Determining the Susceptibility of a Bacterial Strain to an Antimicrobial Drug

In many cases, susceptibility of a pathogenic organism to a specific antimicrobial drug is unpredictable. Unfortunately, it has often been the practice to try one drug after another until a favorable response is observed or, if the infection is very serious, to give several together. Both approaches are undesirable. With each unnecessary drug given, needless risks of toxic or allergic effects arise and the normal flora may be altered, permitting the over growth of pathogens that are resistant to the drug. A better approach is to determine the susceptibility of the specific pathogen to various antimicrobial drugs and then choose the drug that acts against the offending pathogen but against as few other bacteria as possible.

Quantitative tests determine the lowest concentration of a specific drug that prevents the growth of an organism in vitro. This concentration is called the minimum inhibitory concentration (MIC). Interpreting the significance of a given MIC requires knowledge of the level of the drug that can be achieved in the patient. Other simpler tests can be used to determine whether an organism is sufficiently susceptible to a drug for a person to be successfully treated.

Determining the Minimum Inhibitory and Bactericidal Concentrations

The minimum inhibitory concentration is determined by assaying the ability of a bacterial strain to grow in broth cultures containing different concentrations of the antimicrobial. Serial dilutions generating decreasing concentrations ofthe drug are first prepared in tubes containing a suitable growth medium (figure 21.9). Then, a known concentration of the organism is added to each tube. The tubes are incubated for at least 16 hours and then are examined for visible growth or turbidity. The lowest concentration ofthe drug that prevents growth ofthe microorganism is the minimum inhibitory concentration. The fact that an organism is inhibited by a given concentration of drug, however, does not necessarily mean that an infection can be successfully treated with the drug. For example, an organism with a MIC of 20 mg/ml of a drug is susceptible to that concentration in vitro. Nevertheless, the organism would be considered resistant for the purposes of treatment if the level that can be achieved in blood were only 5 mg/ml. Microorganisms requiring inhibitory concentrations on the borderline between susceptible (treatable) and resistant (untreatable) are called intermediate.

The minimum bactericidal concentration (MBC) is the lowest concentration of a specific antimicrobial drug that kills 99.9% of a given strain of bacteria. The MBC is determined by assaying for live organisms in those tubes from the MIC test that showed no growth. A small sample from each of those tubes is transferred to fresh, antibiotic-free medium. If growth occurs, then living organisms remained in the original tube. Conversely, if no growth occurs, then no living organisms remained, indicating that the antibiotic was bactericidal at that concentration.

Determining the MIC and MBC using these conventional methods gives precise information regarding an organism's susceptibility. The techniques, however, are labor-intensive and consequently expensive. In addition, individual sets of tubes must be inoculated to determine susceptibility to each different antimicrobial tested.

Conventional Disc Diffusion Method

The Kirby-Bauer disc diffusion test is routinely used to qualitatively determine the susceptibility of a given organism to a battery of antimicrobial drugs. A standard concentration of a bacterial strain is first uniformly spread on the surface of an agar plate. Then 12 or so discs, each impregnated with a specified concentration of a selected antimicrobial drug, are placed on the sur-

Organism A

Control (no bacteria)

Organism B

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  • bernd
    How to determine the susceptibility of a bacterial strain to an antimicrobial drug?
    3 years ago

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