Antimicrobial chemotherapy, agents and mechanisms

Question 1

Antimicrobial agents

Aimed at

Two types

Answer 1

Controlling specific infecting organisms

Broad and narrow spectrum

Broad spectrum G+ and G- useful

Narrow - primarily useful against only certain species of bacteria

Question 2

Therapeutically useful if target is

Most antibiotics are directed against

Answer 2

Not present in man
Microbe has higher affinity for drug than man

Bacterial cell wall synthesis, protein synthesis, bacterial nucleic acid synthesis

Question 3

Selective toxicity

Expressed by

Drug is safer with

Answer 3

Highly effective against toxin but limited toxicity to humans
Therapeutic index - ratio of toxic dose to therapeutic dose

Higher index

Question 4

Clinically useful characteristics

Answer 4

• It should have a wide spectrum of activity with the ability to destroy or inhibit many different species of pathogenic organisms.
• It should be nontoxic to the host and without undesirable side effects.
• It should be nonallergenic to the host.
• It should not eliminate the normal flora of the host.
• It should be able to reach the part of the human body where the infection is occurring.
• It should be inexpensive and easy to produce.
• It should be chemically-stable (have a long shelf-life).
• Microbial resistance is uncommon and unlikely to develop.

Question 5

Classification of antimicrobials

Classified by

Answer 5

Chemical structure
Target site
Whether they are bactericidal or bacteriostatic

Question 6

Testing antibiotics
Methods
Result

MIC/MBC

Answer 6

Disc diffusion on agar
Clear zones around antibiotic application indicate sensitivity to antibiotic

Minimal inhibitory concentration
Minimal bactericidal concentration
MIC/MBC in liquid

Question 7

Main targets for antimicrobials

Answer 7

Cell wall - peptidoglycan 
Protein synthesis - ribosomes or enzymes 
Metabolic pathways 
DNA 
Membranes 
Enzymes

Question 8

Cell wall

Most antimicrobials

Answer 8

Peptidoglycan outlines lipid bilayer
Unique structure
Cross-linked with amino acids to improve strength

Act against cross-linking and affect interactions between NAM and NAG

Question 9

Main classes of anti peptidoglycan agent are

Answer 9

Beta lactams - penicillins and cephalosporins
Glycopeptides - vancomycin and teicoplanin
Cycloserine - inhibits alanine racemase and D-alanine ligase

Question 10

Cycloserine used for

Targets

Answer 10

TB treatment - structural analogue of D-alanine

Two enzymes involved in th cytosolic stages of PEPTG synthesis

Question 11

Beta lactam antibiotics

Structure

Function

Examples

Answer 11

Bactericidal compounds

Beta lactam ring and inhibit normal cell wall formation

Beta lactam ring can have different structures attached
- penicillins, cephalosporins

Inhibit peptidoglycan formation

1. vancomycin - effective against gram +ve
2. inhibits formation of cross linkages - beta lactam ring binds to penicillin binding protein which usually catalyses formation of cross linking - acts as competitive inhibitor

Question 12

Penicillin
Structure
Function

Answer 12

Mimics structure of D-ala-D-ala
Inhibits formation of peptidoglycan cross links in the bacterial cell wall
- BINDING of the 4 membered B lactam ring of penicillin to DD-TRANSPEPTIDASE penicillin binding protein
Cross links are not catalysed –> cell death

Question 13

Vancomycin
Effective against

Binds to

resistance??

Answer 13

Gram positive organisms

D-alanyl D-alanine dipeptide on side chain of newly synthesised peptidoglycan subunits

subunits not incorporated into cell wall

In resistant strains there s a different amino acid at the end of each chain
vancomycin cannot bind and cell lysis doesn’t occur

Question 14

Protein synthesis

Answer 14

Dna as template
mRNA processed and binds to ribosomes 30s and 50s
tRNA carries amino acids to mRNA
Base pairing occurs and nascent peptide chain grows
Bind to 30s subunit and cause misreading of genetic code
Tetracyclines - inhibit binding of tRNA to mRNA - ribosome complex - amino acids are not transported to form chain

Erythromycin blocks exit of formed chain
Fusidic acid binds to elongation factor
Blocks attachment to chain therefore blocks chain from growing

Question 15

Erythromycin

Answer 15

Binds to a molecule in 50s subunit blocking exit of nascent polypeptide chain
Fusidic acid - elongation factor G - bacterial protein needed for translocation on bacterial ribosome after peptide bond formation during protein synthesis
Fusidic acid binds EFG preventing protein synthesis

