Prof. Dr. Sevgi ERTUĞRUL KARATAY
Antibiogram Tests
Broth Macrodilution Method
This method is performed in test tubes and requires serial dilution. The difference from the microdilution
test is the large volume of antibiotics tested.
Quantitative results can be obtained as MIC values of antibiotics against certain microorganisms can be determined as a result of two-fold serial dilution.
Although it is more sensitive than disk diffusion test,
this methods is more labor intensive.
Antibiogram Tests
Application of Broth Macrodilution Method Antibiotic susceptibility of microorganism to be examined appropriate liquid medium (Mueller Hinton
Agar-MHB) is prepared and put into the test tubes.
The turbidity of the 18-24 hours liquid microorganism culture is adjusted to 0.5 McFarland.
With micropipettes, the antibiotic is placed in the test tubes at the highest dose. Then the other wells are
serially diluted with the same antibiotic.
Antibiogram Tests
Application of Broth Macrodilution Method
The microorganisms are inoculated in test tubes with micropipettes.
The test tubes are incubated at 37 ºC for 18-24 hours.
After incubation, the growth in the test tubes is measured visually. The measured value is compared to the standard values reported by institutions such as CLSI or EUCAST to
determine whether the microorganism is susceptible or resistant to that antibiotic.
Antibiogram Tests
Application of Broth Macrodilution Method
In this experiment, tubes containing no antibiotics but containing microorganisms and media are used as
positive controls.
Similarly, tubes containing antibiotics and media but without microorganisms are used as negative controls.
If there is no growth in the positive control or in the
negative control, the experiment is repeated.
Antibiogram Tests E-test
E-test is a commercial test that combines the features of disc diffusion and microdilution tests into one
application.
Strips which contained with antibiotics at certain concentrations are used in this method.
Since the concentration of antibiotics in the strips is doubled, it is possible to obtain a quantitative result
and determine the MIC value.
The method is similar to the disc diffusion method.
Antibiogram Tests Application of e-test
Antibiotic susceptibility is prepared for the
microorganism suitable solid medium (Mueller Hinton Agar-MHA) is prepared and poured into agar medium.
The turbidity of the 18-24 hours liquid microorganism culture is adjusted to 0.5 McFarland.
The liquid culture is immersed in the swab and waited until it is absorbed. The process is repeated at least 6
times to spread the microorganisms to the whole
medium.
Antibiogram Tests Application of E-test
The E-test strips, whose effect will be determined, are placed on the surface of the inoculated petri dish according to the size of the petri dish and gently pressed onto it with
the help of forceps.
Petri dishes are incubated at the temperature and time (37 ºC, 18-24 hours) at which the microorganism will develop.
After incubation, transparent zones of inhibition around the strip on the surface of the petri dish are checked. The lowest concentration without growth is determined as MIC.
Antibiogram Tests
Ease of application Work load Precision
Disc Diffusion Easy Low Low
Microdilution Moderate Moderate High
E-test Easy Low Moderate to high
