (not very common)

Question 47

What is the easiest way for a bacteria to resist antibiotics?

Answer 47

Keeping the antibiotic outside the cell in the first place (resitricting access of antibiotics)

Question 48

What are the three ways bacteria can restrict access of antibiotics (and keep them outside the cell)?

Answer 48

1) Changes in outer membrane porins (Gram-negatives)
2) Reduce uptake across cytoplasmic membrane (e.g. change receptor)
3) Active efflux of antibiotic: prevents antibiotics from reaching a high enough concentration in the cytoplasm to exert their effect

Question 49

What are the two ways bacteria can enzymatically inactivate an antibiotic?

Answer 49

1) B-lactamases
2) Aminoglycoside-modifying enzymes

Question 50

1) Define B-lactamases
2) What do B-lactamases represent?
3) What is one strategy for countering B-lactamases?

Answer 50

1) B-lactamases: enzymes that cleaves the B-lactam ring
2) β-lactamases represent a major and prevalent resistance mechanism of enzymatic inactivation
3) One strategy for countering β-lactamases is to mix the β-lactam antibiotic with a β-lactamase inhibitor, such as clavulanic acid.

Question 51

How do aminoglycoside-modifying enzymes perform enzymatic inactivation?

Answer 51

Modification enzymes inactivate aminoglycosides by addition of functional groups phosphoryl, adenyl, or acetyl groups.

Question 52

What are the two ways bacteria can modify the antibiotic target?

Answer 52

1) Resistance to B-lactams
2) Resistance to glycopeptides

Question 53

1) What is involved with resistance to B-lactams?
2) What is resistance to B-lactams a major cause of?
3) Why is MRSA unique?

Answer 53

1) Alteration of the binding specificity of the penicillin binding proteins (PBPs); modification of the target
2) Major cause of methicillin resistant Staphylococcus aureus (MRSA)
3) MRSA has a unique transpeptidase that does not bind B-lactam antibiotic

Question 54

1) What is prevented in glycopeptides? What is the goal?
2) What is replaced in resistance from glycopeptides?

Answer 54

1) Vancomycin prevents cross-linking of peptidoglycan by binding to D-Ala-D-Ala; goal is modification of antibiotic binding site
2) Bacteria replace the D-Ala-D-Ala with another dipeptide that does not bind vancomycin: D-Ala-D-lactate and D-Ala-D-serine

Question 55

1) What type of bacterial resistance reduces binding by 1000 fold?
2) What type requires 3 enzymes?

Answer 55

1) Glycopeptide resistance
2) Glycopeptide resistance

Question 56

How does failure to activate a drug cause drug resistance?

Answer 56

To gain resistance, the organism simply loses the ability to convert a prodrug to its active form

Question 57

1) Give an example of a drug some bacteria fail to activate.
2) What type of drug is this? When it works, what is it converted to and how?
3) What does the active form of the drug do? How?
4) When is this drug used? Why?
5) How do bacteria gain resistance to it?

Answer 57

1) Metronidazole (marketed by Pfizer as Flagyl)
2) Metronidazole is a prodrug; it is converted to its active form by the reduction of its nitro group by bacterial nitroreductase.
3) The active form inhibits nucleic acid synthesis by disrupting the helical structure of DNA
4) Used to treat infections of anaerobic bacteria and sensitive protozoan organisms because these organisms have the enzymatic machinery to reduce metronidazole to its active form intracellularly.
5) The organism simply loses the ability to convert the prodrug to its active form

Question 58

What are the 4 parts of the cell antibiotics can inhibit?

Answer 58

1) Cell wall structure (peptidoglycan and membrane)
2) Protein synthesis (30S and 50S)
3) Nucleic acid synthesis (DNA replication and RNA synthesis)
4) Antimetabolites (unique cytosolic precursor pathways)