Antibiotics 3 a

Question 1

Diaminopyrimidines (trimethoprim)

Answer 1

Group of antibiotics acting on tetrahydrofolate synthesis pathway

Question 2

Mechanism of action of diaminopyrimidines (Trimethroprim)

Answer 2

• Inhibits dihydrofolate in the later stages of the tetrahydrofolate biosynthesis pathway
• Trimethoprim: Inhibits activity of hydro folate reductase by binding to the enzyme and preventing binding of substrate dihydrofolate.
• structurally different from bacterial dihydrofolate reductase: has a lower affinity for the isozyme meaning its not a significant target of this antibiotic.
• has a higher affinity for the bacterial enzyme making the drug selectively toxic to bacterial cells and not toxic to mammalian cells.

Question 3

Diaminopyrimidine side effects (trimethoprim)

Answer 3

• Broad-spectrum bacteriostatic antibiotic
• Side effects are rare

Question 4

Co-therapy; trimethoprim and sulphamethoxazole

Answer 4

• Trimethoprim mostly used in treatment of urinary tract infections someitmes in combination with a sulphonamide to act on two locations of the tetrahydrobiosnthesis pathway
• Trimethoprim can be just as effective alone and has fewer side effects.

Question 5

Co-therapy; trimethoprim and sulphamethoxazole - CO-TRIMOXAZOLE

Answer 5

trimethorpim administered in the same tablet as sulphamethoxazole in a 1:5 ratio
- not active against Pseudomonas sp.
- rarely used due to potential toxicity.
- used as an alternative when resistance to B-lactam antibiotics and ciprofloxacin have occurred.
- used to treat and prevent pneumocystis pneumonia (fungal infection) in HIV/AIDS

Question 6

Diaminopyrimidine resistance - acquisition of dihydrofolate reductase isozymes

Answer 6

• Occurs via horizontal gene transfer
• over 30 dihydrofolate reductase isozymes identified causing resistance, often carried on integrons
• integrons found on E.coli cause resistance to sulphonamides and trimethoprim.

Question 7

Other mechanisms of diaminopyrimidine resistance

Answer 7

Reduced permeability of the drug into the cell.
Enhanced efflux of antibiotic from the cell

Question 8

Flouoro quinolones

Answer 8

• Family of synthetically made or naturally occurring broad-spectrum antibiotics.
• ## Prevent bacterial DNA from unwinding and replicating.

Question 9

Flouoro quinolones structure

Answer 9

• R groups are variable depending on which quinolone is being shown
• Known are a fluroquinolone if the fluorine sites in the circled position
• Side chains vary at several positions giving rise to quinolones with different activities.

Question 10

Examples of qiunolones

Answer 10

alidixic acid, norfloxacin and ciprofloxacin

Question 11

Examples of fluoroquinolones

Answer 11

Ciprofloxacin and moxifloxacin

Question 12

Nalidixic acid

Answer 12

• First quinolone antibiotic discovered
• Good activity against gram negative bacteria: notable exception is Pseudmonas aeruginosa.
• Well absorbed when taken orally
• Extensively metabolised in the body prior to excretion via urine
• used exclusively to treat urinary tract infections

Question 13

Fluoroquinolones

Answer 13

• Active against broader ranfe of organisms than quinolones; active against gram negative and positive bacteria including P.aerugiosa
• Largely replaced quinolones

Question 14

What are ciprofloxacin and moxifloxacin used for?

Answer 14

• Treatment of anaerobes of Bacteroides fragilis group ( commensal gut microbes causing blood infections when displaced due to surgery or trauma)

Question 15

Fluoroquinolone administration

Answer 15

• generally orally.
• dosages can remain low due to the drugs good distribution throughout the tissues of the body and their concentration within mammalian cells: particularly useful on fighting intracellular pathogens such as chlamydia, legionella and some mycobacteria.

Question 16

Fluoroquinolone side effects

Answer 16

Generally well tolerated
GI disturbances
Non-specific neurological symptoms
Not recommended for children and pregnant women due to experimental evidence suggesting they may alter deposition of cartilage
Fluoroquinolone derivatives withdrawn from the market due to unexpected toxicity to the patient including severe hepatotoxicity.

Question 17

Fluoroquinolone targets

Answer 17

• acts in gram negative bacteria
• targets type II DNA topoisomerases
• tompoisomerase IV is a secondary target (primary target for gram-positive bacteria)

Question 18

Function of type II DNA topoisomerase

Answer 18

• Also known as DNA gyrases
• Maintain integrity of supercoiled DNA helix during replication and transcription.

Question 19

Fluoroquinolones mechanism of action

Answer 19

• Bind to enzyme-DNA complex preventing it from performing this function: Leads to strain on the DNA unwound and causes breakages in the strains as they are released from the complex without being re-ligated leading to cell death.
• Action of fluoroquinolones is bactericidal.

Question 20

Flouroquinolone: Topoisomerase IV action

Answer 20

• Primarily unlinks two linked strands of DNA.
• After replication: two copies of chromosomal DNA will be linked like a chain
• Topoisomerase IV breaks and repairs DNA at a specific site in order to release two copies from each other for cell division.
• Topoisomerase IV functions to relax supercoiled DNA during replication as does DNA gyrase.

Question 21

Fluoroquinolone resistance

Answer 21

Resistance is multifactorial- mutations can occur together.
Due to mutations occurring on the chromosome that modify the structure of DNA gyrase and topoisomerase IV s protein activity is unaffected and the antibiotic is no longer able to bind to enzyme.
Gram-negative bacteria: fluoroquinolone entry into cells dependent on presence of outer membrane porin proteins
Mutations result in down-regulation of proteins increasing the organisms resistance to drugs
Mutations can cause overexpression of proteins making up the multidrug efflux pumps of bacteria: drugs will be rapidly exported from the cell.

Question 22

Fluoroquinolone use

Answer 22

-Restricted in hospitals to treat drug resistant infections.
-Used to treat animal infections
-Could cause an increase in the incidence of resistant bacteria human population also due to transference of resistance between species if the resistance genes held on mobile DNA such as plasmids.