Antibiotic sensitivity test

  Purpose

  1. This test is used to check the effectiveness of a drug against bacteria.
  2. It is used to check the susceptibility/ sensitivity of bacterium against antibiotic therapy.
  3. This test is used to select the best antibiotic to avoid resistance.


Intrinsic resistance

When we administer a drug (narrow spectrum) that is effective against gram-positive bacteria and the causative agent that is present is gram-negative or when we administer a drug (narrow spectrum) that is effective against gram-negative bacteria and the causative agent that is present is gram-positive then the drug is ineffective against them. This is called intrinsic resistance.

Acquired resistance

When we administer a low dose of a drug, it does not harm bacteria and causes bacteria to undergo mutation. Then bacteria become resistant to that drug. This is called acquired resistance.

Antibiotic sensitivity/susceptibility

The response of bacteria to a drug is called antibiotic sensitivity. Antibiotics must be given for 5-7 days.

Material

  • Petri dish
  • Nutrient agar
  • Culture broth
  • Antibiotic disc
  • Incubator
There are two methods
  1. Swab streaking method or Kirby-Bauer method
  2. Test tube method; pour 1 ml liquid and 0.2 ml culture in a test tube.

Kirby-Bauer method

It is named after the name of two scientists who discovers it in 1950. This test is standardized in 1961.

Procedure

  1. Take a petri dish containing 1 ml agar and 0.2 ml culture.
  2. Place the antibiotic disc in it with the help of a dispenser or sterilized forceps. We can place up to 8 discs in one petri dish.
  3. Incubate at 370 C for 24-48 hours.

Result

Qualitative

The clear zone around disc indicates that bacteria are sensitive to that drug or drug is effective against that specific bacteria; Test is positive

Quantitative

Measure the diameter of the clear zone from the mid and compare with standard chart e.g., for methicilin.

Resistant
Intermediate sensitive
(Response is there but it is not sensitive)
Susceptible
Size of clear zone > 12 mm
Size of clear zone > 13-14 mm
Size of clear zone > 15 mm


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