Antibiotics

Question 1

What are antibiotics?

Answer 1

natural products of fungi and bacteria which live in the soil that kill or inhibit the growth of other microorganisms through natural antagonism and selective advantage

Question 2

How have humans developed antibiotics from natural products of fungi/bacteria?

Answer 2

Natural products undergo fermentation, then modified chemically to have:

• better pharmacological properties/activity
• better antimicrobial effect/activity

Some antibiotics however are totally synthetic (e.g. sulphonamides)

Question 3

Considerations when using antibiotics as therapeutic agents

Answer 3

Selective Toxicity

Therapeutic Margin

Question 4

What is selective toxicity?

Answer 4

ability of drug to kill or inhibit pathogen while damaging host as little as possible:

• target in microbe, not host
• difficult for viruses (intracellular), fungi and parasites

Question 5

How good is selective toxicity of antibiotics?

Answer 5

There are many unique targets in bacteria not present in humans which can be inhibited by antibiotics. We have a range of different antibiotics which have low toxicity because we have very good selective toxicity.

Question 6

What is the therapeutic margin?

Answer 6

margin between the therapeutic (beneficial) dose and the toxic dose of a given substance :
-active dose (MIC) vs toxic effect

*you want to ensure the therapeutic dose is enough dose to kill of the infection (MIC), without inducing too much toxicity

Question 7

What does it mean if the dose between active dose and toxic dose is narrow?

Answer 7

Therapeutic margin is narrow and drug is toxic

Question 8

What does it mean if the dose between active dose and toxic dose is wide?

Answer 8

Therapeutic margin is wide and drug is safe to use (not very toxic)

Question 9

Therapeutic margin of aminoglycosides & vancomycin

Answer 9

Narrow therapeutic margin, meaning they are toxic drugs

• ototoxic
• nephrotoxic

Question 10

How do we monitor a patient on toxic drugs aminoglycosides & vancomycin?

Answer 10

Take blood samples to ensure MIC is being achieved, but drug levels haven’t reached toxic levels (ototoxic and nephrotoxic)

Question 11

What is the minimum inhibitory concentration (MIC)?

Answer 11

the concentration at which you have to give an active dose in order for it to be effective microbiologically

-need to achieve this MIC without inducing too much toxicity

Question 12

What is microbial antagonism?

Answer 12

competition between microbes

Question 13

How do we maintain our normal flora?

Answer 13

Via microbial antagonism, as flora limit the growth of competitors and pathogens

Question 14

Negative effect of antibiotics on flora

Answer 14

Some antibiotics can disrupt the homeostatic balance of our natural commensal flora (e.g. gut, skin), causing loss of normal flora and bacterial/pathogen overgrowth, leading to disease

Question 15

Which disease arises from antibiotics disrupting commensal balance?

Answer 15

Antibiotic Associated Colitis (Pseudomembranous Colitis)

• clindamycin
• broad-spectrum lactams
• fluoroquinolones

Question 16

What causes pseudomembranous colitis?

Answer 16

Overgrowth of Clostridium difficile.

This is a spore-forming organism which causes:

• ulcerations →inflammation
• severe watery diarrhoea
• hospital cross-infection risks (spreads easily)

Question 17

Why is it difficult to treat infections with antibiotics in immunocompromised patients?

Answer 17

Because antibiotics rely partly on the immune system to help clear the infection (do not work alone)

Therefore, antibiotics aren’t very effective during immunosuppression:

• Cancer chemotherapy, transplantations, myeloma, leukaemias
• HIV with low CD4
• Neutropenics, asplenics, renal disease, diabetes, alcoholics
• Babies, elderly etc.

Question 18

How are antibiotics classified?

Answer 18

Type of Activity

Structure

Target Site for Activity

Question 19

How are antibiotics classified by “Type of Activity”?

Answer 19

Bactericidal
Bacteriostatic

Broad-Spectrum
Narrow Spectrum

Question 20

Bactericidal antibiotics

Answer 20

Antibiotics which KILL bacteria

Question 21

When are bactericidal antibiotics used?

Answer 21

Used when the host defence mechanisms are impaired

-required in endocarditis, kidney infection (life-threatening infections)

Question 22

Bacteriostatic antibiotics

Answer 22

Antibiotics that do not actually kill bacteria but rather inhibit their growth

Question 23

When are bacteriostatic antibiotics used?

