L24 & L25. Antimicrobials Flashcards
What are the three broad types of sources for antibiotics?
- Natural
- Synthetic
- Semi synthetic
Antimicrobial agents are classified in three major ways: what are these?
- Source: natural vs. synthetic
- Broad mechanism of action (static vs. cidal)
- Pharmacological class
What are the different ways antibiotics are able to affect bacterial growth?
Bacteriostatic: cause the cessation of growth (replication) without killing or destroying the viability of the bacteria. They enter premature stationary phase
Bactericidal: cause the bacteria to die
Is the use of bactericidal vs. bacteriostatic drugs important?
In most situations the use is not important or distinguishable.
Except in the case of immunocompromised individuals: bacteriastatic drugs are both ineffective and dangerous because the host is unable to clear the static bacteria
What are the tetracyclines? What is the mechanism of action?
A very strong class of antibiotics that are usually used as the last resort option. They are a class of broad spectrum antibiotics
Bacteriostatic class that inhibits bacterial protein synthesis by reversible binding to the 30S subunit of the ribosome
What are the beta-lactam antibiotics?
A class of antibiotics that were the first class discovered They attack the cell wall of bacteria by interfering with peptidoglycan synthesis.
Describe the development of the beta-lactam class of antibiotics
Penicillin G was the first class of Abx discovered: was non toxic so had no side effects. But it was acid labile so could only be injected
Penicillin V was modified to be acid stable so could be oral administered
Ampicillin was developed to be used against gram negative rods
Methicillin was developed in response to S.aureus resistance
Flucoxacililin was the same but with a lower toxicity than methicillin
Carbenicillin was developed against pseudomonas
What is meant by selective toxicity?
It is important for Antibiotics to act on and damage the microbe but not cause damage to the host cells
Some non-selective antibiotics are useful for cancer treatments
What are the major bacterial cell targets for antimicrobials? [5]
- The cell wall
- Cytoplasmic membrane
- Ribosomes
- Nuclei can acid (different genomes)
- Folic acid synthesis pathways
What is the peptidoglycan layer composed of?
Repeating disaccharide units (sugars, one of which is unique to bacterial cells) and peptide cross bridges
Describe the peptidoglycan synthesis pathway of gram positive bacteria
Precursors of the PG layers are synthesised from intermediates in the cytoplasm
The blocks become immobilised on the inner aspect of the plasma membrane and synthesis continues till complete
The complete block is exocytosed (energy dependent) where it is linked to the growing PG chain
A P-P-lipid on the building block binds to the membrane while the D-ala-D-ala links the block to a Glycine on the PG chain
What is important to realise about antimicrobials that act on the synthesis of the PG layer?
These antimicrobials act on ACTIVE processes of the bacteria no thus require the bacteria to be living and synthesising in order to be effective.
Thus administering these drugs in combination with bactericidal or static antibiotics renders these drugs useless
What are the Glycopeptide antibiotics? Give an example
A class of antibiotics that binds directly to the terminal D-ala-D-ala and thus PREVENTS PG CROSS LINKING (elongation)
Eg. Vancomycin
Why is vancomycin not effective on gram negative bacteria?
Because it is a very big and highly charged molecule and thus it cannot permeate the outer membrane
How did Enterococci (gram positive cocci) develop resistance against vancomycin? (VRE)
Replaced the D-ala-D-ala with D-ala-D-lac such that the drug cannot recognise and bind to the chain.
This is encoded by different genes
Vancomycin resistant Enterococci are not considered a major threat to healthcare for a majority of the population. Why? And how do they indirectly cause problems?
Enterococci are not serious pathogens, causing diseases that are not normally fatal to humans.
The problem is if the VRE is able to MRSA causing VRSA
What are the Vancomycin Intermediate Staph Aureus (VISA)?
These are bacteria that are able to make more cell wall components and material to act as a sponge to mop up all the antibiotics so that the actual wall is not harmed.
This is overcome by adding higher concentrations of the drug to outnumber the increased amount of PG but there is a limit in terms of TOXICITY
How do the beta lactams act on the cell wall?
Inhibit enzymatic activity that is necessary for the PG synthesis to occur.
Eg. penicillin and its derivatives
Draw the beta lactam ring structure. Why is it important?
