Antibiotics & Antibacterial Chemotherapy

Question 1

Give the definitions of antibiotics, antibacterial agents and disinfectants/antiseptics?

Answer 1

Antibiotics – natural substances produced by one micro-organism that kill or inhibit the growth of another

Antibacterial agents/ antibacterials – compounds capable of killing/ inhibiting bacteria, including (semi-)synthetic agents

Disinfectants/ antiseptics – compounds that kill microorganisms but are too toxic for internal use in human patients

Question 2

Give the definitions of antibacterial drugs and antibacterial chemotherapy?

Answer 2

Antibacterial drugs – compounds that show selective toxicity against bacterial vs. mammalian cells and can be used in patients

Antibacterial chemotherapy – the use of antibacterial drugs to treat and cure bacterial infections of animals and humans

Question 3

What is the difference between bacteriostatic and bacteriocidal action?

Answer 3

Bacteriostatic - prevent growth of bacteria (majority of antibiotics)
Bactericidal action - actually kills the bacteria

The majority of studies show bactericidal action isn’t superior

Question 4

What is the toxicity selectivity of action within antibiotics?

Answer 4

Growth of infecting organism is selectively inhibited or killed, without damaging cells of host
Based on exploiting differences in structure or biochemistry of the infecting agent and the host
Ideal antibacterial drug would have no adverse effect on the patient but be lethal to the organism

Question 5

Describe the biosynthesis of antibiotics?

Answer 5

Secondary metabolites produced by bacteria and fungi - derives from non-essential metabolism in the organism
e.g. Actinomycetes (bacteria), Penicillium spp. (fungi)
Some organisms make >25 different types of antibiotic

Maybe used to kill competing molecules or in a signalling communication within organisms

Question 6

Comparison of natural products v synthetics?

Answer 6

Name is written with a lower case letter 
Brackets are the class to which it belongs 

Natural products - often large complicated products we could never think of
Natural - daptomycin, vancomycin and penicillin

Synthetic - less complicated
Ciprofloxacin, linezolid, daptomycin

Question 7

Give a background of antibacterial chemotherapy?

Answer 7

85 years old
Our primary means of treating bacterial disease
A major means of preventing bacterial disease
An essential cornerstone of modern medicine
Taken for granted
Increasingly under threat

Question 8

Describe the history of antibacterial chemotherapy?

Answer 8

1676 - van Leeuwenhoek, 1861 - Pasteur, 1867 - Lister, 1876 - Koch, 1910 - Ehrlich

1928 - Fleming, 1935 - Domagk, 1939 - WW2 sulphonamides, 1940 - Florey/Chain, UK/USA war production of penicillin

Question 9

What is the impact and importance of antibacterial chemotherapy?

Answer 9

Rapid decrease in deaths per 10,000 births
Increase in life expectancy from 47 to 80 from 1901 to 2010

Importance:
Modern economies underpinned by antibiotics
Massive benefits to individual health and societies: survival/longevity and quality of life
Modern medicine relies on antibiotics
Treatment of bacterial infection, dentistry, surgery and transplants

Question 10

Within the mechanism of antibacterial action what are recurring themes?

Answer 10

Substrate analogues
Steric hindrance
Enzyme inactivation
Disruption or subversion (destructive/reactive processes)

These aren’t mutually exclusive - as can use more than one simultaneously/sequentially

Question 11

What is the basis for selective toxicity?

Answer 11

Agent does not get into mammalian cells as easily as into bacteria
Agent targets processes/structures not present in mammalian cells
Agents target processes/ structures that are different in mammalian cells
Agent is a pro-drug that is only activated in bacteria

Question 12

What are some targets of antibiotic action?

Answer 12

Nucleotide biosynthesis
DNA replication 
Topoisomerases
RNA transcription
Protein synthesis
Cell wall synthesis

Mainly interferes with generating macromolecules: nucleotides, DNA, RNA, proteins, peptidoglycan

Question 13

What are used to target cell-wall biosynthesis?

Answer 13

B-lactams - penicillin, methicillin

Glycopeptides - vancomycin, teicoplanin
They bind to the substrate

Question 14

Describe antibacterial action targeting nucleotide metabolism?

Answer 14

Tetrahydrofolate pathway is needed in the cell in the de novo synthesis of deoxythymidine monophosphate - used for nucleotides

Targeting this prevents DNA replication
Trimethoprim - acts on the final stage
Sulphonamide - acts on an early stage
SMZ +TMP = co-trimoxazole

Question 15

Targeting nucleotide metabolism - describe sulfa drugs?

Answer 15

Competitive inhibitors and alternate substrates for DHPS
Structural analogs of p-aminobenzoic acid (PABA) - one of the substrates used by dihydropteroate synthase

With the Sulfa drug - acts as a competitive inhibitor and also act as an alternative substrate to produce a dead end complex
This is selective for bacteria cells as humans don’t perform this early stage in the pathway

Question 16

Targeting nucleotide metabolism - describe trimethoprim?

Answer 16

Conversion of dihydrofolic acid to tetrahydrofolic acid
This uses dihydrofolate reductase and NADPH
It can’t act as an alternative substrate = just a competitive inhibitor
Bacterial cells are very susceptible to this drug (around 50 times more than humans

Question 17

Describe antibacterial action targeting DNA?

Answer 17

Quinolones and fluoroquinolones - both synthetic antibacterial drugs
Target DNA gyrase and topoisomerase IV = remodelling DNA

Question 18

Targeting DNA - describe the mode of action of quinolones?

Answer 18

Target two related type II topoisomerases
DNA gyrase
DNA topoisomerase IV (Topo IV)
Both A2B2 heterotetramers that catalyse ATP-dependent DNA double-strand breakage/ re-joining reactions
At 4 base staggered sites

It forms an intermediate - where a tyrosine residue is covalently bound to the 5’ end where the strand is broken
Quinolones bind at this end, undergoing base stacking interactions - preventing a nucleotide base binding there
= promiscuous endonuclease will cut up the bacteria

It also interacts with the enzyme
The keto-acid portion of the quinolone collates the Mg ion
The Mg ion can co-ordinate 4 water molecules, which come close enough to 2 acidic amino aids in the enzyme - allowing hydrogen bonding (water-metal ion bridge)

Question 19

Describe antibacterial action targeting protein synthesis?

Answer 19

Large number of antibiotic classes target protein synthesis
Most act directly on the ribosome
Most are bacteriostatic

Can differentiate between host and infection as ribosome has different compositions between eukaryotes (80S) and prokayotes (70S)

Question 20

Targeting protein synthesis - describe linezolid?

Answer 20

A member of the oxazolidinone class of synthetic agents
Binding site on 50S ribosomal subunit
Initially thought to block subunit assembly
Recent studies suggest a different mechanism of action

Linezolid occupies space normally taken by the aa residue of the A site aa-tRNA
Other amino acids will then not be able to bind

Question 21

Targeting protein synthesis - how do other inhibitors target the ribosome?

Answer 21

Tetracycline - blocks binding of aminoacyl-tRNA to A-site of ribosome

Streptomycin - prevents the transition from translation to chain elongation adn also causes miscoding

Question 22

Targeting protein synthesis - what is the mode of action of fusidic acid?

Answer 22

This is a steroid like compound
It interferes with the function of an accessory protein - EF-G
It recognises the post-translocation complex

It binds to EF-G on the ribosome, so the A-site is blocked - no more amino acids can then bind and protein translation is stopped

Question 23

Targeting protein synthesis - how does mupirocin work?

Answer 23

This inhibits formation of isoleucyl tRNA

It competitively binds to isoleucyl tRNase = substrate analog