Answer 23

Used in many infectious diseases when the host defence mechanisms are intact which clears the infection

Question 24

What are broad spectrum antibiotics?

Answer 24

Antibiotics effective against many types of bacteria

-e.g. Cefotaxime

Question 25

What are narrow spectrum antibiotics?

Answer 25

Antibiotics effective against very few types of bacteria | -e.g. Penicillin G

Question 26

How can we develop antibiotics with different spectrums of activity?

Answer 26

Via pharmaceutical modification E.g.: - 1st gen cephalosporins good at killing gram-positives but not gram-negatives - 2nd gen better at killing gram-negatives, but as a result not as good at killing gram-positives - 3rd gen good at killing gram-negatives, but not gram-positives

Question 27

How are antibiotics classified by "Molecular Structure"?

Answer 27

Families of antibiotics are used to differentiate them from one another -e.g. β-lactam antibiotics

Question 28

What is β-lactam?

Answer 28

β-lactam is the active chemical structure of a chemical compound seen in penicillin and cephalosporin antibiotics.

Question 29

Action of β-lactam antibiotics

Answer 29

Competitive Inhibitors They are structural mimics of natural substrates for bacterial enzymes involved in making the bacterial cell wall/peptidoglycan -cell wall synthesis inhibitors

Question 30

How do some organisms develop resistance to β-lactam antibiotics?

Answer 30

They acquire enzymes called β-lactamases which degrade the β-lactam ring Once this ring is destroyed, β-lactam antibiotics (e.g. penicillin and cephalosporin) no longer have any antimicrobial properties

Question 31

How are antibiotics classified by "Target Site for Activity"?

Answer 31

Antibiotics can be identified depending on what bacterial processes they will inhibit: - cell wall synthesis inhibitors - protein synthesis inhibitors - DNA&RNA processing inhibitors - folic acid metabolism inhibitors - cell membrane inhibitors - free radical generators

Question 32

Cell wall synthesis inhibitors

Answer 32

Target the enzymes which make peptidoglycan β-lactam antibiotics & Vancomycin -inhibit bacterial cell wall synthesis → bacteria die *different in gram-ve and gram+ve bacteria

Question 33

Selective toxicity of cell wall synthesis inhibitors

Answer 33

Good selective toxicity because human eukaryote cells don't have a cell wall

Question 34

Protein synthesis inhibitors

Answer 34

Erythromycin & Chloramphenicol -inhibit 50S ribosomal subunit Tetracycline & Aminoglycosides (Streptomycin) -inhibit 30S ribosomal subunit

Question 35

Selective toxicity of protein synthesis inhibitors

Answer 35

Good selective toxicity because ribosome in bacterial prokaryote different to ribosome in human eukaryote

Question 36

DNA & RNA processing inhibitors

Answer 36

Antibiotics which inhibit bacterial enzymes involved in DNA replication or mRNA formation Quinolones -inhibit DNA gyrase, a topoisomerase involved in coiling and uncoiling of genome during replication Rifampicin -inhibits DNA-directed RNA polymerase → no mRNA, no bacterial proteins→ death

Question 37

Selective toxicity of DNA & RNA processing inhibitors

Answer 37

Good selective toxicity because bacterial enzymes involved in DNA replication and mRNA formation different to enzymes in eukaryotes

Question 38

Folic acid metabolism inhibitors

Answer 38

Folic acid (vitamin) is an important co-factor for many enzymes, so if bacteria can't make folic acid, many enzymes can't carry out reactions and bacteria dies Sulfonamides -blocks dihydropteroate synthetase by competing against natural substrate PABA, inhibiting DHFA production Trimethoprim -blocks dihydrofolate reductase, inhibiting THFA production

Question 39

Selective toxicity of folic acid metabolism inhibitors

Answer 39

Good selective toxicity because humans don't have the enzymes that make folic acid in bacteria

Question 40

Cell membrane inhibitors

Answer 40

Antibiotics which damage bacterial membrane - Colistin - Daptomycin

Question 41

Selective toxicity of cell membrane inhibitors

Answer 41

Poor selective toxicity because bacterial cell membrane is similar to eukaryotic cell membrane

Question 42

Free radical generators

Answer 42

Antibiotics which generate free radicals that mainly damage bacterial DNA Metronidazole -used to treat anaerobic bacteria Nitrofurantoin -used to treat UTIs

Question 43

What has to be taken into consideration when prescribing a cell wall synthesis inhibitor?

