34 Attacking the enemy: antibiotics Flashcards
Interaction between host, microbial pathogen, and antimicrobial agent can be considered as a triangle, with one side inevitably affecting the other sides.
Antimicrobial agents have selective toxicity, what is this?
Antimicrobial agent should act on target site of infection organism, which is absent on host cells.
Easier in prokaryotes, as they are very different. But eukaryotes are more similar to host.
Antiviral agents need to be able to enter host cells without damaging, then attack virus
What are desired antimicrobial properties of ideal drug?
Selective toxicity for microbes, not host
Cidal activity - kills pathogen
Slow emergence of resistance
Narrow spectrum of activity - usually preferred as less resistance/ microbiota disturbance. However sometimes broad spectrum required for polymicrobial infections e.g meningitis
What are desired pharmacological properties of antimicrobial agent
Selective toxicity
Long plasma half life - e.g for once daily dosing
Good tissue distribution e.g CSF
Low plasma protein binding
Oral and parenteral dosing forms
No interaction with other drugs
Antibiotics strictly means natural metabolic product of fungi/ bacteria which kill other microbes.
Many antibiotics are synthetic, or semi-synthetic, so term anti-microbial preferred.
How were antimicrobials discovered in the past, and how has this been modernised?
Previously through random screening of soil microbes to assess for antimicrobials.
Now computer modelling and genomics informs of potential drugs by identifying target sites
What are three ways of classifying antibacterial agents?
Bactericidal or bacteriostatic
By target site
By chemical structure - does not have any practical use alone, but when combined with target site allows us to group antibacterials into specific families
Antibacterial agents can be classified by target site, what are the five main targets
Cell wall synthesis
Cell membrane function
Nucleic acid synthesis
Protein synthesis
Metabolic pathways
Define antibacterial resistance
Organism will not be inhibited or killed by antibacterial agent at concentrations of drug achievable in body after normal dosage.
Some are naturally resistant, as they may not actually have target site for antibacterial to work, or naturally produce an enzyme which inactivates antibiotic
What are three ways in which drug resistance can evolve?
Chromosomal-mediated resistance - mutant selection
Plasmid-mediated resistance - spread of resistant plasmid
Plasmid-mediated resistance on transposon - spread of resistance gene
How does chromosomal mutation result in resistance?
Single chromosomal mutation can result in altered protein e.g ribosome, cell wall which changes target site
Multiple mutations can completely change proteins such as penicillin binding proteins (PBPs) in penicillin resistant pneumococci
These selective advantages allow bacteria to survive and outgrow competition. Can cross-infect other patients
How does plasmid transfer confer resistance?
Plasmid can contain genes which can provide resistance. This is very efficient, and does not rely on chance such as mutation. Multiple genes can be transferred.
Resistance can spread through population quickly
How do transposons confer resistance?
Transposons are “jumping genes”, and in replicative process can be integrated into other areas of DNA or a plasmid
If in plasmid, can be rapidly spread throughout population
Resistance genes can be transferred on chromosome, plasmids, or transposons found in both locations.
In some cases, multiple resistance genes may come together in structure known as integron casette. This can move into variety of DNA molecules, or act indepdendently as mobile genetic element
What do integron cassettes contain?
Excision/ recombination enzyme such as integrase, which allows excision and integration of cassette
Resistance genes
A promoter which directs transcription of cassette-encoded genes
What is name of staphylococcal methicillin resistance cassette
Staphylococcal genes for methicillin resitance are organised into unique cassette termed SCCmec
What are three broad mechanisms of antimicrobial resistance
Altered target site - lowers affinity for drug
Altered uptake -
- altering entry of drug (reduce cell wall permeability)
- pump drug out of cell (efflux)
Drug inactivation - enzymes which modify or destroy antibacterial agent
- e.g beta lactamases, aminoglycoside-modifying enzymes, chloramphenicol acetly transferases
Peptidoglycan component of cell wall is specific to bacteria, so can provide selective toxicity.
Peptidoglycan syntehsis begins in cell cytoplasm, moves to cytoplasmic membrane, then subunits attach to growing peptidoglycan chain.
Which antibiotics are cell wall inhibitors?
Beta-lactams - penicillin, carbopenems, cephalosporins, monobactams
Glycopeptides
Bacitracin - savilon
Fosfomycin
Beta-lactams is a large family of different antimicrobial compounds, all containing beta lactam ring.
