Antibiotics Flashcards
1
Q
What is meant by selective toxicity?
A
Using drugs to kill an invading species without damage to the host
2
Q
What can be used for selective toxicity?
A
- Anticancer
- Antiviral
- Antiprotozoal
- Antibacterial (looking at this in this module)
3
Q
Do selectively toxic compounds work in the same way as disinfectants and sterilising agents?
A
This is NOT the same. As disinfectants and sterilising agents kill everything, where as selective toxic things do not
4
Q
Can antibacterials be selectively toxic to different organisms?
A
Yes
5
Q
Whats required for something to be selectively toxic?
A
To be selective, we need to find molecular mechanisms that are important in the pathogen, but are not present (or are not important) in the host (look for difference sin metabolic pathways between bacteria and host)
6
Q
Are antibiotics naturally occuring?
A
Yes, they are agents made by one microorganism that kill another
7
Q
Give two examples of naturally occuring antibiotics. How has this been changed/ adapted over the years?
A
Penicillin and streptomycin
Over the years they have become more synthetic or semi-synthetic as chemists take precursors of them and then add in characteristics that they want to see
8
Q
What did the scientist Ehrlich hypothesise?
A
- For a series of related compounds toxicity did not parallel curative action (reduce toxicity and improve curative action)
Inactive in vitro:
- Reduced to the As(III) form e.g. highly toxic drug used to be used to treat syphilis (don’t need to know just for history)
9
Q
What were Ehrlich’s concepts?
A
- Drugs contain a haptophore for binding and a toxophore for killing (not terms that we use now but principles are important)
- Drugs could be metabolically activated
- Need to not actually kill the bacteria
- Structure activity relationship
10
Q
Whats the difference between Bactericidal and Bacteriostatic antibiotics?
Provide an example for each
A
Bactericidal- kill invading organism e.g., penicillin
Bacteriostatic- stopping growth of bacteria so body eliminates invading organism e.g. sulphonamide
11
Q
Tell me whats meant by the structure activity relationship (S.A.R)?
A
Structure Activity Relationship (S.A.R.)- should be able to relate biological activity to structure of a compound
12
Q
What are the main sites of antibiotic action?
A
- Metabolism (antimetabolites)
- Cell wall synthesis (penicillin is the best-known example for this category)
- Protein synthesis (eukaryotic vs prokaryotic and different inhibitors)
- Nucleic Acid synthesis (packaging and synthesis is different with eukaryotes and prokaryotes)
13
Q
Prontosil is an antibacterial drug, what was metabolised from it that is more widely used now?
Draw the structure for it
A
In a continuation of Ehrlich’s approach, another product of the dye industry, prontosil, was introduced as an antibacterial agent by Gerhard Domagk in 1935. This compound was metabolized to sulphonamide, a fact which when recognized, led to the development of many new sulpha drugs.
14
Q
How do sulphonamides work?
A
Bacteriostatic
- Why a slow stopping of growth of bacteria when antibiotic added: not a penetration problem as gets in quickly, drug depletes stores of something in the bacteria which stops new synthesis of that. As product is used up, the bacteria stop growing
- Bacteriostatic antibiotics
15
Q
Tell me about each of the groups on the sulphonamides structure…
A
16
Q
Sulphonamide shows similar isosterism to para-amino benzoic acid (pAB), draw the structures for each and what group is different/
A
17
Q
What reaction is para-aminobenzoic acid (pAB) an important initiator of and why is this reaction important in regards to the final product produced?
A
The final product is tetrahydrofolate
Tetrahydrofolate (THF) or tetrahydrofolic acid is a derivative of Vitamin B9 (folic acid or pteroyl-L-glutamic acid), a water-soluble vitamin that serves as a coenzyme for metabolic reactions involving amino acids and nucleic acids.
18
Q
Draw the cascade reaction for the production of tetrahydrofolate from para-aminobenzoic acid?
A
19
Q
Whats the problem with the tetrahydrofolate reaction pathway?
A
Sulphonamide shows isosterism with pAB this means
Sulphonamides compete with pAB for the synthesis of dihydropteroic acid
However, this leads to the generation of a false product which can’t be used for further synthesis of folate
20
Q
What are the uses of folic acid?
A
“one carbon metabolism” – used as a factor in adding methyl groups. Essential for the synthesis of RNA, DNA, amino acids….
If the metabolism is stopped then the synthesis of folic acid is stopped as a consequence
21
Q
If folic acid synthesis is stopped, why does this kill bacteria but not mammals?