Answer 43

Whether the bacteria is a gram+ve or a gram-ve organism

Question 44

Where are the enzymes which make peptidoglycan located?

Answer 44

Enzymes which make the peptidoglycan layer are on the outer side of the inner bacterial cell plasma membrane.

Question 45

Gram+ve bacterial cell wall

Answer 45

Large peptidoglycan layer which contains components of the bacterial cell wall including teichoic and lipoteichoic acid

Question 46

Action of cell wall synthesis inhibitors against gram+ve bacteria

Answer 46

Plasma membrane is a porous structure, and antibiotics penetrate this porous structure to target enzymes which make the peptidoglycan and inhibit them

Question 47

Gram-ve bacterial cell wall

Answer 47

Small peptidoglycan layer which sits within the periplasmic space and has an impermeable outer membrane, with lipopolysaccharides on the outside.

Question 48

Action of cell wall synthesis inhibitors against gram-ve bacteria

Answer 48

Outer membrane is an impermeability barrier, and the only way anything can get across it is through porins. Antibiotics are transported through porins into the outer side of the inner plasma membrane to target enzymes that make peptidoglycan Many antibiotics you can’t use on gram-negative organisms because they can’t get through this impermeable outer membrane barrier.

Question 49

Peptidoglycan synthesis

Answer 49

1) Disaccharide precursor monomers containing 5 peptides, with last two peptides being terminal alanine isomers (terminal D-ala D-ala) are synthesised in the cytoplasm. 2) Precursors will be transported across cytoplasmic membrane by phospholipid carried molecule and into the cell wall 3) Trans carboxypeptidases/PBPs then cleave D-ala and cross-link the peptide chains to the growing peptidoglycan structure (polymerisation)

Question 50

MOA of cell wall synthesis inhibitors (preventing peptidoglycan synthesis)

Answer 50

Cycloserine -inhibits incorporation of terminal D-ala D-ala into cell wall precursor Bacitracin -inhibits dephosphorylation of phospholipid carrier, preventing regeneration of carrier necessary for the synthesis to continue Vancomycin -binds to terminal D-ala D-ala residues and prevent incorporation of the peptides into the growing peptidoglycan β-lactam antibiotics -bind to and inhibit trans carboxypeptidase enzymes/PBPs which catalyse the cross-linking

Question 51

What are penicillin binding proteins?

Answer 51

Cross-linking enzymes (trans carboxypeptidases) which synthesise the peptidoglycan and also competitively bind penicillin and cephalosporin (β-lactam antibiotics)

Question 52

Action of β-lactam antibiotics on PBPs in Gram-ve bacteria

Answer 52

If you come in with a β-lactam antibiotic like a penicillin or cephalosporin, the antibiotic has to be able to get through the outer membrane porin and bind to and inhibit the penicillin binding protein (PBP). This prevents the bacteria from cross-linking, and as a result most bacteria have an autolytic response to not being able to produce its peptidoglycan, lysing itself → death.

Question 53

Bacteria that don't have peptidoglycan layer

Answer 53

Mycoplasma Chlamydia *can't be treated with β-lactam antibiotics

Question 54

Use of antibiotics

Answer 54

· Treatment of bacterial infections · Prophylaxis - given if in close contacts of transmissible infections to reduce carriage rates - prevention of infection (e.g. meningitis, tuberculosis) - peri-operative cover for gut surgery - people with increased susceptibility to infection

Question 55

Route of antibiotic administration

Answer 55

Oral -community infections I/V -serious infections requiring hospitalisation Topical (antiseptic creams, heavy metal ointments) - conjunctivitis - superficial skin infections

Question 56

What does the MIC dose depend?

Answer 56

· Depend on age, weight, renal & liver function of patient and the severity of infection · Depend on the susceptibility of the organism · Will also depend upon properties of the antibiotic e.g. enough to give a concentration high than the MIC at the site of infection

Question 57

When do we give antibiotic combinations?

Answer 57

Before an organism identified in life-threatening infections -E.g. endocarditis, septicaemia Polymicrobial infections -E.g. abscess, GI perforation anaerobes and aerobes To reduce antibiotic resistance (e.g. tuberculosis)