It is important as it mimics the bacterial D-ala-D-ala bond
What is the mechanism of Action of beta lactams?
Target a group of enzymes called he PENICILLIN BINDING PROTEINS anchored in be cell membrane involved in cross linking the bacterial cell wall components. Penicillin binds to them and prevents their ability to act as a catalysing enzyme.
This then leads to death of the cell (BACTERICIDAL) as enzymes are locked and the bacteria senses issues and becomes suicidal with cell swelling and hypertonic until bursting
Why is the antibacterial spectrum of penicillin widely diverse?
Because different bacteria have different penicillin binding proteins and some bacteria have the ability to secrete beta lactamases
What is the main mechanism of resistance to beta lactams conferred by some bacteria?
Some bacteria are able to produce enzymes that destroy the antibiotics Called BETA LACTAMASES that bind to them and destroy the ring.
This enzyme is either plasmid encoded or chromosome encoded
What is the difference between the plasma encoded and the chromosome encoded enzyme beta lactamase. Give examples
PLASMA:
Transferred from bacteria to bacteria
Occurs in 80-90% of staphylococci, >50% of e.coli and up to 50% of haemophilus spp. And N.gonorrhoeae.
CHROMOSOME:
All bacteria have chromosomal beta lactamases but their ability to be resistance depends on amount produced.
Pseudomonas aeuroginosa strains are all INNATELY resistant to early penicillins
What is another mechanism by which bacteria acquire resistance to beta-lactam drugs?
Some bacteria are able to synthesise a new penicillin binding protein (transpeptidase) that is able to carry out the enzyme processes that would normally be inhibited by penicillin.
What is an example of antibiotics that attack the cytoplasmic membrane? Why isn’t this the best target?
Polymyxins
Bacterial cell membranes are very similar to host membranes ie. are very toxic
What are the major antimicrobials that attack the bacterial membranes?
Aminoglycosides (gentamicin, tobramycin, amikacin) and chloramphenicol and tetracyclin
What are the different stages of protein synthesis can drugs act on? Give examples of each stage [5]
- Recognition (aminoglycosides and tetracyclines) - leads to wrong incorporation of amino acid
- Peptidyl transfer (chloramphenicol)
- Translocation (macrolides)
- Isoleucyl-tRNA-synthesis (mupirocin)
- Formation of the initiation complex (oxazolindones)
Aminoglycosides are highly charged and cannot easily penetrate the cell wall. How do they work if their mechanism of action is on ribosomes in the cytoplasm?
Low concentrations of the drug is able to get through the wall and interfere with the structure of the cell. Thus initial interference weakens the wall and allows more drugs so get across. This then leads to cessation of growth
= 2 step action
How is resistance to aminoglycosides achieved? [4 main mechanisms]
- Mutations in the ribosome to prevent drug interaction
- Enzymatic modification of the wall (reduced entry)
- Increased efflux mechanism (ATP pump) pumping out the drug before it can act
- Modifying the outer membrane (not common)
What is important about the mutations in ribosomes and the wall in conferring resistance to aminoglycosides?
They are enzymatic modifications that cannot be overcome by adding more drugs
What classes of drugs affect the nucleic acids affect DNA folding and nucleic acids?
Rifamycins (transcriptions)
Quinolones (DNA folding)
Why is folic acid synthesis a target for antibiotics?
Humans are unable to synthesise their own folic acid (get it from the diet) and so only bacteria contain the mechanisms and synthetic pathways for it: selectivity
What are some examples of antibiotics that interfere with the synthesis of folic acid?
Sulphonamides
Trimethoprim
What are the major mechanisms used by antimicrobial agents to confer resistance? [4]
- Drug inactivation (hydrolysis or covalent modification of the drug)
- Altering the drug target
- Reducing the access of the drug to the target
- Failure to activate a precursor of the drug (prodrugs)
What is an example and the mechanism of action of that example of a drug that requires activation in the body?
Metronizadole: very active against anaerobic organisms
Requires activation by reduction: catalysed by nitro reductase, an enzyme synthesised only by anaerobic organisms.
