Antibiotic sensitivity testing Flashcards
What attributes makes an ideal antimicrobial agent?
- Effective
- non-toxic
- Broad activity
Differentiate between narrow spectrum and broad spectrum. Give examples
Narrow: effective against few kinds of pathogens
ex. Vancomycin only kills gram pos
Broad: effective against many kinds of pathogens
ex. Tetracycline
Define selective toxicity. What 2 things does it depend on
Harmful to the pathogen without being harmful to the host
Depend on:
1. Specific receptor function
2. Inhibition of biochemical events
What are the 5 main mechanisms of action of antibacterial agents
Inhibitors of:
1. Cell wall synthesis
2. Cell membrane function
3. protein synthesis
4. DNA and RNA synthesis
5. Metabolic processes
What is the antibiotic order requirements
- Dose
- Duration
- Indication
What are 3 conventional susceptibility testing methods
Disk diffusion
Broth Dilution
Agar dilution
Define minimum inhibitory concentration MIC
vs minimum bactericidal concentration MBC
Which value is usually higher?
MIC:
- lowest conc. that completely inhibits VISIBLE growth of organism
MBC
- lowest conc. that results in 99.9% REDUCTION in CFU/mL (death)
MBC is always a little higher
Which factors must be standardized when performing tests in the lab? (6)
- Bacterial inoculum size
- Growth medium used (non-selective, non-differential) (pH, cation conc, blood/serum supplements, thymidine content)
- Incubation atmosphere 37 deg
- Incubation temp
- Incubation duration
- Antimicrobial concentration
What are limitations and other factors not taken into account of standardization
Cannot perfectly replicate in vivo environment (in body)
Other factors:
- Antibiotic diffusion into tissues/host cells
- Serum protein binding of antimicrobial agents
- Status of patient immune system
- Virulence of infecting bacterium
How does Broth dilution testing occur?
- Varying concentrations of an antibiotic are tested on a bacteria (doubling dilution, different starting conc)
- Lowest conc of antibiotic that inhibits in vitro growth is MIC
- Based on the conc of the MIC, bacteria is classified as
a. Susceptible (low MIC)
b. Intermediate
c. Resistant (high MIC)
What are advantages and disadvantages of Broth dilution testing
Advantages
- quantitative and qualitative data
- MIC is useful to explain resistance level of a bacteria
Disadv
- Supply intensive (takes up space & resources)
- Some microorganisms don’t grow well in broth
Explain how agar dilution testing
- Series of agar plates, each with decreasing concentration of antibiotic
- Fixed number of bacteria is inoculated on each plate
- Incubation –> colony count
- Results
a. Susceptible (low colony count)
b. Intermediate
c. Resistant (high colony count)
Adv and Disadv of agar dilution testing
Adv
- useful for organisms that don’t grow in broth
Disadv
- VERY labour intensive
- some antimicrobial agents are not stable in agar media
Explain the disk diffusion method? What standardized agar medium is the most common?
Most common: Mueller-Hinton agar
- Dip swab in broth culture, remove excess liquid, swab entire agar in 3 directions (leave no streaks)
- Press antibiotic disks gently to agar with alcohol flamed forceps
- Label plates on the edges
- Invert plates and incubate
- Measure zones of inhibition
a. Susceptible (large diameter)
b. Intermediate
c. Resistant (small diameter)
Adv and disadv of disk diffusion methods
Adv
- Convenient
- User friendly
Disadv
- No interpretative criteria for organisms not on standard tables
- no precise data (need an MIC)
Explain the epsilometer test. Where is the MIC and MBC in this
Strip with decreasing conc of antimicrobial is placed on a plate of bacterial culture
MIC:
- the point where the zone of ihibition intersects with numerical scale
MBC
- the lowest conc of no bacterial growth at all
What tests are using for selection of reference breakpoints
- in vitro
- animal/human PK/PD
- clinical bacteriological outcome data
What x and y values are used in a scattergram to determine breakpoint
Zone diameter vs MIC
What do the 2 horizontal lines on the scattergram indicate?
from the MIC RESISTANT breakpoint to the SUSCEPTIBLE MIC breakpoint
What is the trend between MIC and Zone inhibition
As MIC increases.. Zone of inhibition decreases
What is the purpose of intermediate category
- Provide a buffer between resistant and susceptible categories to prevent interpretive errors
- Implies that susceptibility varies based on if the antimicrobial is concentrated at the site of infection
- Suggests that higher dose can be used when safe to achieve desired efficacy
Define antimicrobial battery
antimicrobial agents chosen for testing a bacterial isolate (“a battery of tests”)
what is an antibiogram useful for? (2)
- Show trends in antibiotic resistance
- Can help making initial antibiotic treatment selection for a patient before individual susceptibility results are available
What are dangers of indiscriminate antibiotic use?
- Changes in the normal flora of the body –> superinfection
- Direct drug toxicity
- Drug resistance development:
-through elimination of drug-sensitive microorganisms from antibiotic saturated environments (hospitals) and replace with drug-resistant microorganisms
