Antibiotic Sensitivity Test

Antibiotics are those compounds which inhibits the activity of bacteria. There are different types of antibiotics. Each type is only effective against certain bacteria. An antibiotic sensitivity test can help find out which antibiotic will be most effective in treating your infection.

Antibiotic sensitivity is the susceptibility of bacteria to antibiotics. Antibiotic susceptibility testing (AST) is usually carried out to determine which antibiotic will be most successful in treating a bacterial infection in vivo.

The goals of testing are to detect possible drug resistance in common pathogens and to assure susceptibility to drugs of choice for particular infections Testing for antibiotic sensitivity is often done by the Kirby-Bauer method. Small wafers containing antibiotics are placed onto a plate upon which bacteria are growing.

If the bacteria are sensitive to the antibiotic, a clear ring, or zone of inhibition, is seen around the wafer indicating poor growth. Other methods to test antimicrobial susceptibility include the Stokes method, E-test (also based on antibiotic diffusion). Agar and Broth dilution methods for Minimum Inhibitory Concentration determination.

Ideal antibiotic therapy is based on determination of the actiological agent and its relevant antibiotic sensitivity. Empiric treatment is often started before laboratory microbiological reports are available when treatment should not be delayed due to the seriousness of the disease. The effectiveness of individual antibiotics varies with the location of the infection, the ability of the antibiotic to reach the site of infection, and the ability of the bacteria to resist or inactivate the antibiotic.

Some antibiotics actually kill the bacteria (bactericidal, whereas others merely prevent the bacteria from multiplying (bacteriostatic) so that the host’s immune system can overcome them. Mueller-Hinton agar is most frequently used in this antibiotic susceptibility test.

Mueller-Hinton agar is also the standard medium used for most broth dilution testing and the conditions of this medium (i.e. pH. Cation concentration and thymidine contents) are well maintained. MHA is best medium for routine antibiotic susceptibility testing (AST) because of the following reasons:

  1. MHA shows acceptable batch-to-batch reproducibility for susceptibility testing
  2. MHA supports satisfactory growth of most non-fastidious pathogens
  3. MHA is low in sulfonamide, trimethoprim, and tetracycline inhibitors (i.e. concentration of inhibitors thymidine and thymine is low in MHA)

Aim For AST

To test the antibiotic sensitivity of the isolate from the clinical sample.

Requirements For AST

  1. Pure test culture suspension
  2. Sterile Mueller-Hinton a gar plates
  3. Poly-discs for Gram positive and Gram negative organism
  4. Spreader and Alcohol


  • Spread 0.1 ml of Pure culture suspension on sterile Mueller-Hinton agar plates.
  • Place the specific poly-discs on the surface of inoculated plates.
  • Incubate the plates at 37°C for 24 hours.
  • Record the results by observing the zone of inhibition and compare the results with CLSI guidelines for recording sensitivity of the isolate.


People Also Ask About Antibiotic Sensitivity Test

1. How are antibiotic sensitivities tested?

Ans – A health care professional will take a blood sample from a vein in your arm, using a small needle. The test is done on MHA medium and antibiotic sensitivity is checked.

2. Which is standard medium for antibiotic sensitivity test?

Ans – Mueller-Hinton agar is the standard medium used for most broth dilution testing.

References and Sources

Further Readings

  1. 10 Instruments Used in Microbiology Laboratory
  2. 8 Qualitative Tests for Protein
  3. Aberration In Lens System
  4. Acid Fast Staining
  5. Algae
  6. Aseptic Transfer Technique
  7. Bacterial Flagella, Fimbriae and Pili
  8. Bacterial Growth and Nutrition
  9. Extremophiles
  10. Fimbriae vs Flagella
  11. Fundamental Microscopy
  12. Growth Curve of Bacteria
  13. MacConkey agar
  14. McFarland Standards
  15. Monochrome Staining
  16. Negative Staining
  17. Ninhydrin Test
  18. Serial Dilution in Microbiology
  19. Spread Plate Technique
  20. Streak Plate Technique
  21. Types of Extremophiles
  22. Xanthoproteic Test