Antimicrobial Susceptibility Testing in Clinical Microbiology

Antimicrobial susceptibility testing (AST) is a fundamental component of modern clinical microbiology that informs therapeutic decisions and underpins surveillance of antimicrobial resistance. AST determines the in vitro response of bacterial pathogens to antimicrobial agents, guiding clinicians to select effective therapies and mitigate the spread of resistance. Standardised methods—including disk diffusion, broth microdilution and automated systems—yield categorical interpretations based on minimum inhibitory concentration (MIC) breakpoints established by international bodies. Two primary guideline authorities, the Clinical and Laboratory Standards Institute (CLSI) and the European Committee on Antimicrobial Susceptibility Testing (EUCAST), define these breakpoints and harmonise testing protocols globally, although regional variation in implementation persists. Recent advances integrate mass spectrometry for rapid species identification and whole-genome sequencing to predict resistance phenotypes, streamlining AST workflows and enhancing resolution at the genetic level. Crucial to surveillance efforts, AST data contribute to national and international resistance monitoring networks, inform empirical therapy in diverse clinical settings and support antimicrobial stewardship initiatives. Novel platforms that combine phenotypic testing with genotypic analysis promise to accelerate turnaround times and improve diagnostic precision, addressing urgent global health challenges posed by multidrug-resistant organisms.

Research from Nature Portfolio

Recent studies have compared the interpretation of susceptibility patterns for opportunistic respiratory pathogens using different guideline frameworks. Investigations into Haemophilus parainfluenzae isolated from human respiratory microbiota have revealed notable discrepancies between CLSI and EUCAST breakpoints, particularly regarding beta-lactam resistance phenotypes. These analyses demonstrated that more restrictive EUCAST categories reclassify a higher proportion of strains as resistant or requiring increased exposure, underscoring the impact of interpretive criteria on reported resistance rates. The findings highlight the necessity for clear communication of guideline choice in clinical reports and the potential implications for dosing strategies and therapeutic outcomes.

Antimicrobial Susceptibility Testing in Clinical Microbiology publication trend

The graph below shows the total number of articles in antimicrobial susceptibility testing in clinical microbiology across all publications each year (not limited to Nature Index journals).

Technical terms

Minimum inhibitory concentration (MIC): The lowest concentration of an antimicrobial that prevents visible growth of a microorganism in vitro.

Breakpoint: A defined MIC or zone diameter threshold used to classify microorganisms as susceptible, intermediate (increased exposure) or resistant to an antimicrobial.

EUCAST: European Committee on Antimicrobial Susceptibility Testing, responsible for setting breakpoint criteria and standardising AST methods in Europe.

CLSI: Clinical and Laboratory Standards Institute, an organisation that develops guidelines for laboratory testing standards including antimicrobial breakpoints.

Disk diffusion: A phenotypic AST method in which antibiotic-impregnated disks are placed on agar inoculated with the test organism and zones of inhibition are measured.

Broth microdilution: A quantitative AST method where serial dilutions of antimicrobials in broth are incubated with the test organism to determine the MIC.

MALDI-TOF MS: Matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry, used for rapid identification of microorganisms based on protein mass spectra.

References

1. Antimicrobial susceptibility testing and reporting practices of public hospital microbiology laboratories in Greece, 2022: A national observational survey and call for action. Eurosurveillance (2023).
2. Diverse Role of blaCTX-M and Porins in Mediating Ertapenem Resistance among Carbapenem-Resistant Enterobacterales. Antibiotics (2024).
3. Galaxy ASIST: A web-based platform for mapping and assessment of global standards of antimicrobial susceptibility: A case study in Acinetobacter baumannii genomes. Frontiers in Microbiology (2023).
4. Prevalence of susceptibility patterns of opportunistic bacteria in line with CLSI or EUCAST among Haemophilus parainfluenzae isolated from respiratory microbiota. Scientific Reports (2020).