Antibiotics and Antifungals (13.03.2020) Flashcards
General Information about bacteria
- Single-cell microorganisms - cell wall & cell membrane
- An entire phylogenetic domain
- ~ 1/3 are pathogenic
Membrane characteristics of different bacteria + name an example for each of the three categories
Gram Positive Bacteria
- Prominent peptidoglycan cell wall, cell membrane below
- E.g. Staphylococcus Aureus
Gram Negative Bacteria
- Outer membrane with lipopolysaccharide (then a peptidoglycan layer and another inner membrane)
- E.g. Escherichia Coli
Mycolic Bacteria
- Outer mycolic acid layer (reasonable peptidoglycan layer below, they don’t fall under g+ or g-)
- E.g. Mycobacterium Tuberculosis
Prokaryotic protein synthesis
- Nucleic Acid Synthesis
a) Dihydropteroate (DHOp)
- Produced from paraaminobenzoate (PABA)
- Converted into dihydrofolate (DHF)
b) Tetrahydrofolate (THF)
- Produced from DHF by DHF reductase
- THF -> Important in DNA synthesis - DNA replication
- DNA gyrase (same as topoisomerase, uses tension)
- Topoisomerase - releases tension - RNA synthesis
- RNA polymerase
- Produces RNA from DNA template
- Differ from eukaryotic RNA polymerase - Protein synthesis
- Ribosomes
- Produce protein from RNA templates
- Differ from eukaryotic ribosomes
Ribosomes in eukaryotes and prokaryotes
P: 30-50s
E: 40-60s
-> protein synthesis
Nucleic acid synthesis in baacteria
- PABA
(via DHOp synthase) ->
- Dihydropteroate DHOp
- > - DHF
(via DHF reductaasae) ->
- THF (Tetrahydrofolate)
DNA replication in bacteria
- uses DNA gyrase
- it is a topoisomerase -> releases tension
RNA synthesis in bacteria
- RNA polymerase -> different in eukaryotes
- makes RNA from DNA template
Bacterial protein synthesis inhibitors - where do they interfere?
- Sulphonamindes -> inhibit DHOp synthase
- Trimethoprim -> inhibits DHF reductase
- Fluoroquinolones inhibit DNA gyrase and topoisomerase IV
- rifamycins -> inhibit bacterial RNA polymerase
- Macrolides, Aminoglucosides, Chloramphenicol and Tetracyclines inhibit bacterial ribosomes.
Rifamycins
- only used to treat TB
- The rifamycins (e.g. Rifampicin) inhibits bacterial RNA polymerase
Sulphonamides
- Sulphonamides inhibit DHOp synthase (nucleic acid synthesis)
- not much used anymore
Trimethoprim
- inhibits DHF reductase
- used sometimes but in combination moslty
Fluoroquinolones
- inhibit DNA gyrase and topoisomerase IV
- e.g. Ciprofloxacin
Macrolides
- e.g. erythromycin
- inhibit prokaryotic ribosomes
Which drugs inhibit prokaryotic ribosomes?
Aminoglycosides (e.g. Gentamicin)
Chloramphenicol
Macrolides (e.g. Erythromycin)
Tetracyclines
Which drugs inhibit bacterial intracellular processes?
Sulphonamides Tetracyclines Trimethoprim Fluoroquinonoles Rifaamycins Aminoglycosides (e.g. Gentamicin) Chloramphenicol Macrolides (e.g. Erythromycin) Tetracyclines
Bacterial Wall synthesis
PtG synthesis -> transportation -> incorporation
- Peptidoglycan (PtG) synthesis
- A pentapeptide is created on N-acetyl muramic acid (NAM)
- N-acetyl glucosamine (NAG) associates with NAM forming PtG - PtG transportation
- PtG is transported across the membrane by bactoprenol - PtG incorporation
- PtG is incorporated into the cell wall when transpeptidase enzyme cross-links PtG pentapeptides
PtG is the most important component of the cell wall.
List the antibiotics that interfere with the bacterial cell wall and where they interfere
- PtG synthesis
- Glycopeptides (e.g. Vancomycin) bind to the pentapeptide preventing PtG synthesis - PtG transportation
- Bacitracin inhibits bactoprenol regeneration preventing PtG transportation - PtG incorporation
- beta-lactams bind covalently to transpeptidase inhibiting PtG incorporation into cell wall
- beta-lactams include:
- Carbapenems
- Cephalosporins
- Penicillins - Cell wall stability
- Lipopeptide - (e.g. daptomycin) disrupt Gram +ve cell membranes
- Polymyxins - binds to LPS & disrupts Gram -ve cell membranes
Name different beta lactams
- penicillins
- carbapenems
- cephalosporins
Simply list the antibiotics that interfere with the bacterial cell wall
- beta lactams (carbapenems, penicillins, cephalosporins)
- glycopeptides
- bacitracin
- lipopeptide
- polymyxins
What do glycopeptides do?
