Susceptible, intermediate, and resistant - the intensity of antibiotic action

Abstract

Introduction: To date, the resistance of infectious agents has been assessed by widely varying criteria in different countries. Therefore, published data on resistance often cannot be meaningfully compared. In Germany, different laboratories can potentially report different results for identical microorganisms, since there is no uniform system for categorization. This situation is unsatisfactory.

Methods: Selective literature review and evaluation of committee reports.

Results: The new ISO standard 20776 for determination of the resistance of infectious agents and the harmonized evaluation system of the European Society for Clinical Microbiology and Infectious Diseases provide a new basis for susceptibility testing. The categorization of infectious agents as "susceptible," "intermediate," or "resistant" to particular antibiotics will become more reliable and will be consistent throughout Europe.

Discussion: For a number of antibiotics, the criteria for evaluation of infectious agents as "susceptible," "intermediate", or "resistant" will change. Comparability with earlier resistance data will be compromised. However, the new evaluation criteria reflect current knowledge on the pharmacokinetics and pharmacodynamics of antimicrobial substances.

Keywords: antibiotic; bacteriological testing; in-vitro diagnostic agent; pharmakokinetics; resistance testing.

Figure

The distribution of MIC values of E. coli isolates cultured from patient specimens at the Leipzig University Clinic in 2006 (n = 2224). The EUCAST ratings (which have already been published for this data set) are indicated by colored borders: red = r, yellow = i, green = s. Bimodal distributions can be seen, in particular, with the penicillins (e.g., wild types for piperacillin up to MIC = 8 mg/L, resistant strains starting at MIC = 16 mg/L), cotrimoxazole (wild types up to MIC = 8 mg/L, resistant strains starting at MIC = 128 mg/L), and the quinolones (e.g., wild types for ciprofloxacin up to MIC = 0.25 mg/L, strains with clinically relevant resistance mechanisms starting at MIC = 4 mg/L).