6.1 Antimicrobial agents (intro + antibiotics)

Question 1

What are antimicrobial agents, give 3 with examples

Answer 1

1) Products to control microorganisms in commercial and
industrial applications

• chemicals in foods, air-conditioning cooling towers, textile and paper products, fuel tanks
  2) Products designed to prevent growth of human pathogens in inanimate environments and on external body surfaces
• sterilants, disinfectants, sanitizers, and antiseptics
  3) Antimicrobial drugs
• chemotherapy

Question 2

Define the following definitions..

a) Antimicrobial agent
b) Antibiotic
c) chemotherapy

Answer 2

a) a chemical that kills or inhibits the growth of organisms
b) a chemical produced by a microorganism that kills or inhibits the growth of another microorganism
b) any chemical-based treatment for diseases caused by bacteria, other microorganisms, parasites and tumour cells

Question 3

What is the relationship between antimicrobial agents and antibiotics

Answer 3

ALL antibiotics are antimicrobials but not all antimicrobials are antibiotics

Question 4

Name six characteristics of an ideal antimicrobial drug

(PSS MR P whats so good about your drug?)

Answer 4

1) reasonably Priced
2) Selectively toxic to the microbe but nontoxic to host cells
3) relatively Soluble; functions even when highly diluted in body fluids
4) Microbicidal rather than Microbistatic
5) doesn’t lead to the development of antimicrobial Resistance
6) remains Potent long enough to act and isn’t broken down or excreted prematurely

Question 5

Define selective toxicity

Define a selective poison

Answer 5

Selective toxicity: drugs that specifically target microbial processes and not the human host cellular processes

Selective poison: targets bacteria not host

Question 6

What are antibiotics?

Name two bacteria and two moulds that produce them

Answer 6

Antibiots are naturally occuring antimicrobials

These are metabolic products produced by bacteria and fungi that usually aid them to reduce competition for nutrients and space

Bacteria: Bacillus, Streptomyces

Molds: Penicillium, Cephalosporin

Question 7

Which part of penicillin’s chemical structure is the key to its function, describe this?

What mold produces penicillin

Answer 7

The Beta-lactam ring is key to penicillin’s function (targeted by B-lactam antibiotics)

Penicillin consists of a 4 membered nitrogen containing structure with the B-lactam ring at its core

Produced by the mold Penicillium chrysogenum

Question 8

What are two modes of action of an antibacterial drug and compare them?

Give two examples of each

Answer 8

1) Bacteriostatic: halts growth of bacteria giving the immune system a chance to clear the infection
* E.g tetracyclines, sulfonamides
2) Bactericidal: kills the bacteria (no more active bacteria left)

• E.g penicillin’s, cephalosporins
• Bacteriolytic drugs also lyse dead bacteria

Bacteriostatic drugs tend to be slower and require a working immune system for effective elimination of the microorganism. Therefore not advisable for use in immunosuppressed conditions or for those suffering from life-threatening acute conditions

** some antibiotics can be both bacterostatic and bactericidal

Question 9

How can you classify antibacterial agents?

What can be said about antibiotcs that share similar structures?

Answer 9

1) Their molecular structure
2) Their spectrum of activity (broad - killing a wide variety of organisms or narrow)
3) Function or mechanism of action (how it works)

*Antibiotics within a structural class usually show similar modes of action and spectrum of activity

Question 10

What is meant by the antimicrobial spectrum of activity?

What is a narrow spectrum antibiotic? + example

What is a broad-spectrum antibiotic? + example

When prescribing when would you use each?

Answer 10

antimicrobial spectrum of activity: is the scope that a drug kills or suppresses the growth of microorganisms.

Narrow spectrum: drugs that only act on one kind or one strain of bacteria

• Isoniazid (TB)

Broad-spectrum: have a wide antimicrobial scope.

• Tetracyclin

*begin with a broad-spectrum when unsure of what the infection is and introduce narrow-spectrum later

Question 11

What is the spectrum of activity for the following bacteria and what is their main target

• Tetracycline
• Cephalosporins
• Streptomycin
• Penicillans
• Isoniazid

Answer 11

Tetracycline: broad spectrum (all except mycobacteria-TB)

Cephalosporins: broad spectrum (1/2 gram + and 1/2 gram -)

Streptomycin: broad spectrum (mycobacteria and gram -)

Penicillans: narrow spectrum (mainly targets gram +)

Isoniazid: narrow spectrum (only targets mycobacteria)

Question 12

Name the five main targets for antibacterials

Answer 12

1) Inhibition of cell wall synthesis
2) Inhibition of protein synthesis
3) Inhibition of nucleic acid synthesis
4) disruption of cell membrane function
5) Inhibition of metabolism (Antimetabolites)

Question 13

Explain the mode of action of drugs that inhibit cell wall synthesis?

