Case 8- antibiotics Flashcards
Antibiotic
A medicine which inhibits the growth of or destroys bacteria
Bacteriostatic
An antibiotic which inhibits multiplication. By stopping the bacteria from growing you contain it till your immune system eventually destroys it
Bactericidal
A types of antibiotic which kills the bacteria, this is normally when the bacteria are reproducing
Antibiotic susceptibility testing
You grow a bacteria on a plate and add the antibiotic to see which kills the bacteria, This determines what antibiotic you should use for a specific pathogen
What do you look for in an antibiotic
- Selective toxicity- kills bacteria but not host
- A high LD50 vs a low MIC and/or MBC
- Favourable pharmacokinetics- reaches target site in body with effective concentration.
- Spectrum of activity- some are broad and kill a wide range of bacteria, whilst others are more narrow and only kill a few. Best to start on broad antibiotics and once you have identified the problem you go on to more narrow ones.
- Lack of side effects- it may not be tolerated if there are too many.
- Little resistance to development- The antibiotic needs to be stable
LD50
Stands for lethal dose 50, the concentration of a substance which would kill 50% of the population
MIC and MBC
MIC is the minimum inhibitory concentration and the MBC is the minimum bactericidal concentration. MIC is about inhibition, MBC is about killing. If you have a low MIC/MBC it means that you only need a low concentration of the antibiotic to stop bacterial growth, avoiding unwanted side effects
What do you need to decide before administering antibiotics?
- Is an antimicrobial agent necessary?
- Dosage and duration of dosage- You want to achieve 4 times the minimum inhibitory concentration (mic) at the site of action.
- Dose interval- don’t want to have to take it that many times a day.
- Route of administration- oral is the most usual and convenient. Parenteral can be used in hospital.
What can antibiotics target in bacteria
They inhibit cell wall synthesis, they can alter cell membranes, inhibit protein synthesis, inhibit nucleic acid synthesis and interfere with metabolic pathways.
Antibiotics- beta lactams
Penicillin, Cephalosporins and Methicillin. They inhibit cell wall synthesis by acting on the peptidoglycan in the cell wall. Later generation Beta-lactams have improved effectiveness against gram negative bacteria.
Antibiotics- inhibitors of cell wall synthesis
Include beta lactams and Vancomycin which is a glycopepetide antibiotic which is a last resort for MRSA. Inhibitors of cell wall synthesis work better against gram positive bacteria. They are bacteriocidal meaning they are effective against actively growing bacteria
Mechanism of action for antibiotics that inhibit cell wall synthesis
The peptidoglycan wall is made of a Polysaccaharide backbone with peptide crosslinks. The peptide crosslinks join with neighbouring peptide molecules and other polysaccharide molecules, this gives them mechanical strength. The peptide cross-links are formed by transpeptidases (Penicillin binding proteins). B-lactams bind to these proteins and inhibit their action. Peptide cross links are not formed, so the cell wall loses its mechanical strength. The membrane can bulge underneath the cell wall to form an emerging Spheroplast. Over time the bacteria loses its shape till it becomes spherical and is a spheroplast. It becomes vulnerable to osmotic change and undergoes cell lysis and bursts.
Beta-lactams in clinical usage
They can be broad spectrum penicillin’s i.e amoxicillin which are also effective against gram-negative bacteria. Another type is extended spectrum penicillin’s i.e. piperacillin which is also active against pseudomonads. You have reversed-spectrum penicillin’s which have a greater activity against gram negatives than gram positives
Antibiotics that inhibit protein synthesis- Chloramphenicol
Binds to the 50S portion of the ribosome and inhibits formation of a peptide bond. Causes the cell to stop growing and producing the proteins it needs
Antibiotics that inhibit protein synthesis- Erythromycin
Binds to the 50s portion of the ribosome and prevents translocation movement of the ribosome along the mRNA, this stops translation and protein formation
Antibiotics that inhibit protein synthesis- Tetracyclines
Interferes with the attachment of the tRNA to the mRNA-ribosome complex
Antibiotics that inhibit protein synthesis- Streptomycin
Changes shape of the 30s portion of the ribosome and causes the code on mRNA to be read incorrectly , a nonsense protein is formed
What types of antibiotics inhibit nucleic acid replication
Fluroquinolones
Ciprofloxacin
An antibiotic which inhibits nucleic acid replication acts by inhibiting the enzyme DNA gyrase, so the genome can not unwind. This prevents negative supercoiling of DNA which is important for normal replication or transcription. It is active against a broad range of bacteria. Resistance is beginning to emerge.