Glycopeptides (e.g. Vancomycin) bind to the pentapeptide preventing PtG synthesis
What does bacitracin do?
Bacitracin inhibits bactoprenol regeneration preventing PtG transportation (bactoprenol transports PtG across the membrane)
What do beta-lactams do?
- bind covalently to transpeptidase
- inhibit PtG incorporation in the cell wall
- transpeptidase enzyme cross-links PtG pentapeptides
What do lipopeptides do?
- disrupt gram+ve cell membranes
- interfere with cell wall stability
- e.g. daptomycin
What do polymyxins do?
- interfere with cell wall stability
- bind to LPS and disrupt gram-ve cell membranes
Why is AB resistance a threat?
- ‘CATASTROPHIC’, ‘APOCALYPTIC’, ‘AS BIG A RISK AS TERRORISM’ UK Chief Medical Officer
- ~ 70% of bacteria developed resistance
- 25000 yearly death rate - Europe & US
What are the causes of AB resistance?
Unnecessary prescription
-> ~ 50% of antibiotic prescriptions not required
Livestock farming
-> ~ 30% of UK antibiotic use in livestock farming (and reducing)
Lack of regulation
-> OTC availability in Russia, China, India
Lack of development
-> Very few antibiotics in recent years (it is not very profitable for pharmaceutical companies to make drugs which are made to keep in case of emergency and try no t to use to prevent resistance) -> financially not favourable
Name the types of AB resistance
- destruction enzymes
- additional targets
- alteration of target
- alteration in drug permeation
- hyper production
Production of destruction enzymes as a resistance mechanism
- beta-lactamases hydrolyse C-N bond of the beta-lactam ring
- Example:
- Penicillins G & V -> Gram +ve
- Flucloxacillin (naturally resistant due to stereo hindrance) & Temocillin -> beta-lactamase resistant
- Amoxicillin -> Broad spectrum AB
- Gram -ve activity
- Co-administered with Clavulanic acid (then it is resistant to the beta-lactamases but not by itself)
What does broad spectrum mean in terms of antibiotics?
an antibiotic that acts on the two major bacterial groups, gram-positive and gram-negative, or any antibiotic that acts against a wide range of disease-causing bacteria.
Resistance mechanism: additional target
- Bacteria produce another target that is unaffected by the drug
- Example: E Coli produce different DHF reductase enzyme making them resistant to trimethoprim
Resistance mechanism: alterations in target enzymes
Alterations in target enzymes
- Alteration to the enzyme targeted by the drug.
- Enzyme still effective but drug now ineffective
- Example: S Aureus - Mutations in the ParC region of topoisomerase IV confers resistance to quinolones
Resistance: alteration in drug permeation
- Reductions in aquaporins
- & increased efflux systems
Examples
- Primarily of importance in gram –ve bacteria
Resistance: hyperproduction
- Bacteria significantly increase levels of DHF reductase
- Example: E Coli produce additional DHF reductase enzymes making trimethoprim less effective
What are the main categories of anti-fungal drugs?
- Azoles: Fluconazole
- Polyenes: Amphotericin
Fungal Infections
- Can be classified in terms of tissue/organs:
- Superficial - Outermost layers of skin
- Dermatophyte - Skin, hair or nails
- Subcutaneous - Innermost skin layers
- Systemic - Primarily respiratory tract
-> down the list the infection becomes more serious
Azoles
- Inhibit cytochrome P450-dependent enzymes involved in membrane sterol synthesis
- Fluconazole (oral) -> candidiasis & systemic infections
Polyenes
- Interact with cell membrane sterols forming membrane channels
- Amphotericin (I-V) -> systemic infections
Drug details (antifungals)
15 anti-fungal drugs licensed in the UK
Two most common categories:
- Azoles: Fluconazole
- Polyenes: Amphotericin
LO1: Antibiotic classes: identify the main classes of antibiotic drugs and distinguish between them in terms of mechanism of action
Intracellular targets
- Nucleic acids - Sulphonamides (DHOp), Trimethoprim (DHFR)
- DNA gyrase - Fluoroquinolones e.g. Ciprofloxacin
- RNA polymerase - Rifampicin
- Bacterial ribosomes – Macrolides (Aminoglycosides, Tetracyclines)
Cell membrane targets
- Peptidoglycan (PtG) synthesis - Vancomycin inhibits pentapeptide
- PtG incorporation - Carbapenems, Cephalosporins & Penicillins inhibit transpeptidase
- Membrane stability - Lipopeptides & Polymyxins
LO2: Antibiotic resistance: recall the mechanisms commonly used by bacteria to become resistant to drugs
- Destruction enzymes - beta-lactamases
- Additional target - Different DHFR enzyme
- Hyper-production - More DHFR enzyme
- Changes to target - Changing DNA gyrase
- Alterations in permeation - Increased efflux mechanisms
LO3: Anti-fungals: differentiate between the drugs used to treat fungal infections
- Azoles - Inhibit ergosterol production
- Polyenes - Bind to ergosterol and create pores
