Antimicrobial Susceptibility Testing (AST) Methods

In this video, Professor Gunnar Kahlmeter briefly discusses some of the types of AST methods.

For the full video please click on the link at the bottom of the page.

Phenotypic AST – based on measuring the activity of the agent against the microorganism, MIC and breakpoints.

Genotypic AST – detection of a resistance gene (mecA, vanA/vanB, etc) in the genome or on transferable genetic elements.

Mechanistic AST – detection of a resistance mechanism – by detecting the product of the resistance gene (enzyme).

Expert rules AST – susceptibility based on empiric knowledge.

Professor Gunnar Kahlmeter goes on to explain the Minimum Inhibitory Concentration (MIC) and its importance.

MIC is the lowest concentration of a drug needed to inhibit cell division in the environment in which the test was performed. It is a broth microdilution and is the basis for all susceptibility tests. All other methods are classed as surrogate methods which are addressed in a subsequent step. Clinical AST should produce a clinical recommendation to determine whether to treat or not and whether standard dosing will suffice or if increased exposure is required.

The MIC is not an absolute value and is affected by factors such as:

• Inoculum
• Incubation time
• The broth used and its composition
• Atmosphere
• pH

Testing many isolates of the same species against a defined agent can determine the wildtype distribution of the species for that particular agent. Wildtype isolates are those that lack any resistance mechanisms. Once the wildtype distribution is established and epidemiological cut-off values (ECOFF) determined, the highest MIC of wildtype distribution, then successive isolates can be categorised as wildtype or non-wildtype.

To view the full video, please click here.

Let us know in the comments section below, which type of method have you heard of or used before?