Microbiology Test

Test : Susceptibility testing, Bacteria


Indications : The test is ordered to determine the susceptibility or resistance of bacteria to a certain dose of anti-bacterial medication. It requires the isolation of bacteria and hence, is a good indicator of the changing trends in bacterial isolates.

Bacterial susceptibility testing includes the following steps:

• Obtaining a bacterial sample

• Identifying the type of bacteria contained in the sample

• Obtaining a control sample from the bacterial sample

• Selecting a set of appropriate antibiotics for testing, based on the type of bacteria in the sample

Isolates of bacteria are required for the testing; the method used in the disc diffusion assay. After the bacterial isolates are obtained they are streaked onto agar plates. Then antibiotic-laden discs are placed on the agar bed containing bacteria. The antibiotic diffuses from the disc and inhibits bacterial growth.

Physiology : Susceptibility testing of bacteria is done to determine the effectiveness of an antibiotic against bacterial infection.

Bacteria have the capability to develop resistance to antibiotics over a period of time and hence, there is a need to monitor this change in susceptibility and develop new and more advanced anti bacterial medications to overcome these microbes.

The test helps clinicians to determine the choicest drug that is best suited in treating bacteria and, to assess the prognosis of an infection after treatment with certain anti-bacterial drugs.

Normal Range : Reported as Minimum Inhibitory Concentration (MIC). Results may be susceptible (S), Intermediate (I), Resistant

Interpretation : The diameter of the zones of inhibition is directly linked to the degree of resistance or susceptibility to the antibiotic.

The zone size is compared to a standardized chart to determine if the bacteria are resistant or susceptible to the drug and to determine the minimum inhibitory concentration (MIC).

MIC is the lowest concentration of a drug that prevents the visible growth of organism after overnight incubation.

Results may be:

Susceptible (S) - Bacteria is treatable for the suggested dose of antibiotic

Intermediate (I) - can be reclassified as sensitive or resistant if retested

Resistant (R) - Infection not treatable as the MIC of the tested antibiotic exceeds safe levels

Sample : Microorganism

Test Method : Disc diffusion assay, Microbroth diltuion.

Related Tests : Abscess, Antibiotic level.

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