Metronidazole
A type of antibiotic that inhibits nuecleic acid formation. Prevents DNA synthesis. It is active against obligate anaerobic bacteria, oxygen is toxic to these bacteria. Metronidazole damages the electron transport proteins. It is also works against protozoa so is an antiprotozoal drug. It interacts with alcohol to give disulphiram-like reactions.
Selective toxicity
When you kill or damage the pathogen without affecting the host. You will target a substance which is present on the bacteria (70S ribosomes) and not on the host. So human cells are not damages
Intrinsic resistance- antibiotics
Occurs naturally and is chromosomally encoded. You treat with another antibiotic that the bacteria is susceptible to. Gram negative bacteria are intrinsically resistant to beta lactamase because their cell wall is thinner. Could be reduced bacterial permeability of bacteria to antibiotics or antibiotic efflux where the antibiotic enters the bacteria and is then pumped out.
Acquired resistance- antibiotics
Results from a mutation in the existing DNA of an organism or the acquisition of new DNA that carries resistance. Could be changes in antibiotic targets by mutation, modification of targets and direct modification of the antibiotic.
Major rule in antibiotic stewardship
Do not initiate antibiotic treatment in the absence of a bacterial infection
What information should you provide about antibiotic stewardship?
- Antimicrobial prescribing levels.
- Patient safety incidents i.e. Clostridium difficile or adverse drug reactions like anaphylaxis. Proper monitoring of the adverse effects of certain antibiotics.
- Education and training on the bad effects of widespread antibiotic usage as well as what’s the appropriate amount of antibiotics.
What should you do when prescribing antimicrobials
When prescribing antimicrobials, prescribers should follow local or national guidelines on prescribing the shortest effective course of the most appropriate dose and route of administration.
Why do antibiotics get less effective over time?
Bacteria eventually gain resistance
Why did the first antibiotics developed gain resistance?
The first antibiotics were naturally occurring so resistance may have developed in the wild and not through human usage
Antimicrobial resistance (AMR)- bad effect
It threatens the effective prevention and treatment of infections caused by bacteria, parasites, viruses and fungi. New resistance mechanisms emerge and spread globally. Bacteria will share resistance mechanisms so lots of bacteria can suddenly become resistant to an antibiotics. AMR bacteria are associated with worse clinical outcomes, death and they consume more healthcare recourses
Important AMR pathogens
- Vancomycin-resistant Enterococci (VRE)
- Methicillin-resistant Staphlococcus aureus (MRSA)
- Extended-spectrum Beta-lactamase (ESBL’s)- producing gram negative bacteria.
- Klebsiella pneumoniae carbapenemase (KPC) producing gram-negative bacteria
- Multi-drug resistant gram negative rods
- Multi-drug resistant tuberculosis
What causes AMR
Inappropriate use of antibiotics, poor antibiotic stewardship, inadequate diagnostic (using a broad spectrum antibiotic when a specific one might be better), excessive use in hospital and agriculture. Animals provide a high yield if they are given antibiotics.
Intrinsic mechanisms of antibiotic resistance- reduced permeability
Gram negative bacteria are more resistant to antimicrobials because the outer membrane forms an increased permeability barrier. However, hydrophilic antibiotics can gain entry through porins. The bacteria is able to sense the danger of the antibiotics and down regulate porin expression so that there will be less entry of the antibiotic. Mutation in porins cause less entry (acquired).
Intrinsic mechanisms of antibiotic resistance- efflux
Many gram negative bacteria can actively efflux pump out antibiotics that are normally effective against gram positive bacteria. Efflux pumps can be narrow or wide spectrum. There is a wide spectrum known as MDR (multiple drug resistance) efflux pumps which can remove lots of antibiotics.
Example of efflux system
E.coli has an AcrB/ AcrA/ TolC efflux system which is well characterised.
