Lecture 17 - Antimicrobials III Flashcards
Antimicrobials that bind to 30s ribosomal subunit
Aminogycosides, tetracyclines
Antimicrobials that target 50s ribosomal subunit
Chloramphenicol, macrolides
What does chloramphenicol target?
Peptidyl transfer
What do macrolides target?
Translocation
What does mupirocin target?
Isoleucyl tRNA synthesis
What do oxazolidones target?
Formation of initiation complex
What was the first aminoglycoside?
Streptomycin
What is tobramycin good for?
Treating pseudomonas
What is gentamicin good for?
A general-purpose anti-G- antimicrobial
Why do aminoglycosides need to be injected?
Not absorbed through epithelium, so if ingested would only be active in the GIT
How are aminoglycosides administered?
injected
Are aminoglycosides bacteriostatic or bactericidal?
Bactericidal
Stages of aminoglycodside bactericidal action
1)
2)
1) Binds 30s subunit, makes recognition of mRNA codons defective. Abnormal proteins form.
2) Abnormal proteins cause membrane to become more permeable. More aminoglycosides enter bacterium, cause complete ribosomal blockade
Why does a small amount of aminoglycoside initially enter a bacterium?
Aminoglycosides are lipid-insoluble.
Have difficulty passing through cell membrane, cell wall
What is a benefit of amikacin?
Resistant to a lot of enzymes which modify aminoglycosides, so as to inactivate them
Three types of bacteria, with respect to growth under streptomycin
1) StrS - Susceptible
2) StrR - Resistant
3) StrD - Dependent
Why are StrD bacteria dependent on aminoglycoside presence?
Ribosomes are defective. Aminoglycosides correct this defect.
Ways that aminoglycosides are modified by bacteria
1)
2)
3)
1) Phosphorylation
2) Adenylation
3) Acetylation
Methods of microbial resistance to aminoglycosides 1) 2) 3) 4)
1) Efflux
2) Modified outer membrane leading to reduced entry
3) Enzymatic modification of aminoglycoside, leading to reduced entry
4) Modified 30s subunit
Example of resistance by drug inactivation
1)
2)
1) Beta lactams by beta lactamases (hydrolysis)
2) Covalent modification - aminoglycosides, chloramphenicol
Example of resistance by altering target of drug
1)
2)
1) Modify target to a less-sensitive form - Beta-lactams, vancomycin
2) Overproduce target - Vancomycin
Example of resistance by reducing drug access to target
1)
2)
1) Reduced entry into a cell - Aminoglycosides
2) Increased efflux from a cell - Aminoglycosides, tetracycline
Example of resistance by failing to activate drug precursor
1)
2)
1) Metronidazole
2) Isoniazid
How is metronidazole activated?
Is reduced in hydrogenosome
What is metronidazole effective aginst?
Obligate anaerobes
Examples of organisms susceptible to metronidazole
Giardia, entamoeba, trachomatis
Bacteria resistant to carbapenems
Enterobacteriaceae
Examples of innate resistance
1)
2)
3)
1) Mycoplasma - lack cell wall
2) Cell wall impenetrability (G- resistant to vancomycin)
3) Pseudomonas - beta-lactam resistance gene on chromosome
Two cycles of phage infection
1) Temperate cycle - gene integrates into genome, cell isn’t lysed
2) Lytic cycle
Example of bacterium with temperate cycle bacteriophage DNA
Vibrio cholerae, with choleratoxin gene from a phage
Difference in ability to share genetic information between conjugation and transformation
Transformation requires that bacteria are closely-related
Conjugation can occur between unrelated bacteria (even between G+ and G- bacteria)
Most important type of bacterial horizontal transfer
Plasmid-mediated
Ways to test bacterial antimicrobial resistance
1)
2)
1) Dilution tests
2) Diffusion tests
What do minimum inhibitory concentration tests test?
Bacteriostatic action
How do you test bactericidal action from a MIC test?
Take samples from inhibited samples, culture them on a gel without antimicrobials. See which grow.
This gives MBC (minimum bactericidal concentration)
MIC test
1)
2)
1) Doubling dilutions of antimicrobial of interest
2) Add bacteria of interest, see which grow, which are inhibited
Potential problems with dilution tests
1)
2)
1) Takes up space. If have to test many samples, can quickly run out of room in lab
2) Patient samples often contain several bacterial species
Solution to issues with dilution tests
Use diffusion tests
Disc susceptibility test
Advantages of disc susceptibility test
Can test several different antimicrobials at once (each on a different disc), on the one plate
Drawbacks of disc susceptibility test
Can’t directly compare diameters of inhibition, as different antimicrobials have different rates of diffusion
Two types of antibiotic susceptibility data
1) Two distinct populations on graph - Beta-lactamase
2) Continuous gradient of susceptibility - Altered penicillin-binding proteins
Break-points for pneumococci
Fully-susceptible MIC 1mg/L
How to determine disc susceptibility breakpoints
Chart MIC (from dilution test) for different bacteria on a graph On same axes, for the same bacteria, chart disc susceptibility test zone diameter.
Negative correlation between MIC and width of zone diameter
Two types of error in testing with MIC and disc susceptibility
1) False susceptible
2) False resistant
Worse type of error
False susceptible
Results in patient being given an antimicrobial that isn’t effective
Less severe type of error
False resistant
Results in patient being denied an effective antibiotic, but given a similarly effective one instead
What is a false susceptible result?
When there is a high MIC, but large zone diameter.
MIC correctly reflects bacterial susceptibility.
Will result in an ineffective antimicrobial testing as efective
What is a false resistant result?
When there is a low MIC, but a small zone diameter.
MIC correctly reflects bacterial susceptibility
Will result in an effective antimicrobial testing as ineffetive
Acceptable error for false susceptible
Under 1%
Acceptable error for false resistant
Under 4%
Drawbacks of E test strip
Expensive, because under intellectual property
