Question

Explain agar dilution and gradient plate technique for susceptibility testing.

Answer 1

Dilutions. Serial dilution of a solution results, after each dilution step, in fewer molecules of the original substance per litre of solution. Eventually, a solution will be diluted beyond any likelihood of finding a single molecule of the original substance in a litre of the total dilution product.

Answer 2

Agar-dilution susceptibility testing

Agar-dilution susceptibility testing is the reference method for measuring the antimicrobial susceptibilities of strains of N. gonorrhoeae. Resistance to antimicrobial agents is measured as the MIC of the agent that inhibits growth of an isolate. Determination of susceptibilities to antimicrobial agents used for therapy should be tested such as the third generation cephalosporins, cefixime and ceftriaxone although other agents, such as ceftobutin, are used in different parts of the world. The methodology for susceptibility testing of N. gonorrhoeae can vary between countries, differing primarily in the base medium used; GC agar base (CLSI, North America), Isosensitest (Australia) and Diagnostic Sensitivity Agar (Europe). For therapeutic antimicrobial agents this has historically been most important for the concentration of penicillin considered resistant; which can differ from ≥2 mg/L (GC base agar) and ≥1 mg/L (Isosensitest and DST agar). Other antimicrobials agents are affected less but this should be remembered when comparing different methodologies.