What is beta-lactam mechanism of action?
Which organisms do beta lactams-not work on?
Bind to penicillin binding proteins (PBPs) prevent cross-linking of bacterial cell wall
Those without cell wall e.g mcoplasma
Impermeable wall e.g TB
Intracellular pathogen e.g brucella, chlamydia, legionella
What drugs are part of beta-lactam family?
Penicillin
Cephalosporins - cefalexin, cefuroxime, ceftazidime
Carbapenems - meropenem
Monobactam - aztreonam
Cephamycins - cefoxitin
Resistance to beta-lactams can occur in three ways - altered target site, resistance in access to target site, production of beta lactamases (drug inactivation)
How does MRSA become resistant?
Synthesise additional Penicillin-binding protein (PBP2a) which has lower affinty for beta lactams, and co can continue cell wall synthesis when other PBPs are inhibited
mecA gene
Other bacteria such as strep pneumoniea, neisseria gonorrhoea, hameophilus an also utilise PBP changes to resist beta-lactams
MRSA also produces beta-lactamase
How do gram negative cells resist beta-lactams?
Beta-lactams normally diffuse through porins in outer membrane. Mutation in porin gene results in decreased permeability of outer membrane, generating resistance.
Also gains cross-resistance to unrelated antibiotics that use same porins
How to beta-lactamases provide resistance
Hydrolyse beta-lactam ring, to yield inactive compound
Hundreds of beta-lactamses exist, some are more specific to certain beta-lactams. Some drugs are hydrolysed by very few enzymes (carbapenems), whereas others (ampicillin) are much more easily inactivated
What is ESBL and why is it bad?
Extended-spectrum beta-lactamases
Can broadly inactivate most beta-lactam compounds, so difficult to treat
Gene can be carries in plasmids
How does co-amoxiclav inactivate beta-lactamases?
Clavulanic acid contains beta-lactam rings, and act as suicide inhibitors, binding to beta-lactamses and prevent them from destroying beta-lactams
Little bactericidal activity on its own
Other drugs like tazocin have piperacillin + tazobactam. Tazobactam inhibits beta lactamases
What are toxic effects of beta-lactams
Type 1 hypersensitivity reaction occurs in 0.5% - 4% of patients, although anaphylaxis occurs up to only 0.04% occasions
Rash is common
If allergic to penicillin, often allergic to cephalosporins as well
Neurotoxicity and seizures can occur if beta-lactams improperly doses for body weight/ kidney function - unconsciousness, mycolonic spasms, hallucinations
What is general spectrum of action of these beta-lactams
Penicillins
Cephalosporins
Carbapenems
Monobactams
Penicillins - mostly gram positive, some gram negative cover. Tazocin has good gram neg cover
Cephalosporins - gram positive. But 4th/5th generation have activity against gram negatives
Carbapenems - gram positive and negative
Monobactams - gram negative
Give examples of different generations cephalosporins
First - cephalexin
Second - cefuroxime
Third - cefotaxime, ceftazidime, ceftriaxone
Fourth - cefepime
Fifth - ceftolozane, ceftaroline
Glycopeptides include vancomycin and teicoplanin. Very large molecules, so cannot penetrate gram negative cell walls. Not absorbed by GI tract or cross BBB as large molecules. Can be used for meningitis as increased permeability of BBB. Excretion via kidney
What is their mechanism of action?
Bactericidal
Inhibit cell wall synthesis by binding to terminal
D-alanine-D-alanine at end of pentapeptide chain in growing bacterial cell wall, preventing further subunits joining to cell wall
Glycopeptides act at earlier stage on cell wall production that beta-lactams, so not useful to combine these.
When are glycopeptides indicated?
Treatments of gram positive bacteria, especially those resistant to beta-lactams e.g MRSA
Allergy beta-lactams
C. Difficile - although risk of promoting emergence of glyocopeptide-resistant enterococci in gut flora
What are side effects of glycopeptides?
