M: Antibiotic Action 2 - Week 9 Flashcards
Describe the mechanism of action of aminoglycosides (2). What does this give rise to? (3)
Aminoglycosides bind to the cytosolic, membrane-associated bacterial ribosome. Aminoglycoside presence in cytosol disturbs peptide elongation at the 30S ribosomal subunit, giving rise to:
- inaccurate mRNA translation
- misreading of the mRNA code
- biosynthesis of truncated or proteins with altered amino acid composition
In regards to Aminoglycosides, describe the following:
A: Bactericidal or Bacteriostatic?
B: What is the spectrum of microorganisms they work on
C: Level of toxicity
D: When to use
A: bactericidal
B: gram -ve aerobic bacteria (also gram +ve bacteria)
C: significant toxicity
D: reserve for serious infections
Why don’t we generally use aminoglycosides to treat gram +ve bacteria?
Because other, less toxic antibiotics are more suitable
Name 4 aminoglycosides
- gentamicin
- tobramycin
- amikacin
- neomycin
In regards to Tetracyclines, describe the following:
A: How do they work/mechanism?
B: Bactericidal or Bacteriostatic?
C: Level of toxicity
D: How often is it used? What does this suggest?
A: bind 30S subunit and prevent alignment of tRNAs
B: Generally bacteriostatic
C: Low toxicity
D: Widely used, suggesting that resistance may be a problem
Name one condition that tetracyclines are useful for. Is there anything important to note here?
useful for Chlamydia. Note: that we may need to treat at other sites, so topical treatment may be added to the oral treatment
Name 3 side-effects that can occur with tetracyclines
- nausea
- phototoxicity
- discoloration of teeth in children under 6 yrs old
In regards to Chloramphenicol, describe the following:
A: How does it work/mechanism?
B: Bactericidal or Bacteriostatic?
C: Level of toxicity
D: Broadness of spectrum of microorganisms it affects
E: Form of administration
A: inhibits peptidyl transferase, blocks chain elongation
B: Bacteriostatic
C: significant toxicity
D: Broad spectrum, highly effective
E: topical preparations available
Name one potential side effect of Chloramphenicol
irreversible aplastic anaemia
In regards to Macrolides, describe the following:
A: Name 2 of them
B: Mechanism of action
C: Broadness of spectrum
A: Ezithromycin, Azithromycin
B: Bind 50S subunit, prevent translocation of peptides
C: Broad spectrum
Name 2 antimicrobial agents that are particularly useful in treating chlamydia
- tetracyclines
- macrolides
For antimicrobial agents that target the nucleic acid of microorganisms: name the 3 ways they can act, and provide an example for each
1: Inhibition of synthesis of precursors - sulphonamides, trimethoprim
2: Inhibition of DNA replication - quinolones
3: Inhibition of RNA polymerase - rifampicin
How can antimicrobial agents inhibit the synthesis of nucleic acid precursors? i.e. What pathway do they act on?
by acting on microbial folate synthesis
How do the following agents act to inhibit nucleic acid precursors:
A: Sulphonamides
B: Tremethroprim
A: Sulphonamides - inhibit folate synthesis by enzyme inhibition
B: inhibits folate required for the synthesis of purines and pyrimidines by enzyme inhibition
In regards to Quinolones:
A: What part of DNA replication process do they affect?
B: What is this part responsible for?
C: Are quinolones bactericidal or bacteriostatic?
D: Describe their spectrum of activity
A: DNA gyrase
B: is required to supercoil the bacterial DNA
C: bactericidal
D: Good activity against pseudomonas, but resistance is rapid and increasing in isolates from other sites
Name 2 examples of fluoroquinolones
- ciprofloxacin
- ofloxacin
Describe the spectrum activity and resistance levels of fluoroquinolones. How do we maintain maximal efficacy of fluoroquinolones?
Broad spectrum activity with relatively little microbial resistance.
- to maintain maximal efficacy of these preparations, fluoroquinolones should not be used when alternative, equally effective agents could be used instead
List 6 questions you should be asking when considering the use of an antimicrobial agent
- is it an infection?
- is an antibiotic actually needed?
- what is the likely pathogen, and which are the major antibiotics active against this pathogen?
- with regard to these antibiotics, which are the safest and narrowest spectrum?
- how should the antibiotic be provided (dose, interval, duration?)
- if one antibiotic is good, are more better?