Resistance: altering the reductase
List some examples of antibiotic resistant bacteria
MRSA Penicillin Resistant Pneumococci PRP VRE VISA Carbapenem resistant Enterobacteriacae CRE Multidrug resistant Gram Negative Rods Hypervirulent Clostridium difficile Multidrug resistant M.tb (MDR-TB) and Extensively resistant XDR-TB
What is the difference between intrinsic and acquired resistance?
Intrinsic means the bacteria has its own chromosomally encoded anti-antibiotic agents
Acquired means the bacteria were once susceptible bus has had a change in genetic make up to make it resistant (mutation or horizontal transfer)
What kind of bacteria is intrinsically resistant to Vancomycin and why?
All Gram negative bacteria
Because the outer membrane protects the PG layer
What drug is P. aeruginosa intrinsically resistant to? Why?
Early penicillin
Because it has chromosomally encoded beta lactamase
Why is E.coli (and some other microorgansims) inherently resistant to Sulphonamide and trimethoprim?
Because these drugs attack the folic acid synthesis pathway
Some drugs like E.coli do not synthesis folic acid and are thus resistant in the same way we are.
What are the three major ways of transferring genomic information between bacteria?
- Transformation
- Phage Mediated Transduction
- Plasmid-mediated
Explain the process of transformation
Fragments of donor DNA are taken up by COMPETENT cells and homologous combination occurs to get the piece of fragment DNA iintegrated into the genome
What is meant by competent bacteria?
Some bacterial cells are able to take up DNA (competent) while others are not
Why must HOMOLOGOUS recombination occur? What does this imply about what bacteria are able to transform to/from one another?
Homologous recombination only occurs between related bacteria (to be able to reorganise itself, cut and replicate and incorporate into the genome). Thus genetic transfer can only be down between cell of the same type and strain.
What is the most resistant type of bacteria?
Normal microbiota because we are always taking antibiotics and thus they are resistant to many host environments and mechanisms as well as antibiotic mechanisms
Explain the process of bacteriophage transduction
Bacteriophages: viruses that infect bacteria. They have a tail that is able to penetrate bacterial cell walls and inject enzymes and DNA into the bacteria. The enzymes enable integration such that homologous recombination is NOT required.
Give examples of phage bacteria
Diptheria and cholera toxins are both encoded by phage DNA
Shiga Toxin makes E.coli 0157 a killer pathogen (haemorrhagic E.coli) - important as a temperate phage and the addition of antibiotics causes a switch to virulent and makes it worse
What are the two types of phages, explain them
Temperate Phage: use the machinery to replicate itself and then the phenotype encoded by the phage is expressed and the bacteria is UNHARMED
-can be encouraged to become virulent
Virulent Phage: replicates itself in the bacteria using bacterial mechanisms and then lyses the bacteria from within and the phage leaves the cell to repeat the process
What is the basis for phage typing? What does this mean for Bacteriophage transfer?
Bacteriophages are restricted to the range of bacteria they are able to infect.
Thus phage DNA is able to be transferred between bacteria usually of the same species only.
What is abnormal phage and what happens to this?
Occasionally an abnormal phage is formed (very rare by mutation) and it is useless because it cannot replicate. But it can potentially still be injected and transferred to other cells and potentially encode and transfer resistance
Describe plasmid mediated conjugation
Plasmids are independently replicated loops of DNA that can use the bacterial mechanisms to express its genes.
Conjugation is when plasmids move from one bacterial cell to another
How does a plasmid move from one cell to another?
Physical contact is required between cells.
The plasmid encodes a cytoplasmic bridge which links the cells together and the plasmid makes a copy of itself as it passes through the bridge and DNA is transferred
Some plasmids are non-conjugative but often utilise a bridge made by another plasmid to get across
What types of bacteria can plasmid transfer occur?
Between ANY two UNRELATED bacteria as long as the donor is conjugative
What is a multi-resistance plasmid? Why are they dangerous?
Plasmids that have found a way to gradually incorporate genes into them by INTEGRONS (cassettes) inserting next to each other and a SINGLE PROMOTOR is able to initiate the expression of all of the,
1 plasmid = multiple resistance
Treating the patient with one kind of antibiotic can cause the bacteria to sense a problem and begin to express the plasmid making the bacteria WAY more resistant