Red man syndrome - histamine release if administered too quickly. Can cause lip swelling and hypotension - anaphylactoid reaction
Nephrotoxicity
Ototoxicity
What are mechanisms whereby organisms can be resistant to glycopeptides
Gram negative naturally resistant to glycopeptides as too large to move through outer membrane
Acquire resistance genes -
- vanA - transmissible
- vanB - transmissible
- vanD - non-transmissible (chromosomal change)
VRE associated with plasmid uptake of resistance genes
Has also been seen in staphylococci
Which antibiotic classes are inhibitors of protein synthesis
30S inhibitors 2
50S inhibitors 6
Inhibit RNA synthesis from DNA 2
30S inhibitors
Aminoglycosides
Tetracyclines
50S inhibitors Chloramphenicol Fusidic acid Lincosamides Macrolides Oxazolidinones (linezolid) Streptogramins
Inhibit RNA synthesis from DNA
Rifampicin
Fidaxomycin
Which protein inhibitors prevent mRNA formation from DNA?
Fidaxomycin
Rifampicin
Both do not act on ribosome.
They inhibits DNA-dependent RNA polymerase
Which protein inhibitors prevents binding of new tRNA to acceptor site?
Tetracyclines
Act early on protein production pathway - inhibit 30S subunit
Which protein inhibitors prevents formation of peptide bonds/ chain elongation?
50S inhibitors
Chloramphenicol Fusidic acid Lincosamides Macrolides Oxazolidinones Streptogramins
Act later on protein production pathway - inhibit 50S subunit
Aminoglycosides interfere with ribosome 30S subunit, preventing binding of fmet-tRNA.
What are examples of these three classes of aminoglycosides
- Streptidine-containing
- 4,6-distrubted 2-deoxystreptamines
- 4,5- dissubstitued 2-deoxystreptamines
- Streptidine-containing -
- streptomycin. Oldest one, now just used for TB treatment
- 4,6-distributed 2-deoxystreptamines -
- gentamicin - broad spectrum
- tobramycin - similar gent, better against pseudomonas
- amikacin - useful if resistant to gentamicin
- 4,5- disubstitued 2-deoxystreptamines
- neomycin - too toxic for parenteral use, useful topically for decontamination e.g burns/ ulcers
How are aminoglycosides administered?
Do they penetrate bone/ tissue/ BBB?
How are they excreted?
IV/ IM
Occasionally streptomycin intrathecal for TB meningitis
Not absorbed well from gut
Do not penetrate tissue and bone
Do not cross BBB
Renally excreted
What are side effects of aminoglycosides?
Nephrotoxic
Ototoxic
What are uses for aminoglycosides
Only use in severe infection
Gram neg septicaemia including:
- pseudomonas
- HAI - e.g HAP/ VAP
- IE
- device infections - catheter, cannula
- pyelonephritis
- intra-abdominal infection
What do aminoglycosides not work on?
Streptococci
Anaerobes
What are mechanisms by which organisms develop resistance to aminoglycosides?
Aminoglycoside-modifying enzymes inactivate antibiotic. Carried on plasmids. Most common mechanism
Can alter ribosome protein so cannot bind
Can alter cell wall permeability, so cannot cross cell membrane
Tetracyclines are bacteriostatic.
What are examples of these?
Which ones give less side effetcs?
Tetracycline Doxycycline Demclocycline Minocycline Tigecycline
Doxycycline/ minecycline are more completely absorbed in GI tract, so give less side effects.
What are tetracyclines mechanism of action?
Inhibit protein synthesis by binding to small ribosomal subunit preventing tRNA binding
Can work on eukaryotic and prokaryotic cells, but uptake much greater in prokaryotes, so tetracyclines have more selective action
How are tetracyclines excreted?
Bile
Urine
What is activity of tetracyclines?
Can penetrate host cells, so useful against intracellular bacteria e.g chlamydia, mycoplasma, rickettsiae
What is mechanism by which organisms become resistant to tetracyclines?
Why is tetracycline resistance widespread
Resistance gene carried on transposon
Allows bacteria to efflux drug out of cell
Resistance widespread, as drugs used commonly, and also used in cattle as growth promoters in animal feed
When should tetracyclines be avoided?
What are side effects
Pregnancy/ Children under 8 - brown staining teeth in fetus/ children
GI upset
Photosensitivity
What is mechanism of action of chloramphenicol?
Prevents peptide bond synthesis by binding to 50S subunit, inhibiting peptidyl transferase
Bacteriostatic
How is chloramphenicol administered?
Topically
Orally
IV
Well dsitrbuted in body, and penetrates host cells
