Antimicrobial Chemotherapy Flashcards
Antibiotics: not just for humans
• ~3/4 of global antibiotic use is in livestock
• Use on plants is common but accounts for less amounts
• Purpose: prevention and treatment of disease, growth promotion
Penicillin, the 1st antibiotic
1928 Sir Alexander Fleming
Penicillium notatum “mold”
1939-40 Isolation of Penicillin
Sir Howard Florey & Ernst Chain
Isolation of Penicillin
Sir Howard Florey & Ernst Chain
First use of penicillin
World War II ~1942-1945
Coconut Grove Fire, Boston 1942
492 people died First use of penicillin
Streptomycin
• Selman Waksmam (1943-44)
• produced by the soil bacterium Streptomyces griseus
• 1st antibiotic discovered through a systematic screen
• Screen of 10,000 strains of soil bacteria and fungi for antibacterial activity
Antimicrobials: why natural products?
Antibiotic have been made by bacteria for over 40 millions years
Antibiotic-producing micro-organisms: soil organisms such as Actinomyces (fungi) and Streptomyces (mold)
• Most antibiotic classes originated from natural compounds
-Streptomycin (aminoglycosides)
-Tetracyclines
-Daptomycin (lipopeptides)
Bactericidal vs static activity
Not an absolute distinction as the killing effects vary with the methods used, doses and bacteria species
AST
Antibiotic susceptibility testing
-key diagnostic test in clinical microbiology lab to determine antibiotic susceptibility and identify resistance in bacterial pathogens causing infections
-Also used in the lab (e.g. to determine antibiotic selection)
-Can be qualitative or quantitative depending on the method
-Specific MIC values (thresholds) are used to determine Resistance vs Susceptibility for each bacterial specie x drug combination
Cons of AST
• Requires culture from pure bacterial colonies - cannot be done directly on clinical samples
• is labour intensive and slow (up to 72h from beginning of sample processing)
Common test of AST
Etest diffusion test
Kirby Bauer disc diffusion test
Dilution susceptibility test
MIC
Minimal Inhibitory Concentration
• MIC: minimal concentration of a drug that inhibits growth of a particular organism.
• High MIC = Resistant.
• Multiple different methods: most measure the zone of growth inhibition, or the dilution of drug that inhibits visible growth
MIC are widely used measures of antibiotic susceptibility in clinical microbiology labs and define whether a bacteria/fungus strain is resistant or not to a particular agent
mechanism of action
• target [cell-wall, ribosome, etc.]
• spectrum of activity [gram positive / gram negative] (some only target one of these)
• bacteriostatic vs. bactericidal
• resistance (next class)
chemical structure
• synthetic / semi-synthetic / natural product
• delivery [oral vs. injected vs. topical]
• side effects
Antibiotics targets
DNA replication: Quinolones
Cell wall synthesis: Beta-lactam
Protein synthesis: Aminoglycosides and Macrolides
Beta lactams
• Cell wall synthesis inhibitor
• Bactericidal antibiotic
• Common beta-lactam ring
• Several different classes with different spectrum of activity
• Beta-lactams mimic the 3D structure of the dipeptide D-Ala-D-Ala component of peptidoglycans, compete binding with penicillin binding protein
• Generally well tolerated with limited side effects
Ex. Penicillins, Cephalosporins, Monobactam, Carbapenem
Many of the cell wall inhibitors are not active against gram-negatives
because they can’t penetrate the outer membrane
Prokaryotic protein synthesis
Prokaryotes: 70S ribosomes
• 30S small subunit
• 50S large subunit
Eukaryotes 80S ribosomes
• 40S small subunit
• 60S large subunit
Antibiotics are mostly selective for prokaryotes, and thus display “limited” side-effects when used as drugs
Major protein synthesis inhibitors
30S inhibitors: Tetracyclines, Aminoglycosides
50S inhibitors: Macrolides
Aminoglycosides
• Bind to 30S ribosomal subunit
• Bactericidal activity
• Several downstream mechanism of action
-Inhibitors protein translocation
-Induce mistranslation leading to loss of cell-wall integrity
• Broad spectrum against gram negative and gram positives but inactive against obligate anaerobes (because need a cell wall transporter that lack in obligate anarobes)
• Somewhat toxic to human, limit clinical use due to side effects
• Can only give by injection
How to choose antibiotics
• Spectrum: narrow vs broad
• Mode of delivery: oral, injectable or intravenous
• Dosing: pharmacokinetics and pharmacodynamics
• Antibiotic resistance
• Clinical indication: disease vs microbiology-driven choice
• Clinical efficacy: clinical studies, microbiological potency, pharmacokinetic
• Side effects and toxicity
• Cost and availability
Antibiotics good or bad
Profound disturbances to the microbiota
Short-term effects:
• fungal/yeast overgrowth
• Clostridium difficile infection (colitis)
Mid-term effects:
• Antibiotic resistance
Long-term effects: ???