Provide 3 valid reasons to use antibiotic combinations
- To cover the range of organisms which might reasonably be expected to be present
- To achieve synergy (e.g. a penicillin + an aminoglycoside for resistant pseudonomas infection)
- To cover the possibility of the presence of antibiotic resistant mutants in the population (e.g. in mycobacterial infections)
What does the therapeutic guidelines recommend for treatment of blepharitis caused by staph? (4)
Lid hygeine: remove debris, warm compress, gentle scrub of lashes, antibiotic efficacy uncertain (chloramphenicol 1% ointment 1-2x daily, 1wk)
What does the therapeutic guidelines recommend for treatment of chronic blepharitis assoc. with rosacea? (2)
Lid hygeine, doxycyline orally (daily for 8 wks)
What does the therapeutic guidelines recommend for EMPIRICAL treatment of conjunctivitis? (2)
“Delayed prescription” approach: chloramphenicol or framycetin drops for empirical treatment
What does the therapeutic guidelines recommend for treatment of conjunctivitis? (when indicated/non-empirical) (3)
gentamycin, tobramycin, and quinolone drops when indicated
What does the therapeutic guidelines say for the treatment of conjunctivitis associated with chlamydia? (1)
successful treatment requires systemic therapy with azithromycin (orally)
What does the therapeutic guidelines recommend for treatment of HSV keratitis?
Acyclovir 3% ointment (5x daily, 14 days)
Describe 3 methods of antibacterial treatment for bacterial keratitis. Under what situations do you use either of these treatments?
- ciprofloxacin 0.3% eye drops: 1 drop into affected eye, every hour (incl. overnight)
- oflaxacin 0.3% eye drops: 1 drop into affected eye, every hour (incl. overnight)
- cefazolin 5% + gentamycin 0.9% eye drops: 1 drop into affected eye, every hour (incl. overnight)
- these topical treatments should be used in the case that referral is delayed
How do we ensure that bacterial keratitis patients are compliant with their topical antibacterial treatment? Why might there be an issue with compliance in the first place?
Since patients are required to instill 1 drop every hour, even overnight, this can be difficult to manage as the patient would have to wake up every hour when sleeping.
- to resolve this, we generally make the patient stay in a hospital overnight, so the nurses can apply the drop hourly
Does oral or intravenous antibiotic therapy have a role in the management of bacterial keratitis?
No, unless there is spread of the condition to the sclera
Name 4 consequences that could arise from not using antimicrobials correctly
- Treatment failures (due to inappropriate antimicrobial choice or antagonistic drug-drug interactions)
- Side effects
- Cost/Expense
- Increased level of antimicrobial resistance
Name 3 antibiotics that bind the 50s subunit of the ribosome
- macrolides
- chloramphenicol
- clindamycin
Name 2 antibiotics that bind the 30s subunit of the ribosome
- aminoglycosides
- tetracyclines
Define Minimal Inhibitory Concentration (MIC)
The lowest concentration of an antimicrobial agent that will inhibit the growth of an organism under test over a suitable defined time interval
Define Minimal Bactericidal Concentration (MBC)
The lowest concentration of an antimicrobial agent that will kill an organism under test over a suitable defined time interval
Name 2 methods for measuring Minimal Inhibitory Concentration (MIC)
- Tube method
- Disc diffusion method
Explain the tube method for measuring MIC (3)
- dilutions of the antibiotic under test are made in a suitable broth, and a standard number of bacteria from the patient are added
- after overnight incubation, tubes are scored for growth
- the last dilution of antibiotic which inhibits growth is the minimal inhibitory concentration (MIC)
Explain the disc diffusion method for measuring MIC (3)
- a standard number of bacteria from the patient are inoculated onto an agar plate
- discs impregnated with appropriate concentrations of different antibiotics are added
- after overnight incubation, zone diameters are measured and correlated with MIC
How does MIC correlate with disc diffusion zone diameter?
The smaller the diameter, the more resistant the microbe and so the higher the minimal inhibitory concentration (MIC)
Define the following terms:
- Susceptible
- Intermediate
- Resistant
Susceptible: infection with that organism will respond to treatment of recommended dose of an antimicrobial
Intermediate: infection will be treated if higher than recommended dose or if infection localised in body region where the drug is physiologically concentrated
Resistant: antimicrobial does not treat infection within the range of achievable doses
For what MIC values would pseudomonas aeruginosa be recognised as:
- susceptible to ticarcillin antimicrobial?
- resistant to ticarcillin?
Susceptible: if MIC less than or equal to 64mg/ml
Resistant: if MIC greater than or equal to 124mg/ml
For what MIC values would pseudomonas aeruginosa be recognised as:
- susceptible to tobramycin?
- Intermediate to tobramycin?
- resistant to tobramycin?
Susceptible: If MIC less than or equal to 4mg/ml
Intermediate: If MIC is 8mg/ml
Resistant: If MIC is greater than or equal to 16mg/ml
Why is the recommended dose (and therefore MIC values) for tobramycin so low?