A
- We cannot make folic acid – it is an essential part of our diet
- Bacteria synthesise their own folic acid, and do not have mechanisms for absorbing it (can’t cross their membranes or cell walls by diffusion of active transport)
22
Q
Sulphonamide derivatives…
A
23
Q
The seach for other antimetabolites has been less successful. Why do sulphonamides work so well?
A
- They generate a false product (exhausting the supply of the pteridine precursor) – so compounds before the block do not build up and compete. (generates an irreversible step which generates a false product)
- Bacteria have no uptake mechanisms for the product.
- We do not synthesise folate – though it is an essential part of our diet. pAB is not present in our cells.
24
Q
What generally blocks sulphonamides actions?
A
Their action is blocked by the final products of folate metabolism – serine, thymine etc.
25
Sulphonamide is described as being **bacteriostatic,** but when can it become **bactericidal?**
They are usually bacteriostatic (stop it growing as seen in growth curve above) – but become bactericidal in the absence of thymine **“thymineless death”**
26
What is broken down to release pAB?
Procaine (a local anaesthetic)
27
Why are sulphonamides less widely used now a days and give examples to support this?
They are less widely used as a result of **resistance** (due to increased pAB level, increased uptake of folate, reduced drug uptake, or drug metabolism due to point mutations of other resistance mechanisms).
28
Give some examples of what sulphonamides are used to treat?
* Bacterial meningitis
* UTIs
* bronchitis
* eye infections
* pneumonia
* ear infections
* severe burns
* traveler's diarrhea
* and other conditions
29
Do sulphonamides work against both gram +ve and -ve bacteria?
yes
30
Give examples of two other drugs that act by the same mechanism as sulphonamide
Give their structure
31
Tell me about procaines involvment in the before mentioned reaction?
* procaine can overcome the actions of sulphonamides (esterase that breaks the procaine down which competes with sulphonamide and increases the cell stores of pAB and then more likely to make the folic acid)
* procaine doesn’t last very long as is a short acting anaesthetic
* procaine competes with the action of sulphonamide
32
Whats another drug that targets the synthesis of folate?
* dHFR is the enzyme involved in the synthesis of folate
* methotrexate inhibits dHFR which results in less folate being produced
* methotrexate is a non-selective drug that inhibits that pathway covered above of folic acid
33
is dHFR found in eukaryotes and prokaryotes?
dHFR is our enzyme and also bacterial (but protein structure of us and in bacteria isn’t the same even though it has the same action)
34
Tell me about the drug **Trimethoprim?**
* Used in combination with suphamethoxazole (1:20) as “septrin”. Inhibiting two points on the same pathway
* Inhibitor of dHFR
* Useful antibiotic
35
Tell me about the drug **Pyrimethamine ?**
36
Inhibitors of the folate pathway and the knock on effects it can have
37
Is penicillin bactericidal or bacteriostatic?
Bactericidal
38
What is Septrin a mixture of?
Septrin is a mixture of sulphonamide with trimethoprim
39
Who discovered penicillin?
Alexander Flemming
40
Tell me 4 things that penicillin is involved in?
1. Bactericidal
2. Prevents cell wall synthesis
3. Cell lysis
4. In isotonic media, protoplasts are generated
41
What are the two types of bacteria?
Gram positive
Gram negative
42
Label this Gram +ve and -ve bacteria
43
Outline of the bacterial cell wall synthesis process
44
Peptidoglycan is the macromolecule found in bacterial cell wall. What polymers does it contain and tell me about its cross links
45
Whats the first stage of bacterial cell wall synthesis
46
Whats the second stage of bacterial cell wall synthesis ?
47
Whats the third stage of bacterial cell wall synthesis?
48
Whats the fourth stage o bacterial cell wall synthesis?
49
Whats the fifth stage of bacterial cell wall synthesis?
50
Whats the sixth stage of bacterial cell wall synthesis?
51
Whats the seventh stage of bacterial cell wall synthesis?
52
Whats the eigth stage of bacterial cell wall synthesis?
What is this blocked by?
Transfer of the disaccharide unit to the existing cell wall (releasing C55-P-P)
Blocked by
* **Vancomycin**
* **Ristocetin**
53
How does Vancomycin work?
Whats the danger of using this antibiotic and how it this overcome/ dealt with at the moment?
