Antimicrobial chemotherapy – agents and mechanisms of action

Question 1

narrow spectrum

Answer 1

only affect certain organisms

Question 2

broad spectrum

Answer 2

action against broader spectrum of microbial agents

Question 3

what are most antibiotics directed against

Answer 3

• bacteria cell wall synthesis (peptidoglycan)
• bacteria protein synthesis (- ribosomes
• enzymes)
• bacterial nucleis acid synthesis
• DNA
• membranes
• enzymes
• metabolic pathways

Question 4

when are antimicrobials useful

Answer 4

if the target is not present in man

if the microorganism has higher affinity for the drug than man

Question 5

selective toxicity

Answer 5

must be highly effective against microbe but minimal toxicity in humans
- expressed by drugs therapeutic index
larger the index, after the drug for human use

Question 6

what characteristics should useful antibiotics have

Answer 6

1) wide spectrum of activity with the ability to destroy or inhibit many different species of pathogenic organism
2) non toxic to host, and without undesirable side effects
3) non allergenic to host
4) not eliminate normal flora of host
5) be able to reach part of the human body where the infection is occuring
6) inexpensive and easy to produce
7) chemically stable (have long shelf life)
8) microbial resistance is uncommon and unlikely to develop

Question 7

classification of antimicrobials

Answer 7

1) chemical structure
- eg B lactam ring
2) target site
3) bactericidal or bacteriostatic
- cidal kill, static inhibit growth
- distinction often blurred

Question 8

testing antibiotics/effectiveness

Answer 8

Disc diffusion on agar

• lawn of bacteria grown on the plate
• disc soaked in antibiotics placed on plate
• if sensitive, will get a clear zone surrounding disc

in liquid
MIC
MBC
testing

Question 9

MIC testing

Answer 9

minimal inhibitory conenctraion

min concentration of antimicrobial needed to stop it growing

Question 10

MBC

Answer 10

-minimal bactericidal concentration, min needed to kill bacteria

Question 11

peptidoglycan cell wall structure

Answer 11

• needs to be cross linked
• amino acid cross links eg d-ala, d-glutamate
• rests on lipid bilayer
• Mae of N acetyl glucosamine
• N acetyl muramic acid

Question 12

main classes of antimicrobial agents

Answer 12

1) B lactams
- penicillins
- cephalosporins
2) Glycopeptides
- vancomycin
- teicoplanin
3) cycloserine
- inhibits alanine racemase and D alanine Ligase
- TB treatment

Question 13

B lactams and what do they mostly bind o

Answer 13

• bactericidal compounds
• contain a B lactam ring and inhibit normal cell wall formation

PBP (penicillin binding proteins )

Question 14

PBP penicillin binding protiens

Answer 14

D-D transpeptidases
involved in peptidoglycan synthesis
normally present in bacterie

Question 15

how does B lactam antibiotic inhibit peptidoglycan formation

Answer 15

NMA and NAG form cross links
- cyclosporine breaks up cross linking
vancomycin binds to alanine in the growing cross links therefore prevents peptidoglycan cross linking

penicillin bind to PBP )as it is an analogue)
inhibits formation of peptidoglycan cross links by binding b ring to enzyme DD transpeptidase
- no cross links can occur

Question 16

what is vancomycin effective against

Answer 16

gram + organisms
binds to D alamyl D Alaine dipeptide on side chain of new subunits
prevents them being incorporated into cell wall y penicillin binding proteins (PBP)

Question 17

what do tetracylines do

Answer 17

inhibit tRNA binding to 50s subunit( of tRNA to mRNA)

broad spectrum
incorporated into developing bone and teeth

Question 18

erthromycin

Answer 18

blocks exit of nascent chain from ribosome

Question 19

fusidic acid

Answer 19

binds elongation factor G (EFG)

Question 20

aminoglycosides

Answer 20

act on 30s subuit (ribosomes)
misreading of genetic code

effective against aerobes and facultative anaerobes

not against anerobes

• given IV or IM

side effects
- nephrotoxicity
ototoxicity

Question 21

macrolides

Answer 21

bind to 50s subunit blocking exit of nascent pp chain
bacteriostatic
- used in pts with penicillin allergy

Question 22

agents that affect DNA

Answer 22

• DNA gyrase helps steady the DNA when it it becoming unravelled
  1) Quinolones
• affect DNA gyrase
  2) rifamycins
• affect DNA dependant RNA polymerase
  3) metronidazole
• strand breakage

Question 23

nitrooimidazoles

Answer 23

e.g. metronidazole
disrupts DNA helix
only works in anerobic organisms

• activated in cell by redox enzyme pyruvate-ferredozin oxidoreductase
• in anaerobes, ferredoin is an e transported molecule that reduced (gives e- to) metronidazole (only functional in anerobic bacteria)
• this single e- transfer reduced nitro group of met. creating highly reactive anion – disrupts DNA helix

Question 24

folic acid synthesis

Answer 24

Folic acid enzymes
- needed for amino acid synthesis
Some antibiotics can interrupt these enzymes (eg sulfonamides)
- active against gram +and – bacteria

antiobiotics bind to dihydropteroate synthetase (structural analogues)
- stops PABA turning into amino acids

Question 25

how does antibiotic resistance occur

Answer 25

• chromosomal mutation
• some coded by plasmid DNA
  Some plasmids are transmissible

Question 26

antibiotic resistance definition

Answer 26

A organism that is not inhibited or killed by an antibacterial agent at concentrations of the drug achievable in the body after normal dosage

Question 27

transfer of antibiotic resistance

Answer 27

Transposons

• can carry resistance genes and jump between chromosome and plasmid
  1) chromosomally mediated resistance
• mutant selection
• resistant bacteria can divide and grow
  2) plasmid mediated resistance
• spread of resistance plasmid
• donor gives plasmid to recipient bacteira (forms a transconjugant

Question 28

integrons

Answer 28

• multiple resistant genes are sometimes organised into genetic elements

contain gene for recombination enzyme to allow insertion

Question 29

why might antimicrobial agents not work

Answer 29

1) The target is structurally altered (by mutation)
- lower affinity for the antibacterial
- penicillin binding proteins (can change structure, therefore penicillin wont bind as much anymore)
2) the target is over produced
- dihydropteroate synthetase (PABA to aa) (bacterial target) and sulphonamide
- will overwhelm the antibiotic
3) the drug is not activated
- aerobes and metrzdazole
- antibiotic wont become active
4) drug is removed
Enzyme destruction
- B lactamase (produced by bacteria)
- aminoglycoside resistance – 3 modifying enzymes that can modify the antibiotic by changing its structure (eg phosphorylation)
Efflux
- tetracyclines, quinolones
5) drug cannot gain entry to the cell
- outer membrane barrier
- lack of transport mechanism

Question 30

what can antivirals target

Answer 30

1) penetration/uncoating
- amantadine (influenza)
- prevent fusion of viral envelope with cell membrane
2) taking over cell machinery
Transcription
- nucleoside analogues – zidovudine, acyclovir
- zidovudine acts as a substrate for and inhibitors of viral reverse transcriptase (enzyme needed to create DNA copy of its RNA, necessary for integration into host genome)
- acyclovir – inhibits HSV DNA polymerase
Translation
- anti sense morpholinos (oligomer molecules that block target sequence in RNA by binding
3) Post translation inhibition
Protease inhibitors
- protease cleaves viral polyproteins into structural proteins required for viral replication
- protease inhibition- immature, defective viral particles