Antimicrobial susceptibility testing

- basic elements -

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)

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

Disc difusion antimicrobial susceptibility test – Kirby Bauer -

Various sizes of growth inhibition around different antibiotics

Interpretation: zone size diametercompared to cutoffsfor each antibiotic-microbe pair

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)

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

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)

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