Antibiotics crisis
Globally
Now
• >1.3M yearly deaths directly attributable to antibiotic resistance
By 2050
• health cost >10M deaths annually
• global economic cost $100 trillion
Canada
• ~1 in 4 infections are already resistant to first line antibiotics
A post-antibiotic era
an end to modern medicine as we know it.
Origin of antibiotics
Part of evolution
Human behavior and environment accelerates the emergence, selection and spread of antibiotic resistance
Origin of antibiotic resistance II
Fortuitous advantageous mutations
Examples:
• Mutations of antibiotic target (such as gyrB): direct
• Mutation of regulator that increases expression of efflux pumps: indirect
Expropriating genes
• Kidnapping resistance mechanism from antibiotic producing organisms (such as beta- lactamase, ermC)
• Subverting enzymes to perform resistance tasks
• Plasmids encoding one or many resistance genes are important in spreading resistance
How does antibiotic resistance spread within a bacterial population
Vertical Gene Transfer
Resistance trait passed on from one generation to another, with clonal expansion. Resistance trait can be a mutated gene, or resistance genes can be encoded on chromosome or plasmids
Horizontal Gene Transfer
Resistance trait transferred to a different bacteria
Transfer occurs mostly by conjugation of plasmids, but can also occur by transformation in naturally competent bacteria
Resistance to any antibiotic can arise VERY quickly
Mechanisms of antibiotic resistance
-Impaired受损的 influx
-efflux
-target mutation
-target modification
-drug modification & drug degradation
Specific antibiotics can have multiple mechanisms of resistance
Specific mechanisms can confer resistance to more than one class of antibiotics
Resistance due to low permeability
gram-negative bacteria and mycobacteria（even lower permeability than gram negative）
hydrophobic and impermeable and acts as a selective barrier
Hydrophilic molecules and some hydrophobic molecules must enter through size-limited pores
Primarily responsible for intrinsic resistance
Intrinsic resistance
Min resistance value of one species
Antibiotic resistance due to drug efflux
• Antibiotics are pumped out of the cell (outer and inner membrane) and therefore cannot reach their targets
• Major mechanism in gram negative bacteria
• Complex “multi-purpose” efflux pumps
• Can cause multi-drug resistance
• Can be upregulate by antibiotics
Antibiotic resistance due to target mutation or modification
• Can be due to genetic mutation of the target gene
-gyrB (topoisomerase) mutations and quinolone resistance
• Can be due to enzymatic modification of the target structure
-Erm (methyl transferase) modification of 50S subunit and macrolide resistance
• Can be due to acquisition of a variant target with low affinity to the antibiotic
-Penicillin Binding Protein PBP2 with low affinity penicillin
Beta-lactam degradation by beta-lactamases
• Enzymes secreted in periplasm that are highly effective at cleaving beta-lactam rings
• Different beta-lactamases exist with different spectrum… now there are extended spectrum (ESBL) that can cleave cephalosporins and metallo-beta-lactamases can cleave carbapenems.
Concerns: can be horizontally transferred
Trying to make beta-lactamases inhibitor, give the inhibitor with antibiotics as the treatment
Aminoglycosides modification
• Inactivation of aminoglycoside by adding additional moieties:
-AMP by adenylyl transferases
-PO3 by phosphoryl transferases
- acetyl by acetyl transferases
• This prevent proper binding of aminoglycosides to the 30S subunit
How human behaviour promote antibiotic resistance
• Antibiotic overuse
-Health, food and animal industries
• Antibiotic misuse
-Improper indications
e.g treating viral infections with antibiotics
• Insufficient length of treatment.
E.g. Tuberculosis 结核病
Antibiotic use and overuse
livestock: enter environment
human pee: enter environment
india: cheap antibiotics
Move around the world
Bring antibiotics to everywhere
How to find new antibiotics
What type of compounds?
• Improve existing compounds: chemical modification or combine therapy
• Natural products
• Synthetic compounds
• New targets
• Unknown targets but known drugs
How to get a new compound?
• Isolate and purify natural compounds
• Chemical or biological synthesis
• FDA-Approved non-antimicrobial drugs
The antibiotic pipeline
• No new drugs were approved between 2003 and 2011
• No new classes approved in >20 yrs
• No new targets
• Not that many drugs
• Minimal products from big pharma
Penicillins
Beta lactum
Cephalosporins
Beta lactum
Monobactam
Beta lactum
Carbapenem
Beta lactum
Streptomycin
Aminoglycosides
Gentamicin
Aminoglycosides
Used a lot for selection
Amikacin
Aminoglycosides