How do efflux system work
Most pumps use a proton gradient to drive out antibiotics from the cell. As a proton moves into the cell down its concentration gradient it uses the energy to pump the antibiotic out of the cell. Can also use energy from ATP or an Na+ gradient
Acquired mechanisms antibiotic resistance- direct modification
Destruction or addition of chemical groups that inactivate the antibiotic
Antibiotic resistance bet-lactam bacteria
Though direct modification that inactivates the antibiotic. Originally they where specific against one or a couple Beta-Lactam antibiotics but now they can be against a wide range. They secrete enzymes making the antibiotic inactive. So, now Extended spectrum B-lactamases (ESBL) secrete enzymes which active against many Beta-Lactams. ESBL pathogens include E.coli, K.pneumoniae and many others
Aminoglycoside
Can inhibit protein production interfering with 30s ribosome subunits, they are potentially useful in gram negative infection. But they are prone to inactivation by the addition of chemical groups by bacterial enzymes (Acetyltransferases, Phosphotransferases, Nucleotidyltransferases). They will add an acetyl group, a phosphate group and a nucleotide respectfully. They become antibiotic resistant through direct modification
Acquired mutations of antibiotic resistance- mutation of target
Most antibiotics bind to targets with a higher affinity. If you change the target via mutations the antibiotic cant bind. The target can carry on with normal function. By changing the structure of the bacteria, the antibiotic can not bind to it but it can still carry on with its normal function.
Acquired mutation of antibiotic resistance- modification of target
Does not require a mutational change, it could just be the addition of a chemical group. The antibiotic no longer binds to the bacteria. Happens a lot in antibiotics that target ribosomes
How do you get an intrinsic mutation for antibiotic resistance
Normally due to a chromosomal mutation i.e. in the bacteria’s own DNA
How do you get an extrinsic mutation for antibiotic resistance
Involves the transfer of genetic material between bacteria
The ways bacteria can acquire resistance through mobile DNA?
- Transfer of free DNA that contains antibiotic resistance genes by transformation.
- Transfer of bacterial DNA that contains antibiotic resistance via a bacteriophage - transduction.
- Transfer of bacterial and plasmid DNA between mating cells by conjugation.
Antibiotic resistance- transformation
It requires a recipient cell that is competent (can take up DNA). The antibiotic resistance gene can then move from the donor cell to the recipient cell and be integrated into the chromosome.
Antibiotic resistance- transduction
The bacteriophage infects the donor cells, takes the resistance DNA and then infects the recipient cell and donates the DNA containing antibiotic resistance.
Antibiotic resistance- conjugation
Requires direct contact between bacteria. Transposons are small jumping genes that often encode antibiotic resistance. It can jump from plasmid to chromosomal DNA and back again. The bacteria do not have to be the same species, many gram negative bacteria can transfer to other gram negative bacteria. Particularly problematic with gram negative B-lactam resistance.
Antibiotic resistance- propagation
In a mixed population of bacteria, some will have acquired resistance. When antibiotics are introduced to the bacteria, the antibiotic will select resistance bacteria by killing non-resistant bacteria. The antibiotic resistance bacteria will then proliferate. This is how antibiotic resistance propagate. Antibiotic resistance can spread in farms and healthcare settings due to the widespread use of antibiotics
NICE guidlines on antibiotics
- Prescribe the shortest effective course.
- The most appropriate dose- smallest effective dose
- Use the most appropriate route of admission
Nice- use of antibiotics
1) Take into account the risk of antimicrobial resistance for individual patients and the population as a whole
2) In hospital take sample before prescribing and review the prescription when results are available
3) In primary care for recurrent or persistent infections take samples when prescribing
4) For non-sever infections, consider taking microbiological samples before make a descision about prescribing, as long as its safe to withhold treatment till the results are available
What to look at in a patients history when prescribing antibiotics
- Possible interactions with other medicines, food, drink
- Patient’s other illness i.e. renal impairment
- Allergies
Broth dilution test
Innoculate the same amount of the bacterial isolate into different dilutions of the same antibiotic. The lowest dilution that the bacteria does not grow in is the Minimum Inhibitory Concentration. Typically carried out in 96 well microlitre plates
Disk dilution
You grow the bacteria on an agar plate, you then add different antibiotics and measure the radius which is free of bacteria, The widest radius is the best antibiotic
Antimicrobial gradient
A strip is put on an agar plate full of bacteria. The strip has increasing concentration of the antibiotics. The MIC is determined by the intersection of the organism growth with the strip. This is measured using the scale on the strips.
