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close this bookBasic Laboratory Procedures in Clinical Bacteriology (WHO; 1991; 128 pages)
View the documentPreface
View the documentIntroduction
open this folder and view contentsQuality assurance in microbiology
close this folderPart I. Bacteriological investigations
open this folder and view contentsBlood
open this folder and view contentsCerebrospinal fluid
open this folder and view contentsUrine
open this folder and view contentsStool
open this folder and view contentsLower respiratory tract infections
open this folder and view contentsUpper respiratory tract infections
open this folder and view contentsSexually transmitted diseases
open this folder and view contentsPurulent exudates, wounds, and abscesses
open this folder and view contentsAnaerobic bacteriology
close this folderAntimicrobial susceptibility testing
View the documentIntroduction
View the documentGeneral principles of antimicrobial susceptibility testing
View the documentClinical definition of terms “resistant” and “susceptible”: the three-category system
View the documentIndications for routine susceptibility tests
View the documentChoice of drugs for routine susceptibility tests in the clinical laboratory
View the documentThe modified Kirby-Bauer method
View the documentDirect versus indirect susceptibility tests
View the documentTechnical factors influencing the size of the zone in the disc diffusion method
View the documentQuality control
open this folder and view contentsPart II. Essential media and reagents for isolation and identification of clinical pathogens
View the documentSelected further reading
View the documentSelected WHO publications of related interest
View the documentBack Cover
 

General principles of antimicrobial susceptibility testing

Antimicrobial susceptibility tests measure the ability of an antibiotic or other antimicrobial agent to inhibit bacterial growth in vitro. This ability may be estimated by either the dilution method or the diffusion method.

The dilution test

For quantitative estimates of antibiotic activity, dilutions of the antibiotic may be incorporated into broth or agar medium, which is then inoculated with the test organism. The lowest concentration that prevents growth after overnight incubation is known as the minimum inhibitory concentration (MIC) of the agent. This MIC value is then compared with known concentrations of the drug obtainable in the serum and in other body fluids to assess the likely clinical response.

The diffusion test

Paper discs, impregnated with the antibiotic, are placed on agar medium uniformly seeded with the test organism. A concentration gradient of the antibiotic forms by diffusion from the disc and the growth of the test organism is inhibited at a distance from the disc that is related, among other factors, to the susceptibility of the organism.

There is an approximately linear relation between log MIC, as measured by a dilution test, and the inhibition zone diameter in the diffusion test. A regression line expressing this relation can be obtained by testing a large number of strains by the two methods in parallel (see Fig. 7 and 8).


Fig. 7. Graphic representation of the relationship between log2MIC and the inhibition zone diameter obtained by the diffusion test using discs containing a single concentration of antibiotic


Fig. 8. Interpretation of zone sizes as susceptible, intermediate, and resistant by their relationship to the MIC

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