Susceptibility in microbiology refers to the degree to which a microorganism can be inhibited or killed by a particular antimicrobial agent.
Explanation
Determining antimicrobial susceptibility is essential for selecting effective therapy and monitoring the emergence of resistance. A susceptible organism lacks or has minimal mechanisms to counteract the drug’s action, so growth is inhibited or cells are killed at achievable concentrations. Susceptibility testing methods include disc diffusion (Kirby–Bauer), broth microdilution, agar dilution and automated systems, each measuring the inhibitory concentration of antibiotics against clinical isolates. Results are interpreted using standardized breakpoints established by organizations such as CLSI or EUCAST, categorizing isolates as susceptible, intermediate or resistant. Factors influencing susceptibility include the permeability of the cell envelope, the presence of efflux pumps, enzymes that inactivate drugs (β‑lactamases), target mutations and biofilm formation. In a clinical context, patient factors such as drug pharmacokinetics and site of infection also determine whether an organism deemed susceptible in vitro will respond to therapy. Surveillance of susceptibility patterns guides empirical treatment and informs stewardship programs.
Examples and testing
Streptococcus pyogenes remains uniformly susceptible to β‑lactam antibiotics, so penicillin is the treatment of choice for pharyngitis. Methicillin‑resistant Staphylococcus aureus (MRSA) is resistant to most β‑lactams but may be susceptible to vancomycin or linezolid; susceptibility testing helps choose between these agents. Gram‑negative bacteria like Pseudomonas aeruginosa exhibit variable susceptibility profiles, often requiring testing against multiple classes such as cephalosporins, carbapenems and aminoglycosides. Laboratories perform Kirby–Bauer disc diffusion by placing antibiotic-impregnated discs on an agar plate inoculated with the test organism and measuring zones of inhibition. Broth microdilution determines the minimum inhibitory concentration (MIC) that prevents visible growth. These results are compiled in antibiograms to monitor local susceptibility trends.
Understanding susceptibility enables rational antimicrobial selection and helps combat the spread of resistance.
Related Terms: Antimicrobial susceptibility testing, Minimum inhibitory concentration, Resistance, Antibiogram, Sensitivity