Give 2 chacteristics of this drug that makes it a good antibiotic

Answer 13

1) they target peptidoglycans in cell wall causing it to become weakened while the cell is growing.
2) the weakening of the cell wall increases permeability
3) the difference in osmotic gradients causes water to enter the cell
4) results in lysis of the cell (bactericidal)

Chacteristics: bactericidal and low toxisity to humans (because we do not have a cell wall)

Question 14

Give 4 drugs that target cell wall synthesis and specifically explain how

Answer 14

1) Penicillin – binds and blocks peptidases involved in cross-linking the glycan molecules
2) Cephalosporins – same as penicillin
3) Vancomycin – hinders peptidoglycan elongation
4) Cycloserine – inhibits the formation of the basic peptidoglycan subunits

Question 15

Cell walls of Gram + and + bacteria contain peptidoglycan layers (thick= + or thin = -), describe the structure of the peptidoglycan layer

Which 2 antibiotics target these structures?

Answer 15

Peptidoglycan contains sugar (glycan) chains, which are cross-linked by short peptide bridges.

The glycan chains consist of alternating units of NAG and NAM joined by glycosidic linkages

• NAG: N-acetylglucosamine (GlcNAc)
• NAM: N-acetylmuramic acid (MurNAc)

Penicillan and Cephalosporin bind and block theses peptides involved in cross-linking the glycan molecules

Question 16

How do B-lactam antibiotics work (eg penicillin)?

Answer 16

They target the PBPs (penicillin binding proteins) which are a group of enzymes that are found anchored to the cell memebrane involved in the cross-linking of the cell wall

The B-lactam ring portion of penicillin binds to these PBPs rendering them unable to perform their role in cell wall synthesis

This leads to death of the cell by osmotic instability or autolysis

Question 17

How may some bacteria aquire resistance to penicillin?

Answer 17

Some bacteria may evolve to produce a beta-lactamase gene which encodes beta-lactamase/penicillinase

These are enzymes that destroy the beta-lactam ring of the antibiotic, making the penicillin ineffective

Question 18

Compare Natural penicillins to Semisynthetic penicillins

Give 2 types of semisynthetic penicillins that can be used to overcome resistance + examples

Answer 18

Both share a common core structure (beta-lactam ring) but the chemical modification on the side chain of the ring changes.

Natural penicillins: have a narrow range of activity
Semisynthetic penicillins: overcome disadvantages

1) Penicilinase-resistant penicillins
* Eg. Methicillin (there is reduced use due to MRSA)
2) Penicillins + β-lactamase inhibitors

• Eg. co-amoxiclav (Augmentin)
• combines amoxicillin with clavulanic acid which inhibits penicillinase

Question 19

What mold produces Cephalosporin?

Compare the molecular structures and spectrum range of penicillin and cephalosporin

How are semi-synthetic verisons of cephalosporin grouped and what can be said about this?

Answer 19

Produced by the mold Acremonium Cephalosporium

Both have a beta-lactam ring, but penicillin is attached to a 5C ring whereas cephalosporin has a 6C ring

Cephalosporin is a broader spectrum drug

Semi-synthetic versions are grouped as generations (new generations are less susceptible to β -lactamases)

Question 20

What spectrum of activity do drugs inhibiting protein synthesis have?

What is their mechanism of action and which mode of action do they tend to be?

What common problem can occur with these types of drugs and why?

Answer 20

These are broad-spectrum

1) they target prokaryotic ribosomes (not eukaryotic) hence display selective toxicity.
2) many bind to either the 30S or 50S subunits of the intracellular ribosomes causing disruption of the bacteria’s normal metabolism
3) leads to death of the organism or inhibition of its growth and multiplication.

** They tend to be bacteriostatic

Common problem is toxicity problems

Question 21

Give 4 antibiotics that target protein synthesis and explain specifically how

Answer 21

1) Aminoglycosides: Streptomycin, neomycin, gentamycin

binds to the 30s ribosomal subunit chaning its shape causing misreading of mRNA

2) Tetracyclines

binds to the 30s ribosomal subunit, preventing attachment of the tRNA to the RNA ribosome complex, preventing additon of amino acids to the elongating peptide chain

3) Macrolides: Erythromycin

binds to the 50s ribosomal subunit preventing movement of ribosome along the mRNA, inhibiting translocation

4) Chloramphenicol:

binds to the 50s ribosomal subunit and inhibits the formation of peptide bonds

Question 22

What was streptomycin formerly used to treat?

Answer 22

The plague caused by the Yersinia pestis bacteria

Question 23

Give the two major drug targets for inhibitors of nucleic acid synthesis and state how they do this

State whether these are broad or narrow spectrum and give an example of each including what treatment they are used for

Answer 23

1) β-subunit of bacterial RNA polymerase

• Inhibits RNA synthesis
• Narrow spectrum: (mycobacteria-anti TB and gram +)
• eg. Rifamycins
• Used in treatment of TB

2) DNA gyrase/topoisomerase

• Interferes with separation/reannealing of dsDNA
• Broad spectrum -> bactericidal
• eg. Quinolones and fluoroquinolones
• Used in urinary and respiratory tract infections

Question 24

What are Rifamycins used to treat?

