Antimicrobial susceptibility testing determines the effectiveness of antimicrobial agents against microorganisms. Beyond standard disk diffusion and broth dilution, advanced methods provide mechanistic information about resistance and enable targeted therapy.
Automated systems such as VITEK and BD Phoenix use miniaturized broth microdilution in multi-well cards with colorimetric or turbidimetric detection. Results are available in 4 to 12 hours with MIC values for a comprehensive antimicrobial panel. These systems integrate expert rules for resistance mechanism detection.
The Etest uses a plastic strip with a predefined antimicrobial gradient. It combines the convenience of disk diffusion with the quantitative MIC of dilution methods. The intersection of the inhibition ellipse with the strip reveals the MIC. Etest is useful for fastidious organisms and when individual MIC values for specific drugs are needed.
Carbapenemase detection has become critical for infection control. The modified carbapenem inactivation method detects carbapenemase production in Enterobacteriaceae, Pseudomonas, and Acinetobacter. The Carba NP test detects carbapenem hydrolysis colorimetrically. Molecular tests including PCR and microarray detect carbapenemase genes including KPC, NDM, VIM, IMP, and OXA variants.
Extended-spectrum beta-lactamase confirmation uses the synergistic inhibition by clavulanic acid. Combined disk tests with and without inhibitor confirm ESBL production. AmpC beta-lactamase detection uses boronic acid or cloxacillin inhibition.
Methicillin resistance in staphylococci is detected by cefoxitin disk diffusion or PBP2a latex agglutination. Vancomycin resistance in enterococci is confirmed by vanA/vanB PCR. Macrolide-lincosamide-streptogramin resistance phenotypes are differentiated by the D-zone test.
Genotypic methods including whole genome sequencing provide comprehensive resistance gene profiling. Next-generation sequencing detects known and novel resistance determinants, typing markers, and virulence genes directly from clinical samples or isolates.
Interpretive criteria follow CLSI or EUCAST guidelines. The choice of breakpoints affects clinical categorization. Epidemiological cut-off values distinguish wild-type from non-wild-type populations independent of clinical outcome.