* binds to substrate, and **prevents enzymes binding to substrate**. Vancomycin **wraps around D-Ala D-Ala terminus** and stops enzymes form getting into the inside of the bacterial cell wall preventing further reactions
* **Resistance** to vancomycin means that the substrate need to be changed (the D-Ala D-Ala changed). **Replaces the terminal D-alanine with D-lactate** (changes the NH group in the structure which gets rid of the H bond with vancomycin).
* **Antibiotic of last resort so resistance doesn’t emerge from it**
* There is some vancomycin derivatives to overcome the resistance, but they aren’t used in clinical practice yet
54
What is the main principle that Vancomycin and Ristocetin work by?
These bind to the D-Ala-D-Ala end and prevent this substrate from occupying its site on the enzyme
55
Whats the ninth stage of bacterial cell wall synthesis?
56
Whats the tenth stage of bacterial cell wall synthesis?
57
What type of antibiotic is penicillin?
How does it work?
A beta-lactam
* Occupies the active site of the enzyme
* Covalent
* Irreversible binding
58
Pencillin activates autolytic enzymes, what does this lead to?
What happens under normal conditions?
Penicillin also activates autolytic enzymes, leading to lysis of the bacterium
Under normal circumstances, peptidoglycan precursors trigger the activation of autolytic cell wall hydrolases
59
inhibition of crosslinking by beta-lactams causes what?
What happens as a result of this?
* Inhibition of crosslinking by beta-lactams causes a **build-up of peptidoglycan precursors**, which triggers the digestion of existing peptidoglycan by autolytic hydrolases without the production of new peptidoglycan
* As a result, the bactericidal action of beta-lactam antibiotics is further enhanced and eventually the structural integrity of the cell wall decreases to the point where lysis occurs
60
New beta-lactam derivatives are required for what?
* Increased acid stability
* Resistance to beta-lactamases
* Increased activity against Gram -ve bacteria
61
Beta-lactam derivatives
62
How many pencillin binding proteins are there in E.Coli?
6
63
What are the 6 PBP (penicillin binding proteins) in E.Coli, what is their role?
**PBP-1:** transpeptidase (does the cross-linking reaction)
**PBP-2:** shape maintenance (restricting where cross links are forms)
**PBP-3:** septum formation (where one cell divides, invagination to form daughter cells)
**PBP-4-6:** control the amount of cross-linking and hydrolysis of the terminal D-Ala
64
What does Mecillinam have an affinity for?
Mecillinam affinity for PBP-2 (Mencillinam has increased activity against Gram -ve microorganisms at expense of activity against Gram +ve’s)
65
What do **beta lactamases** do?
Break the -lactam ring and give resistance to penicillin
66
What are the 4 groups of beta-lactamases?
* Penicillinases
* Cephalosporinases
* Extended-spectrum beta-lactamases (ESBL)
* Cabapenems
67
Tell me about **penicillinases**
Penicillinases: inactivate penicillin’s but do not degrade cephalosporins, aztreonam, or carbapenems.
68
Tell be about **Cephalosporinases**
Cephalosporinases preferentially inactivate cephalosporins and aminopenicillins, but do not affect other penicillin’s (carboxy- and ureido-penicillin’s), aztreonam, and carbapenems.
69
Tell me about **ESBL**
Extended-spectrum -lactamases (ESBL) inactivate all -lactams except carbapenems.
70
Tell me about **C****arbapenems**
Carbapenems (Several different classes) inactivate carbapenems and, depending on the enzyme, other -lactams as well
71
Tell me about the inhibitor of beta-lactamase
72
Tell me the following about eukaryotes and prokaryotes...
* ribosomal subunits they are composed of
* What are the proteins are the subunits made of
73
mRNA translation in bacteria
74
Tell me about the antibiotic **streptomycin?**
* The first antibiotic to **inhibit Gram negative bacteria**
* The first antibiotic **against tuberculosis**
* An **aminoglycoside antibiotic** (other aminoglycosides include gentamycin, kanamycin, neomycin)
75
Is streptomycin bactericidal or bacteristatic ?
**Bactericidal** (unusual for an inhibitor of protein synthesis)
76
What does streptomycin bind to?
30S
May have other actions on cell membrane as well as on protein synthesis
77
Tell me about the evidence for streptomycin action?
* Accumulation of 70S monosomes in a frozen initiation complex
* it does not inhibit elongation.