Are they broad of narrow spectrum

What is their mechanism of action?

Answer 24

Rifamycins are used in the treatment of TB

They are narrow spectrum: (mycobacteria-anti TB and gram +)

They inhibit nucleic acid synthesis by targeting the B-subunit of RNA polymerase which inhibits RNA synthesis

Question 25

What are Quinolones and fluoroquinolone used to treat?

Are they broad of narrow spectrum

What is their mechanism of action?

Answer 25

Used in urinary and respiratory tract infections

They are broad spectrum antibiotics are are bactericidal

They inhibit nucleic acid synthesis by interfering with separation/reannealing of dsDNA

Question 26

What is the only drug that disrupts the cell membrane function and what is its sepctrum of activity and why?

What is its mode of action?

Explain its mechanism of action

What are 2 dangers of this drug and therfore what can be said about its administration

Answer 26

Polymyxin B: targets gram negatives as this has a thin PGN layer

Tends to be Bactericidal

1) It mimics the overall structure of the phospholipid bilayer as it has a polar head group and hydrophobic tail
2) this allows it to enter disrupt the bacterial PM
3) results in the leakage of solutes that are essential for the cell’s survival which results in cell death

Dangers:

• can be neurotoxic
• lack of selective toxicity: nonspecific for cell membranes of any type hence can target eukaryote membranes

often only used topically or in combination with bacitracin and/or neomycin which are broad spectrum

Question 27

What are the two major targets of drugs that inhibit metabolic pathways and which is more common?

Give 2 examples of each and list its mode of action

Answer 27

1) Folic acid synthesis (more common)

• Eg. Sulphonamides and Trimethoprim
• Bacteriostatic mode of action

2) Mycolic acid synthesis or incorporation
* Isoniazid (INH) and Ethambutol

Question 28

What is the mechanism of action of Sulphonamides and trimethoprim

What bacteria can these NOT target and why?

Why do they not affect human cells

Answer 28

Prevent synthesis of folic acid, which is required for synthesis of purines and nucleic acids (bacteristatic)

• Sulphonamides: target DHPS (dihydropteroate synthase)
• Trimethoprim: targets DHFR (dihydrofolate reductase)
• Both are important in folic acid synthesis

Does not affect human cells or certain bacteria which can use preformed folic acid

Question 29

What is the mechanism of action of Isoniazid and Ethambutol?

Answer 29

Targets mycolic acid synthesis or incorporation which Inhibits synthesis of mycobacterium cell wall

Question 30

Name the four methods that can determine microbial sensitivity

Answer 30

1) disk diffusion method
2) dilution method
3) serum killing power
4) automated methods

Question 31

What is the purpose of the Disk Diffusion method?

Explain how it would be done and what the results would indicate

Answer 31

Purpose is to determine whether an antibacterial will be effective against a particular strain of bacteria/its microbial sensitivity

1) test strain bacteria plated on nutrient agar
2) discs with antibiotic put on plate
3) antibiotics diffuse out

Results:

• a confluent growth of bacteria (bacterial lawn) will be seen where growth is not inhibited
• “zones of inhibition” (no growth) will be seen when strain is susceptible to that antibiotic concentration in agar

Question 32

In the disc diffusion test what does the diameter of the zone of inhibition indiate?

Answer 32

Diameter of zone of inhibition is a measure of susceptibility

Question 33

Describe the Dilution method

Answer 33

1) Sequential dilutions of antibiotics onto bacteria
2) observe how the bacterial growth is affected amongst different antibiotics or concentrations

Question 34

What is the significance of the ‘innovation gap’?

Answer 34

Period of time between the 60s and 2000s where no new antibiotics were developed, this led to the chance of bacteria developing more resistance

Question 35

When selecting an antimicrobial you must use emerical therapy and definitive therapy

Explain what is meant by each of these

Answer 35

Empirical therapy:

• used initially when the infecting organism is not yet identified
• use a single broad spectrum agent

Definitive therapy

• used when the microorganism is identified
• use a narrow spectrum, low toxicity regiment to complete the course of treatment

Question 36

List 6 factors you must know about a microorganism to select the most appropriate antimicrobial agent

Answer 36

1) organisms identity: is it gram + or - ?
2) the organism’s susceptibility to a particular agent
3) the site of infection: how will we get the drug there (topical, oral, Intramuscular? (consider protein binding, lipid solubility MW and does it need to cross BBB?)

4) patient factors (renal/hepatic problems, age, sex, pregnancy,
lactation and immune system)

5) the safety of the agent- do the benfits outweight the risks?
6) the cost of the therapy