Antibiotic sensitivity test procedure

Antibiotics are natural or semi-synthetic agents which kill bacteria and other microbes at small doses. The number of antibiotics used in present health care system are many yet not all of them are suitable for every kind of infection.

For example rifampicin is best for treatment of tuberculous infection while penicillin is best for syphilis. But these two antibiotics are completely ineffective in vice verse manner. This knowledge of selectivity of an antibiotic to eradicate a specific microbe is essential  for effective antimicrobial therapy.

Antibiotic sensitivity test:

Antibiotic sensitivity test is a procedure done to identify the susceptibility of a microbe to a suitable antibiotic. This is determined by the extent of inhibition of growth of a specific microbe when treated with a set of antibiotics. The antibiotic which inhibits the growth to a large (greater) extent is the concluded to be the best one to treat the infection by the said microbe.

This antibiotic sensitivity test is done by two techniques namely.

  • Agar Plate method
  • Dilution methods

Agar plate methods use nutrient agar as a substrate for growth of microbe under the test for its antibiotic sensitivity. The microbe culture is uniformly spread on the surface of nutrient agar. Then filter paper disks of 1 cm radius are dipped in different antibiotic solutions and placed over the surface of inoculated agar in the plate. The plate is then incubated at suitable temperature like 25 degrees for a week and checked for the growth of microbe around the paper disks. The paper around which there is maximal inhibition of growth of microbe is said to have maximum effect on the microbe growth. This antibiotic is suitable to treat the infection by the microbe in humans and/or animals.

Dilution methods: Here liquid nutrient media is used to dilute the microbial culture in suitable cylindrical tubes like test tubes. Same dilution of microbes is present in all tubes. Then fixed concentration of different antibiotics are added to each tube with microbial culture by labeling them. After  a week of incubation the growth in each tube is measured in terms of turbidity of the solution. The tube with less turbidity indicates effective growth inhibition of microbe by the antibiotic.

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