* In hybrid cells sensitivity is dominant over resistance
* Resistant mutants
30SR + 50SS = resistant
30SS + 50SR = sensitive
* Map to protein S12 (in 30S subunit) Lys 42 to arg, the asp or lys 87 to arg but streptomycin does not bind to the isolated S12
78
Misreading of the genetic code (caused by streptomycin)
* Generally misreading at the 5’-end or middle of the codon
* Can suppress the effects of nonsense and missense mutations
* Can isolate conditionally streptomycin dependent strains (also S12 Lys42 to Gln)
79
What are actions of streptomycin?
* Streptomycin induces errors in translation. It binds to the decoding centre and increases errors
* Binding of a correct AA-tRNA normally induces a conformational change in the 30S subunit, to a closed state, where hydrolysis of EFTu•GTP is activated
* In the presence of streptomycin, some incorrect AA-tRNAs can induce this conformation change
* Streptomycin stabilises the conformation which has a high affinity for ααtRNA – so increases the binding of some wrong tRNAs
80
Tell me about the selective toxicity of streptomycin?
it does not bind to 80S ribosomes
81
What is streptomycin potent against?
What is this used in because of this?
**Potent against Gram –ve bacteria:** Used in conjunction with ß-lactam antibiotics (to increase permeability)
82
Tell me about the resistance of streptomycin?
* Modifying enzymes
* Altered ribosomes
* Decrease uptake
83
Tell me about the side effects of streptomycin?
* (which are not related to its action on protein synthesis)
* Muscle weakness – decreased release of acetyl choline (binding to Ca2+ sites on the presynaptic membrane)
* Reversible ototoxicity
84
Tell me the following about the antibiotic **Chloramphenicol**
* produced by?
* broad spectrum?
* bactericidal or bacteriostatic?
* binds to?
* inhibts?
* Produced by Streptomyces venezuelae
* Broad spectrum antibiotic
* Bacteriostatic (reversible)
* Binding to 50S subunit
* Inhibits peptidyl transferase
85
What does Chloramphenicol stabilise?
Stabilizes polysomes – inhibits elongation
86
Does Chloramphenicol have resistant mutants?
What does it bind close to?
* No resistant mutants that map to protein changes
* Binds close to L16, L2, L24, L7
87
Tell me about the mechanism of action of Chloramphenicol?
* Does not affect aa-tRNA binding, but affects interaction with the amino acid end of the aminoacyl-tRNA
* Inhibits peptidyl transferase binding to A2451 and A2452 residues in the 23S rRNA of the 50S ribosomal subunit.
88
Tell me about the selective toxicity of Chloramphenicol?
Selective Toxicity: does not bind to 80S ribosomes
89
Tell me about the resistance of Chloramphenicol?
Resistance: Chloramphenicol acetyl transferase (3-OH)
90
Tell me about the side effects of chloramphenicol
* Gray baby syndrome excreted by glucuronidation not developed in neonates
* Reversible anaemia – bone marrow depression
* Low incidence (about 1 in \> 20,000) of aplastic anaemia, which can develop weeks after stopping the treatment – therefore only used for eye infections and life-threatening conditions
91
What can the antibiotic Tetracyclin interact with? Tell me about what to be careful about when prescribing it?
Can interact with Mg2+ and Ca2+. If too much, then can form a ppt and can’t be absorbed. So, shouldn’t be used with high levels of Mg2+ or Ca2+
92
Tell me some characteristics of tetracycline?
* Broad spectrum antibiotic
* Bacteriostatic
93
Tell me about the mechanism of action of tetracycline?
* Binds to 30S subunit (via 16S rRNA and S4 or S18)
* Blocks binding of incoming aminoacyl-tRNA to the A-site
94
Tell me about the selectivity of Tetracycline?
Selectivity: Inhibits both 80S and 70S ribosomes (though greater effect on 70S) – selectivity is due to bacterial uptake
95
Tell me about the side effects of tetracycline?
* Interaction with divalent metals: Ca2+, Mg2+ so should not be taken with milk or antacids.
* May stain developing teeth.
* Some skin photosensitivity
* May cause suprainfection
96
What type of antibiotic is Erythromycin?
A macrolide antibiotic; produced by Streptomyces erythreus. Similar to Clarithromycin (synthetic)
97
Tell me some characteristics of Erythromycin?
* Mainly active against Gram positive bacteria
* Bacteriostatic
* Inhibits translocation (50S subunit)
* Used especially for those who cannot take penicillin
* Antibiotic of choice for Legionnaires disease
98
Tell me about the selectivity of Erythromycin?
Selectivity: Does not bind to 80S ribosomes
99
Tell me the mechanism of action of erythromycin?
