Susceptibility Calculators

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Antimicrobial susceptibility testing (AST) determines whether a pathogen is susceptible, intermediate, or resistant to specific antibiotics. The minimum inhibitory concentration (MIC) is the lowest concentration of antibiotic that inhibits visible bacterial growth and is the quantitative measure of susceptibility. AST results guide clinical treatment decisions — selecting effective antibiotics, optimizing doses, and detecting resistance patterns for infection control. The two main methods are broth microdilution (gold standard) and disk diffusion (Kirby-Bauer test).

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Minimum Inhibitory Concentration (MIC)

MIC = the lowest concentration of antibiotic that completely inhibits visible bacterial growth after 18–24 hours of incubation at 35°C. Expressed in μg/mL or mg/L.

MIC is determined by:

  • Broth microdilution: Two-fold serial dilutions of antibiotic in liquid medium + standard inoculum (~5 × 10⁵ CFU/mL). Incubate 18–24 hours. MIC = lowest concentration with no turbidity.
  • Agar dilution: Antibiotic incorporated into agar plates at different concentrations. Inoculate and read next day.

Disk Diffusion (Kirby-Bauer Test)

Antibiotic-impregnated paper disks placed on agar inoculated with the organism. After overnight incubation, the zone of inhibition (diameter in mm) is measured around each disk. Zone size inversely correlates with MIC. Zone size is compared to CLSI or EUCAST breakpoints to classify as S, I, or R.

Susceptibility Categories (CLSI)

  • Susceptible (S): Standard dosing is likely to be effective
  • Intermediate (I): May respond to high-dose therapy or body site accumulation
  • Resistant (R): Standard doses unlikely to be effective

E-Test (Gradient Strip)

An antibiotic-impregnated strip with a continuous concentration gradient. When placed on an inoculated agar plate, an elliptical inhibition zone forms. The MIC is read where the zone edge crosses the strip — combining ease of disk diffusion with quantitative MIC determination.

Glossary

Minimum Inhibitory Concentration (MIC)
The lowest concentration of antibiotic completely inhibiting visible bacterial growth after 18–24 hours at 35°C. Expressed in μg/mL. The quantitative measure of antimicrobial susceptibility — used to classify organisms as S, I, or R using CLSI/EUCAST breakpoints.
Disk Diffusion (Kirby-Bauer Test)
An antimicrobial susceptibility test using antibiotic-impregnated paper disks on agar inoculated with the test organism. Zone of inhibition diameter (mm) is compared to breakpoints to classify as Susceptible (S), Intermediate (I), or Resistant (R).
CLSI Breakpoints
Concentration thresholds (μg/mL) set by the Clinical and Laboratory Standards Institute defining Susceptible, Intermediate, and Resistant categories for specific antibiotic-organism combinations. Updated annually based on pharmacokinetic, pharmacodynamic, and clinical outcome data.

Frequently Asked Questions

MIC is the lowest concentration of an antibiotic that completely inhibits visible bacterial growth after 18–24 hours at 35°C. Expressed in μg/mL. Determined by broth microdilution (serial dilutions in liquid medium) or agar dilution. MIC is the quantitative measure of susceptibility — compared against CLSI or EUCAST breakpoints to classify organisms as Susceptible (S), Intermediate (I), or Resistant (R) to each antibiotic tested.

Antibiotic-impregnated paper disks are placed on Mueller-Hinton agar inoculated with a standardized inoculum of the test organism. After overnight incubation at 35°C, antibiotic diffuses outward from the disk — bacteria cannot grow where the concentration exceeds their MIC. The diameter of the clear zone of inhibition around each disk is measured in mm and compared to CLSI breakpoint tables to classify the organism as Susceptible, Intermediate, or Resistant.

Susceptible (S): the pathogen's MIC is at or below the susceptible breakpoint — standard antibiotic doses are expected to achieve concentrations that inhibit growth at the infection site. Resistant (R): MIC exceeds the resistant breakpoint — standard doses are unlikely to be effective even at maximum tolerated doses. Intermediate (I) (now often called 'Susceptible with increased exposure'): may be effective with high-dose therapy, when the antibiotic concentrates at the infection site, or via continuous infusion.

The E-test (Epsilometer test) uses an antibiotic-impregnated plastic strip with a predefined MIC gradient (e.g., 0.016–256 μg/mL). When placed on an inoculated agar plate, an elliptical inhibition zone forms. The MIC is read where the zone's edge intersects the strip — providing a quantitative MIC reading without the labor of serial dilutions. E-tests are used when precise MIC determination is needed for specific organism-antibiotic combinations, or when broth microdilution panels don't include the required antibiotic.