Commercial Modification Of Antimicrobial Susceptibility Test

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Figure 21.9 Determining the Minimum Inhibitory Concentration (MIC) of an Antimicrobial Drug

The lowest concentration of drug that prevents growth of the culture is the MIC.

face of the medium (figure 21.10). During incubation, the various drugs diffuse outward from the discs at a rate inversely proportional to their size, forming a concentration gradient of drug around each disc. A clear zone of inhibition around an antimicrobial disc reflects, in part, the degree of susceptibility of the organism to the drug. The zone size is also influenced by characteristics of the drug including its molecular weight, stability, and concentration in the disc.

Antibiotic Disc For Coli

Special charts have been prepared correlating the size of the zone of inhibition to susceptibility of an organism to the drug. Based on the size of the zone, organisms can be described as susceptible, intermediate, or resistant to the drug. These seemingly simple categories reflect the extensive work of researchers who tested the antimicrobial sensitivity of a variety of bacteria that had a wide range of drug susceptibilities. For each organism, they determined the minimum inhibitory concentration of a particular antimicrobial drug and the size of the zone of inhibition around that antimicrobial disc. The correlation of MIC and zone size, along with the determination of the achievable blood levels of each of the various drugs, enabled the Kirby-Bauer disc diffusion test to become a standard method to guide physicians in selecting an appropriate antimicrobial drug.

Commercial Modifications of Antimicrobial Susceptibility Testing

Commercial modifications of the conventional methods offer certain advantages. They are less labor-intensive, and the results

Figure 21.10 Kirby-Bauer Method for Determining Drug Susceptibility The size of the zone of inhibition surrounding the disc reflects, in part, the sensitivity of the bacterial strain to the drug. Because zone size is influenced by characteristics of the drug such as molecular weight, however, a chart must be consulted that correlates zone size to susceptibility.

Chapter 21 Antimicrobial Medications

PERSPECTIVE 21.1 Measuring the Concentration of an Antimicrobial Drug in Blood or Other Body Fluids

There are many situations in which it is necessary to determine the concentration of an antimicrobial drug in a patient's blood or other body fluid. For example, patients who are being administered an aminoglycoside must often be carefully monitored to ensure that the concentration of drug in their blood does not reach an unsafe level, particularly if they have kidney or liver dysfunction that interferes with normal elimination. Likewise, new drugs must be tested to determine achievable levels in the blood, urine, or other body fluids.

A technique called the diffusion assay is used to measure the concentration of an antimicrobial in a fluid specimen.The test relies on the same principle as the Kirby-Bauer test, except in this case it is the concentration of drug, not the sensitivity of organism, being assayed. A culture of a stock organism that is highly susceptible to the drug is added to melted cooled agar, and the mixture is poured into an agar plate and allowed to solidify.This results in a solid medium that is uniformly inoculated throughout with the sensitive organism. Cylindrical holes are then punched out of the agar, creating wells. Standards, or fluids containing known concentrations of the drug, are then added to some of the wells, while others are filled with the body fluid being tested. Following

Figure 1 Biological Assay to Determine the Concentration of an Antimicrobial Drug in a Patient's Body Fluid overnight incubation, zones of inhibition form around the agar wells, the sizes of which correspond to the concentrations of the drug (figure 1). The higher the concentration of antimicrobial, the larger is the zone of inhibition.The zone sizes around the standards are measured, and from this a standard curve is constructed by plotting the zone sizes against the corresponding drug concentration. A line relating zone size to concentration is obtained, from which the concentration of the antimicrobial drug in the body fluid can be read.

Drug Susceptibility Test

(a) Standards and patient's serum are added to agar that has been seeded with susceptible strain of bacteria.

(b) A standard curve that correlates the size of the zone with the concentration of antibiotic is constructed. The concentration of the antimicrobial drug in the body fluid can be read from the line relating zone size to concentration can be read in as little as 4 hours. One system utilizes a small card with miniature wells containing specific antimicrobial concentrations. The highly automated system inoculates and incubates the cards, determines the growth rate by reading the turbidity, and uses mathematical formulas to interpret the results and derive the MICs in 6 to 15 hours (figure 21.11). Another system uses

Figure 21.11 Automated Tests Used to Determine Antimicrobial Susceptibility The miniature wells in the card contain specific concentrations of an antimicrobial drug. An automated system inoculates and incubates the cards, determines the growth rate by reading turbidity, and uses mathematical formulas to interpret the results and derive the MICs.

Figure 21.11 Automated Tests Used to Determine Antimicrobial Susceptibility The miniature wells in the card contain specific concentrations of an antimicrobial drug. An automated system inoculates and incubates the cards, determines the growth rate by reading turbidity, and uses mathematical formulas to interpret the results and derive the MICs.

a microdilution tray containing various concentrations of antimicrobials that is manually inoculated.

The E test, a modification of the disc diffusion test, utilizes a strip impregnated with a gradient of concentrations of an antimicrobial drug. Multiple strips, each containing a different drug, are placed on the surface of an agar medium that has been uniformly inoculated with the test organism. During incubation the test organism will grow, and a zone of inhibition will form around the strip, but because of the gradient of concentrations, the zone of inhibition is shaped somewhat like a teardrop that will intersect the strip at some point (figure 21.12). The MIC is determined by reading a number off the numerical scale printed on the strip at the point where the bacterial growth intersects it.

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  • gigliola
    How to determine minimum inhibitory concentration from zone of inhibition graph?
    2 years ago

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