* Binds to 50S subunit; proteins L15 and L16 required for binding; binds close to the chloramphenicol site. Resistance maps in protein L4 and in rRNA
* Blocks the tunnel through which the new peptides leave the peptidyl transferase centre.
* Very high affinity for 70S ribosomes of 10^-8 M
100
Tell me the side effects of erythromycin?
* Erythromycin inhibits the cytochrome P450 – especially CYP3A4.
* Producing organism is protected by dimethylation of a specific adenine it’s in 23S rRNA – A2058Ec
101
What is Fusidic acid produced by?
Produced by Fusidium coccineum
102
What does fusidic acid inhibit?
EFG
103
What does fusidic acid prevent ?
Prevents recycling of the EFG-GDP complex
104
Is Fusidic acid Bacteriostatic or bactericidal?
Bacteriostatic – Gram positive bacteria; active against MRSA
105
What is fusidic acid mainly used in?
creams and eyedrops
106
What does mutation from fusidic acid arise from?
Resistance arises from a single point mutation – should not be used in monotherapy. (synergistic with penicillin)
107
Many drugs act on nucleic acids but are those generally selective or non-selective?
They are generally non-selective
108
What are the antibiotics that are often used in chemotherapy?
* daunomycin
* actinomycin
* bleomycin
* cis-platin
109
What is the antibiotic **rifampicin** produced by?
110
Is rifampicin bactericidal or bacteriostatic?
Bactericidal
111
What group of antibiotics is rifampicin part of?
ansamycin
112
What conditions is rifampicin especially useful against?
TB and leprosy
113
What is rifampicin mainly active against and what does it inhibit?
Mainly active against Gram positive (permeability) and inhibits bacterial RNA polymerase
114
Whats the mechanism of action of rifampicin?
* Inhibitor of bacterial RNA polymerase
* Binds to the beta-subunit (rpoB gene) (of the α2betabeta’σ holoenzyme)
* Acts at the initiation stage – does not inhibit elongation of mRNA synthesis (compare streptolydigin which inhibits both)
* Does not block formation of the first phosphodiester bond but prevents further additions. The binding site does not overlap with the sites for interaction with DNA or for synthesis of RNA but with sites for the new RNA product.
* After 2-3 phosphodiester bonds have formed further elongation is not blocked
* (Sensitivity dominates over resistance)
115
What does resistance rapidly develop from?
A single point mutation in the beta-subunit
116
What should resistance not be used in?
monotherapy
117
What do the βD516V, βH526D, βH526Y, and βS531L substitutions account for?
75% of clinical isolates of rifamycin-resistant *Mycobacterium* tuberculosis
118
RNAP-rifampin complex
119
What is Rifampin a potent inducer of?
(NB. Rifampin and rifampicin are the same drug)
Inducer of cytochrome P450 – CYP2C9 and CYP3A4 (which metabolise warfarin)
120
Name a quinolone
Nalidixic acid
121
What does Nalidixic acid inhibit and bind to?
Its an inhibitor or DNA gyrase
Binds to the A-subunit
122
What is Nalidixic acid active against?
Gram positive and gram negative bacteria
123
Is Nalidixic acid bacteriostatic or bactericidal?
Bacteriostatic at low concentrations, bactericidal at high concentrations
124
What is Nalidixic acid used for?
Especially used for urinary tract infections
125
What structure does DNA gyrase have?
A2B2
126
Whats the role of DNA gyrase?
unwinds double stranded DNA and so introduced negative supercoils into circular DNA
127
Is DNA gyrase ATP-dependent?
yes
128
DNA gyrase
129
quinoline antibiotics
130
Name 3 fluoroquinolone antibiotics
* ciprofloxacin
* coumermycin
* Novobiocin
131
What does **ciprofloxacin** bind to?
What does it act against?
What does it inhibit?
How can resistance be obtained?
* Binds to the A-subunit of DNA gyrase
* Active against Gram +ve and Gram -ve bacteria
**Anthrax** is a serious infectious disease caused by gram-positive, rod-shaped bacteria known as Bacillus anthraci
* Inhibits DNA gyrase and topoisomerase IV
* Resistance can be obtained with point mutations in the A subunit e.g. Ser83 to Trp
132
Tell me the following about Coumermycin and Novobiocin
* What does it inhibit?
* What are they competitive inhibitors of?
* What are they prescribed for?
* Inhibit DNA gyrase by binding to the B-subunit
* Competitive inhibitors of ATP binding.
* No longer marketed
*
