Antimicrobial Susceptibility Test Method By Paramedical Info



The antimicrobial susceptibility test is also known as the antimicrobial sensitivity test this test is most useful and effective against certain antimicrobial bacteria and fungus. Antibiotic susceptibility regulates the susceptibility of bacteria to an antibiotic. Antimicrobial susceptibility finds out the ability of the drug to kill the antimicrobial bacteria and fungus.

Antimicrobial susceptibility tests are of two types:-

  1. Diffusion Test
  2. Dilution Test

1. Diffusion Test

  1. Stokes disc diffusion method
  2. Kirby-Bauer disc diffusion method

2. Dilution Test

  1. Broth dilution method
  2. Agar dilution method
Disc diffusion tests are most widely used to determine the susceptibility of isolates of pathogenic bacteria to antibiotics.

Stokes Disc Diffusion Antimicrobial Susceptibility Method 

The test bacterium is inoculated on the central one-third and control on upper and lower thirds of the Muelle-Hinton agar plate. However, in the modified Stokes disc diffusion method, the test bacterium is inoculated Over the upper and lower thirds of the plate and control on the central one third.
Antibiotic discs are applied between the standard and test inocula. The plates are then incubated at 37°C for 16-18 hours.


The reporting of results is done by comparing the zones of inhibition of control and test bacterium. Measure the zone size i.e. the distance in mm from the edge of the disc to the zone edge. It is interpreted as follows:


The zone of test bacterium is equal to, or larger than, that of the control strain. If the zone size of the test bacterium is smaller than that of the control, the difference between the two should not be more than 3 mm.

Imtermediate Sensitive

The zone size of the test bacterium ought to be in any event 2 mm and the distinction between the zone of test and control strain ought to be at any rate 3 mm. The zone size of the test bacterium is more modest than 2 mm.

Kirby-Bauer Disc Diffusion Antimicrobial Susceptibility Method

The test bacterium is inoculated over the entire agar surface of the Mueller-Hinton agar plate. On a plate of 100 mm diameter, seven antibiotic discs may be applied, one in the center and six in the periphery. The plates are then incubated at 37°C for 16-18 hours. The zones of complete growth inhibition around each of the discs are measured.
The diameter of the disc is included in this measurement. The interpretation of zone size into sensitive, intermediate, or resistant is based on the interpretation chart. Control strains should be tested each time when a new batch of discs or agar is used.

Epsilometer or E-Test Or Antimicrobial Susceptibility Test 

Epsilometer or E-test is an antimicrobial susceptibility test to detect a minimum inhibitory concentration (MIC) of antibiotics. It is a recent modification of the agar diffusion susceptibility test. It utilizes a retentive strip with a known inclination of anti-infection focus along its length. The strip is put on the agar plate vaccinated with the test organism. The least inhibitory fixation (MIC) is recorded as the most minimal grouping of the slope which hinders the development of the creature.
Spread the love

Leave a Comment