Lecture 6 Flashcards
How are antibiotics delivered in humans/animals
Humans:
- Pills
- IVs
- Injections
- Can be consumed via antibiotic treated meat
Animals:
- Cow feed
What are antibiotics and how are they used
- They’re a substance derived from a fungi, bacteria or other organism which can inhibit or destroy the growth of microorganisms
- Used in prevention/treatment of infectious disease
What are secondary metabolites
Complex organic molecules that are non-essential to organism growth or reproduction
Antibiotic resistance mechanisms
- Inactivation of antibiotic - enzymes degrade/modify antibiotic by hydrolysis of functional group
- Target group alteration - Alters antibiotic target to reduce binding often by mutations
- Target bypass - Function of antibiotic target accomplished by a new protein
- Increased/Decreased efflux - down/upregulation of porins in cell membrane
- Target protection - physical association of target protection protein with antibiotic target
Inhibition of cell wall synthesis by Beta-lactam antibiotics
- Cell wall composed of NAG-NAM chains cross linked by peptide bridges between NAM subunits
- New NAG/NAM subunits inserted into the wall by enzymes, allowing growth. Normally, other enzymes link new NAM to old NAM subunits via crosslinks
- Penicillin causes unattached NAM subunits
- Cell bursts from osmotic pressure as integrity of peptidoglycan not maintained
Resistance to beta lactam antibiotics
Beta lactamases - degrades beta-lactam ring, inactivating antibiotic
Bypasses antibiotic by providing alternative PBPs
For example:
Penicillin -> Penicilloic acid by disabling beta ring
Mode of action of different antibiotics
Streptomycin: Aminoglycoside that binds 30S subunit of ribosome to cause mRNA misread and incorrect amino acids
Tetracycline - Binds 30S subunit and blocks tRNA docking - stops amino acids being added
Chloramphenicol - Tetracycline that binds 50S subunit and inhibits peptidyl transferase - amino acids can’t be joined
Clarithromycin - Macrolide that binds 50S subunit and blocks mRNA movement - halts elongation
Antisense nucleic acids - bind mRNA and prevent ribosome attachment - no translation
Metabolic pathway in antibiotic action
PABA -> Dihydrofolic acid -> Tetrahydrofolic acid -> Purines and pyrimidines -> DNA/RNA
PABA -> dihydrofolic acid inhibited by sulfonamides
Names examples of antibiotics that kill/inhibit bacteria, fungi, protozoa, helminths and viruses
Bacteria: tetracycline, streptomycins etc
Protozoa/fungi - azoles
Helminths - Niclosamide, Praziquantel
Viruses - Acyclovir, Ribavirin
Antibiotic consumption in England
2007 - 35 million prescriptions (19M for penicillin)
Aim: 15% reduction by 2024
Bacterial resistance development
Drug-sensitive cell and drug-resistant mutant exposed to drug
Sensitive cells inhibited by drug
Remaining population grows over time
Antimicrobial drug resistance
Causes:
- Incorrect prescriptions
- Non-adherence
- Counterfeit drugs
- Anti-infective drugs in animals/plants
- Loss of effectiveness
- Community-acquired e.g TB, pneumococcal
- Hospital acquired e.g. MRSA, enterococcal
Consequences:
- Prolonged hospital admissions
- Higher death rates
- More expensive and toxic drugs
- Higher healthcare costs
Explain resistomes
Collection of antibiotic resistance genes and precursors in both pathogenic and non-pathogenic bacteria
Soil, air, water
Types of resistance
Passive - antibiotic has no target and cannot enter the cell. Antibiotic has no action
Mutation - Target site changes so antibiotic ineffective
Acquired - Actively acquired resistance e.g. via plasmids
Mutation in streptomycin
Streptomycin binds 16S rRNA of 30S subunit
Inhibits protein synthesis
Mutation in 16S rRNA gene by point mutation, insertion, inversion, deletion or duplication changes active site and renders streptomycin ineffective
Acquired resistance
Often caused by horizontal gene transfer from other organisms by transformation, transduction or conjugation
Often plasmid encoded aided by transposons/integrons
Horizontal gene transfer
Transformation - free DNA in environment and competent receptor - donor dead/alive
Transduction uses bacteriophages
Conjugation - Cell to cell contact - plasmid free in cytosol or on chromosome of living donor
Conjugation
- Donor pilus attaches to recipient
- Pilis retracts the two bacterial cells together and mating bridge forms
- Relaxome nicks plasmid at origin of transfer and DNA polymerase helps construct the complementary strand as ssDNA is transferred
- Plasmid copied during transforming so both cells end up with a copy
Modular mobile elements
- Mobile gene cassette carries 1 or more resistances
- Integron promotes dissemination of GCass - Drives Amr gene expression
- Comosite transposons are flanked by IS/IR, complex transposons has own transposase/rec’ase and conjugative transposon has own transfer
pFBAOT6
- IncU resistant plasmid
- 87748bp
- 94 coding sequences, 12 not assigned a function
- Core functions in first 31kb
- Genetic load region is 54kb - Class I integron, transposable elements, 43kb transposon
Why are cells in biofilms more resistant to antibiotics
- Reduced drug permeability
- Low metabolic activity
- Persister cells
- AMR gene exchange
MRSA overview
- Oppotunist pathogen - 25-30% population have S. aureus in system
MRSA is present in UK and US and in 1974 2% of hospital S. aureus infection
MRSA community and hospital infections are different
94,000 severe infections and 19,000 deaths a year via MRSA in 2007
What else is MRSA resistant to?
- Ampicillin, penicillins and cephalosporins
- Spread by horizontal gene transfer
- SA requires resistance to beta-lactam antibiotics through PBP2a
- Treated using vancomycin but resistant strains are already emerging
Percentage of hospital acquired infections that are MRSA 1999-2015
More MRSA infection in 2015 than 1999
Far less MRSA infections in 2015 than 1999 in UK due to MRSA reporting and prevention measures
