Exercise 6: Antibiotic Susceptibility Testing Flashcards
Where were antibiotics derived from?
Naturally-occurring antibacterial substances usually secreted by soil fungi
Explain the difference between MIC and MBC
MIC - is the lowest concentration of antibiotic required to inhibit growth
MBC - is the lowest concentration of antibiotic required to kill microorganisms
For an antibiotic to be effective, the MBC should be within two doubling dilutions of the MIC
What are the main methods used to test for the MIC?
- Tube dilution method (uses visible turbidity as the index of growth)
1. Add an antibiotic solution to the first tube
2. Add TSB to all remaining tubes
3. Serially dilute the antibiotic solution
4. Finally, add culture into each tube - Disc diffusion method
1. A uniform lawn culture of an organism is made on ISO-Sensitest agar
2. Antibiotic discs are placed on the agar
3. Zone around disc = SENSITIVE
No zone around disc = RESISTANT
Give the target site in the bacterium for penicillin
Peptidoglycan in the cell wall
Explain whether you would use the same range of concentrations for all antibiotics when determining the MIC?
No you would not as each antibiotic varies in strength, meaning that each antibiotic would have a unique concentration at which it inhibits the growth of a bacterium.
Distinguish between an inhibitory and a cidal effect
Inhibitory effect - inhibits growth of microorganism
Cidal effect - kills microorganism
Suggest how the inhibitory effect of the penicillin solution used in the MIC is neutralized when transferred onto NA in the MBC procedure.
- The NA has nutrients, so it supports the growth of the organism
- The antibiotic also gets diluted after it has been streak plated
Comment on the relationship between the MIC and the MBC of an antibiotic. Ideally, should they be similar or dissimilar?
- They should be relatively similar - within two doubling dilutions of each other
- The MBC should be at a higher dilution than the MIC
Under what circumstances might the tube dilution method be preferable to a disc diffusion test, given that the MIC and the MBC is more complex?
- Tube dilution method determines both the MIC and MBC
- The disc diffusion method does not give MIC/MBC; only demonstrates how sensitive/resistant bacteria is to an antibiotic
In the disc diffusion test, if penicillin and tetracycline both have the same zone of inhibition, does this mean that the organism is equally susceptible to each?
No, as the zone of inhibition depends on a variety of factors including:
- Concentration of antibiotic
- Rate of diffusion of antibiotic particles in agar
Explain why the concentration of each antibiotic in the disk is different?
The organism being used and the medium which it is being cultured on both factor into deciding which concentration of antibiotic is most effective against the organism.
Discuss the need for standardization and controls in the antibiotic susceptibility tests
They enable comparison between samples and reproducibility in further experiments
Discuss the action of penicillin on bacterial cells.
- Is a type of β-lactam that acts on the cell wall of gram positive bacteria (as the LPS of gram negative bacteria prevent access of penicillin to their cell wall)
- It inhibits the cross-linking between the layers of peptidoglycan by binding to penicillin-binding proteins, thereby interfering with cell wall sythesis and causing the bacteria to lyse under osmotic pressure
Are E. coli and S. epidermis sensitive or resistant to Penicillin and Tetracycline?
- E. coli - Is resistant to penicllin (due to it being a gram negative bacterium with LPS in its outer membrane); sensitive to tetracycline
- S. epidermis - Is sensitive to both penicillin and tetracycline