Because tobramycin is a relatively toxic drug. Any higher would be too toxic
List 4 mechanisms of resistance to antimicrobial agents
- Drug inactivation
- Altering the target of drug action
- Reduce access of drug to target
- Failure to activate inactive precursor of drug
Name 2 ways microbes can inactivate drugs
- by hydrolysis (e.g. beta-lactams)
- by covalent modification (e.g. aminoglycosides, chloramphenicol)
Name 2 ways microbes can alter the target of drug action
- modify target to a less sensitive form (e.g. b-lactam, vancomycin)
- overproduce target (e.g. sulphonamides, trimethoprim)
Name 2 ways microbes can reduce access of a drug to its target
- reduce entry into cell (e.g. aminoglycosides)
- increase efflux from cell (e..g aminoglycosides, tetracyline)
Provide 2 examples of when microbes cause failure to activate an inactive precursor of a drug
- e.g. metronidazole, isoniazid
Describe 3 ways microbes can acquire resistance to beta-lactams
- Produce beta-lactamases
- Alteration of PBPs (penicilling binding proteins)
- Reduced entry into cell
Where do gram +ve vs gram -ve bacteria secrete beta lactamases once produced?
Gram +ve: into extracellular space
Gram -ve: into periplasmic space (located b/w the cytoplasmic membrane and the outer membrane)
Name 2 bacteria that are able to alter PBPs (penicillin binding proteins)
- some staphylococci e.g. MRSA
- strep. pneumoniae
What does bacterial alteration of PBPs result in? Does this effect the eye?
causes resistance at other sites, but not in the eye
What type of bacteria are able to reduce the entry of beta-lactams into the cell?
Some gram -ve bactria
Explain the function of the enzyme Beta-Lactamase
it breaks a bond in the beta-lactam ring of penicillin (or other beta-lactam). As a consequence, the penicillin no longer looks like D-ala-D-ala and therefore no longer acts
How can bacteria acquire resistance to glycopeptides e.g. vancomycin? Explain how (2 ways). Name a bacteria that can do this (2)
Through alteration of the target: the terminal D-ala-D-ala is mutated into D-ala-D-lactate (can occur in enterococci)
Or through production of excess target (peptidoglycan) (can occur in staphylococci)
When bacteria acquire resistance to glycopeptides, does transpeptidation still occur?
yes
What type of resistance does the production of excess target (peptidoglycan) by staphylococci confer?
Intermediate resistance (to glycopeptides)
Describe 4 mechanisms of bacterial resistance to Aminoglycosides
- Enzymatic modification: leading to inactivation of aminoglycoside
- Modified outer membrane: leading to reduced re-entry of the aminoglycosides
- Efflux of the aminoglycosides by bacteria
- Ribosomal mutations: leading to blinding of the aminoglycoside
List 3 enzymatic modifications that can be made to aminoglycosides
- acetylation
- adenylation
- phosphorylation
How does a bacteria acquire resistance to chloramphenicol? (2)
- bacterial enzyme ‘chloramphenicol acetyl transferase’ (CAT) acetylates the drug at 2 sites
- the acetylation of chloramphenicol renders it inactive, preventing it from binding the ribosomes
Describe 2 methods a bacteria can use to resist Macrolides
- Increased efflux from cell: macrolides are pumped out of cell
- Modification of target: via methylation of 23s ribosomal RNA, which renders the 50s subunit resistant to drugs
Describe the predominant method bacteria can use to resist tetracyclines
- predominantly by increased efflux from the cell
Describe 2 methods a bacteria can use to resist Quinolones
- Increased efflux from cell (seen in both gram + and -)
- Modification of target: mutation in DNA gyrases and topoisomerases causes reduced binding of the antibiotic
Why should fluoroquinolones not be used when alternatives exist?
Because fluoroquinolones have broad-spectrum activity with relatively little microbial resistance. Alternatives should be used when possible in order to maintain maximal efficacy of fluoroquinolones
How do antibiotics select for cells that are resistant? (3)
Antibiotics select for cells which are resistant due to:
- innate insensitivity
- mutation of selective genes
- acquisition of genetic material by: transformation, transduction, conjugation, transposition
List 3 intrinsic components to the genetic basis of resistance
- cell wall impermeability (vancomycin and gram -ves)
- lack of target
- chromosomal resistance gene (pseudonomal beta-lactamase)
List 2 acquired components to the genetic basis of resistance
- mutation
- horizontal gene transfer
Name 4 ways how horizontal gene transfer b/w bacteria occurs
- Transformation
- Phage-mediated transduction
- Plasmid-mediated conjugation
- Transposition
List 5 potential features of resistant cells/bacteria
- prevent entry of drug
- pump drug out of cell (efflux)
- inactivate drugs
- modify the drug target
- use an alternative pathway