Question 16

Aminoglycosides

Which pathway do facultative anaerobes use

Not effective against

Not absorbed from the…

Administration
Side effects

Answer 16

Contain aminocyclitol ring linked to sugar
Effective against aerobes and facultative anaerobes

Either - oxygen dependent or oxygen independent
Optimal in aerobic conditions

Anaerobes as bacterial uptake needs oxygen or nitrate dependent electron transport
Anaerobes will not be surviving in this environment

Gut

• IV or IM for systemic tx
• side effects are nephrotoxicity, ototoxicity

Question 17

Tetracyclines

Function

Features

Answer 17

Inhibit binding of tRNA to mRNA ribosome complex

Bacteriostatic

• all broad spectrum
• penetrates mammalian cells to reach intracellular organisms
• incorporated into developing bone and teeth
• restricted use due to resistance
• not recommended in children

Question 18

MAcrolides

Answer 18

Binds to 50s subunit blocking exit of produced pp chain

Family of large cyclic molecules all containing macrocyclic lactone ring

• bacteriostatic
• erythromycin most commonly used

Erythromycin used for penicillin allergic patients
- penetrates mammalian cells to each IC organisms

Question 19

Agents affecting DNA - list

Target and mechanism

Answer 19

Quinolones - DNA gyrate

Rifamycins - inhibits DNA dependent RNA synthesis - high affinity for bacterial RNA polymerase - poor affinity for mammalian - prevents formation of new RNA

Metronidazole - disrupts DNA - only works when it is reduced - only in anaerobes –> only effective against anaerobes

Question 20

Nitroimidazoles

Function

Examples

Answer 20

Disrupt DNA

Metronidazole
Tinidazole
Antiparisitic and antibacterial

Question 21

Metronidazole is

Active only against

Answer 21

Inactive
- activated in cell by redox enzyme pyruvate-ferredoxin oxidoreductase

• In anaerobes, ferredoxin is an e- transporter molecule that reduces (gives electrons to) Metronidazole
• This single electron transfer reduces nitro group of met. creating highly reactive anion – disrupts DNA helix - highly reactive
• -> intermediate is short-lived and decomposes

Strictly anaerobic organisms
Produce low redox potential for reduction of drug

Question 22

Metabolic acid- folic acid synthesis

Answer 22

Folic acid enzymes - needed for AA synthesis
Some antibiotics interrupt enzymes
Active against gram +ve and gram -ve

Question 23

Antibiotic resistance

Definition

Cause of resistance

Answer 23

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

Chromosomal mutation
Some coded for by plasmid DNA
Some plasmids transmissible
Transposons can carry resistance genes and jump between chromosome and plasmid

Question 24

Transfer of resistance

Answer 24

Multiple resistance genes organised into INTEGRONS

INTEGRONS contains gene for recombinant enzyme to allow for insertion

Question 25

Antimicrobial resistance Mechanism What happens to target Final result Removal of drug via

Answer 25

Target is structurally altered by mutation --> lower affinity for antibacterial e.g pbps Target is overproduced -- too product Drug is not activated e.g aerobes and metronidazole Drug can be removed by enzymatic destruction e.g beta lactamase --> destroy beta lactam ring in beta lactam antibiotics Antibiotic can be EFFLUXED out of cell e.g tetracyclines drug cannot gain entry to cell - outer membrane barrier and lack of transport mech

Question 26

Beta lactamase Amino glycosides

Answer 26

Hydrolyses beta lactam ring have relatively uncommon resistance Structure is changed by modifying enzymes e.g acetylase --> change structure by phosphorylation

Question 27

Antivirals Methods of killing at different stages

Answer 27

Few Narrow spectrum Virastatic not cidal Interfere with viral machinery without affecting host 1. Penetration/uncoating e.g amantadine for influenza - prevent fusion of viral envelope with cell membrane 2. taking over cell machinery --> transcription - nucleoside ANALOGUES which act as substrate and inhibit viral reverse transcriptase and DNA polymerase i.e. zidovudin and acyclovir --> translation - morpoholinos that block target sequence in RNA by binding or ribozymes which cut viral RNA

Question 28

Post-translation inhibition

Answer 28

Protease inhibitors Protease cleaves viral polyproteins into structural proteins required for viral replication Protease inhibition - immature defective viral particles --> HAART highly active antiretroviral therapy