Antibiotic Sensitivity Test

 


1. Objective

The objective of this test was to determine the sensitivity of bacterial isolates to various antibiotic agents, helping guide effective antimicrobial therapy.

 

2. Principle

The test was based on the inhibition of bacterial growth by antibiotics. The Kirby-Bauer disc diffusion method was used, where antibiotic-impregnated discs were placed on an agar plate inoculated with bacteria. The zone of inhibition around each disc indicated the effectiveness of the antibiotic.

3. Materials

  • Pure bacterial isolate

  • Mueller-Hinton agar plates

  • Antibiotic discs (e.g., ampicillin, ciprofloxacin, ceftriaxone)

  • Sterile swabs 

  • McFarland standard (0.5)

  • Incubator (35–37°C)

  • Ruler or caliper

  • CLSI guideline chart for interpretation

4. Procedure

  1. A standardized bacterial suspension (0.5 McFarland) was prepared.

  2. The suspension was evenly spread onto Mueller-Hinton agar using a sterile swab.

  3. Antibiotic discs were placed on the agar surface using sterile forceps.

  4. The plate was incubated at 35–37°C for 16–18 hours.

  5. The diameter of inhibition zones was measured in millimeters and compared to standard CLSI charts.

5. Result (Example)

AntibioticZone (mm)Interpretation
Ciprofloxacin28Sensitive
Ampicillin12Resistant
Gentamicin20Intermediate
Ceftriaxone30Sensitive

6. Uses

  • Identified effective antibiotics for infection treatment

  • Prevented antibiotic resistance

  • Helped in hospital infection control

  • Guided targeted therapy over empirical treatment

7. Conclusion

The antibiotic sensitivity test was a critical diagnostic tool in clinical microbiology that helped ensure rational and effective use of antibiotics, reducing the risk of treatment failure and resistance development.


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