Antibiotic Susceptibility Testing

Tube dilution tests A series of culture tubes are prepared, each containing a liquid medium and a different concentration of a chemotherapeutic agent. The tubes are then inoculated with the test organism and incubated for 16-20 hours at 35C. After incubation, the tubes are examined for turbidity (growth). Minimum Inhibitory Concentration (MIC): Is the lowest concentration of chemotherapeutic agent capable of preventing growth of the test organism. Minimum Bactericidal Concentration (MBC): Is the lowest concentration of the chemotherapeutic agent that results in no growth (turbidity) of the subcultures.

The agar diffusion test (Bauer-Kirby test): In this test, the in vitro response of bacteria to a standardized antibiotic-containing disc has been correlated with the clinical response of patients given that drug. The MIC for each agent was then compared to the usually-attained blood level in the patient with adequate dosage. Categories of "Resistant,“ "Intermediate," and "Sensitive" were then established.

Disk diffusion test

Epsilo-meter test It’s a new technique for direct detection of MIC, a graduated increasing concentration of the antibiotic is fixed along a rectangular plastic test strip which is applied to the surface of an inoculated agar plate, after over night incubation a tear drop shaped inhibition zone is seen. The zone edge intersect the graded test strip at the MIC of the antimicrobial.

E Test

E Test

Effects of Combinations of Drugs Synergism occurs when the effect of two drugs together is greater than the effect of either alone. Antagonism occurs when the effect of two drugs together is less than the effect of either alone.

Kirby-Bauer animation http://learn.chm.msu.edu/vibl/content/antimicrobial/kirbybauer/index.html