This document discusses antimicrobial susceptibility testing, which determines how well a microbe will respond to different antibiotic drugs. There are two main methods: disk diffusion testing, which uses disks containing antibiotics placed on an agar plate, and minimum inhibitory concentration (MIC) testing, which finds the lowest concentration of antibiotics that prevents microbial growth. The results of these tests guide the selection of effective antibiotic treatment based on whether a microbe is susceptible, resistant, or intermediately resistant to various drugs.

1. Antimicrobial susceptibility
testing
- basic elements -

2. Definition and Purpose
• In vitro tests that measure the growth of an
isolated microbe in the presence of particular
drug or drugs (antibiotics) in order
to predict the in vivo success or failure of
antibiotic therapy
• The results - guide the choice of antibiotics (+
clinical information and experience)

3. Methods for antimicrobial susceptibility
testing (1)
• Disk difusion (Kirby Bauer):
– swab bacterial suspension on agar plate
– place small filter paper disks impregnated with
a standard amount of antibiotic
– incubate overnight
– zone of inhibition of bacterial growth =
measure of susceptibility (predefined cutoffs):
• large zone = susceptible
• small / no zone = resistant
• zone between the above = intermediate

4. Disc difusion antimicrobial susceptibility test
– Kirby Bauer -
Various sizes of
growth inhibition
around different
antibiotics
Interpretation:
zone size diameter
compared to cutoffs
for each
antibiotic-microbe
pair

5. Methods for antimicrobial susceptibility
testing (2)
• MIC (minimum inhibitory concentration) =
minimum concentration of antibiotic that will
inhibit the growth of a microorganism
Measurement methods:
- Broth dilution
- E-test (Epsilometer test)

6. MIC (continued): Measurement methods
Broth dilution:
- Bacteria inoculated in culture broth + antibiotic (various
concentrations) and incubated
- MIC = the lowest concentration of antibiotic which
inhibited bacterial growth

7. MIC (continued): Measurement methods
E-test:
- plastic strips impregnated with
decreasing antibiotic
concentrations (log scale)
- strips applied on agar
microbial culture and
incubated
- eliptic inhibition zone intersects
the MIC value scale (μg/ml) at
the level of MIC
- shape resembles Greek letter
”Epsilon” (ε)

8. Methods for antimicrobial susceptibility
testing (3)
• MBC (minimum bactericidal concentration) = minimum
concentration of antibiotic that will kill 99.9-100% of the
microbes of a tested bacterial strain
- Less used than MIC; more time consuming
- Indicated in:
- severe infections in immunocompromised patients,
- infections in anatomic sites hard to reach with antibiotics e.g.
endocarditis, osteomyelitis