In vitro susceptibility testing plays an important role in this process. Sensitivity testing can be performed qualitatively, semi-quantitatively or by using molecular biological methods.
The results of qualitative methods are usually indicated as one of the following: "sensitive (S)", "intermediate (I)" and "resistant (R)". The widely used classical agar diffusion method (Kirby-Bauer test) can be used for fast growing bacteria. Here, antibiotic-impregnated platelets are placed on agar media that have been inoculated with the pathogen to be tested.
Semiquantitative methods determine the minimum concentration of a substance necessary to inhibit the growth of a specific pathogen in vitro. This minimum inhibitory concentration (MIC) is expressed as a numerical value, which is then assigned to one of three categories (S), (I) or (R). The broth-dilution method is the gold standard in most reference laboratories worldwide.
An alternative method for detection MIC uses a polyester strip (E-test) impregnated with a concentration gradient along its entire length.
Sensitivity testing can be performed using an automated computerized system. The sample data are recorded electronically and the corresponding antibiotic test card is assigned. Nucleic acid-based methods involve molecular biology procedures similar to those used for pathogen identification and to detect known resistance genes or mutations by PCR and WGS. Although a number of such genes are known, their presence does not automatically imply in vivo resistance. Also, because new mutations or other resistance genes may be present, their absence does not guarantee antibiotic susceptibility. For these reasons, routine phenotypic susceptibility testing methods remain the standard approach to assess bacterial susceptibility to antimicrobial drugs.