Antimicrobial Susceptibility Testing Flashcards
Destroys microorganisms by inhibiting their development or action
Antimicrobial agents
Produced by microorganisms that inhibit the growth of other microorganisms
Antibiotics
Agents that inhbit the growth of microorganisms but relies on the host immune system to follow through to destroy organisms
Bacteriostatic agents
Agents that kill the microbe and causes cell lysis
Bacteriocidal agents
Agents with limited spectrum of action. Example: penicillin
Narrow spectrum
Spectrum of activity of penicillin
Gram positive bacteria
Agents that action against gram positive and gram negative bacteria. Example: tetracycline
Broad spectrum
Disadvantage of using broad spectrum agents
They also destroy/inhibit normal flora
Infections that results afater the treatment of the primary infection
Superinfection
3 organisms capable of superinfection
- Pseudomonas aeruginosa
- Candida albicans (fungus)
- Staphylococcus aureus
2 types of bacterial resistance
- intrinsic resistance
- acquired resistance
Type of bacterial resistance. Characteristic of organism before antibiotic use
Intrinsic resistance
Bacteria that has intrinsic resistance to novobiocin
Staphylococcus saprophyticus
4 bacteria that have intrinsic resistance to polymyxins
- Proteus
- Providencia
- Edwardsiella
- Morganella
Type of bacterial resistance that is necessitated by prior exposure to the antimicrobial agent. May result from chromosomal mutations or from plasmids.
Acquired resistance
3 ways for the transfer of resistance
- conjugation
- transduction
- transformation
3 specific mechanisms of resistance
- modification of the target site
- inactivation of the antimicrobial agent
- decreased permeability of the cell wall
Medium used with Kirby-Bauer Disk Diffusion Test
Mueller-Hinton Agar
Specific disk diffusion susceptibility testing method which uses MHA and paper disks impregnated with specific antibiotics are place on the agar and then the zone of inhibition is measured
Kirby-Bauer Disk Diffusion Test
An indicator of the relative susceptibility of of the organism to the antibiotic in the Kirby-Bauer disk diffusion test
Diameter of the zone of inhibition
4 types of results from Kirby-Bauer disk diffusion test
- susceptible
- resistant
- intermediate
- moderately susceptible
Result in the KB disk diffusion test. The antibiotic appears to be effective against the bacteria because growth is inhibited.
Susceptible
Result in KB disk diffusion test. The antibiotic appears to be ineffective against the bacteria tested because growth is not inhibited.
Resistant
The lowest concentration of antibiotic that inhibits in vitro bacterial growth
Minimum Inhibitory Concentration (MIC)
Relationship of the diameter of the zone of inhibition to the logarithm of the MIC of the antibiotic
Inverse linear relationship
Peptone source of Mueller-Hinton Agar
Casein hydrolysate
Component of MHA that neutralizes fatty acids
Cornstarch
Effect of increased concentration of calcium and magnesium ions on aminoglycosides
Decreased activity of aminoglycosides against P. aeruginosa
Effect of increased concentration of calcium and magnesium ion on tetracycline
Decreased activity of tetracycline against all bacteria
Effect of increased pH on tetracycline activity
Decreased tetracycline activity (false resistance)
Effect of decreased ph on tetracycline activity
Increased tetracycline activity (false susceptibility)
Ideal pH of the MHA
pH 7.2-7.4
Effect of thicker agar depth on zone of inhibition
Smaller zones (false resistance)
Effect of thinner agar depth on zone of inhibition
Larger zones (false susceptible)
Ideal agar depth of MHA
4 mm
Standard where the density of the inoculum is adjusted to
0.5 McFarland Standard
3 components of the McFarland Standard
- barium
- chloride
- sulfuric acid
Effect of inoculum that is too light
False susceptible
Effect of inoculum that is too heavy
False resistant
How many antibiotic disks are placed on the agar plate?
12 different disks (8 disks forming an outer ring and 4 disks in the center)
Temperature at which antibiotic disks are stored for long periods of time
-14 degress Celcius
Temperature for working supply of antibiotic disks
2-8 degress Celcius
Incubation period for the KB disk diffusion test
35 degree Celsius for 16-18 hours in ambient air
