4.2 The approach used to determine the resistance mechanism

Once AST has confirmed that a bacterial isolate is resistant to antimicrobial(s), two approaches can be used, either separately or in combination, to identify the resistance mechanism involved ‒ phenotypic methods and genotypic methods.

Phenotypic tests focus on characteristics of the bacteria (i.e. phenotype) that can be readily observed or tested for in the laboratory. They use the results of these observations/tests to infer the resistance mechanism (and thus the gene) involved. In contrast, genotypic tests identify the gene responsible for the resistance directly.

You will learn more about these two approaches in the following sections.

Figure 9 Tests to identify resistance mechanisms can be performed in a clinical microbiology laboratory.