How Do Doctors Determine Antibiotic Sensitivity?

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  • Written By: Jillian O Keeffe
  • Edited By: Shereen Skola
  • Last Modified Date: 24 October 2019
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When a patient has a bacterial infection, doctors may take samples of the bugs from the person so that the microbiology laboratory can identify the species. As specific antibiotics only kill certain groups of bacteria, and as different strains of bacteria may be immune to particular antibiotics, an antibiotic sensitivity test may also be necessary. This is also performed in the laboratory, and takes a day or more before the results are readable. Either a petri dish test, or a test using nutrient liquids, can tell a microbiologist how sensitive the infection is to particular antibiotics.

One way of identifying the bacterial species that causes an infection is to grow it in the lab into a large population that can have tests performed on it. These tests include such characteristics as visual appearance on nutrient plates, ability to grow without oxygen, or appearance under the microscope. Advanced labs also have the option of genetic testing of the infectious bug, which can be much quicker and more accurate than the traditional tests.


Often a doctor does not need to know exactly what type of infectious organism is present in the patient, but only needs to know which antibiotics can cure the condition. In these situations, the identifying step need not be included at all, but the antibiotic sensitivity test is employed alone. For all of the regular antibiotic sensitivity tests, the doctor needs to take a sample from the patient that includes some of the bacteria.

Then the microbiologist in the hospital laboratory takes the sample, adds it to nutrients and helps the bacteria in it to grow in population. Once a sufficient amount of bacterial cells are available, he or she can expose these to antibiotics, to see if they are killed, and if they are, how efficiently this works. Various types of antibiotic sensitivity tests are options for microbiologists, which can use solid nutrient agar dishes or liquid nutrients in tubes.

Laboratories often use the disk diffusion method to determine antibiotic sensitivity. In this procedure, the microbiologist picks a petri dish full of a nutrient solid, and transfers large amounts of the infectious bacteria onto the surface of the nutrient. Under the right warmth and oxygen conditions, the bacteria will grow in this petri dish.

To show which substances can kill the bacteria, the microbiologist then places little paper-like disks onto the dish. Each individual disk is soaked with a specific antibiotic, that may or may not kill the bacteria already on the plate. After the dish goes into an incubating machine for a day or more, the analyst looks at the pattern of bacterial growth. Bare patches around certain disks indicate that these antibiotics killed or prevented the growth of bacteria. The antibiotic with the biggest empty circle around it is the most efficient at killing the bacteria, and so may be the one chosen to treat the infection.

An alternative method to dishes with solid nutrients is the broth tube method. In this test, the microbiologist fills tubes with a broth that contains all the necessary nutrients for the bacteria to grow. Then he or she adds the sample and different antibiotics, in varying concentrations, to individual tubes. The tubes that contain clear, unchanged liquid represent the antibiotics that killed the bacteria, and the ones with cloudiness represent the drugs that let the bugs grow.


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