What is a minimum inhibitory concentration (MIC)?

The MIC is the lowest concentration of an antimicrobial that prevents visible growth of an organism under standardized conditions; it is the quantitative result that dilution methods aim to determine.

Why are there different AST methods?

Dilution methods give a numeric MIC, diffusion methods such as disk diffusion are simple and inexpensive, and automated systems offer speed and throughput; laboratories choose among them based on the organism, the drugs tested, and available resources.

What do susceptible, intermediate, and resistant mean?

They are interpretive categories assigned by comparing a measured MIC or zone diameter to breakpoints set by standards bodies; they summarize the likelihood that an organism will respond to an agent under usual conditions and are defined methodologically rather than as treatment advice.

Antimicrobial Susceptibility Testing Methods

Antimicrobial susceptibility testing (AST) is the set of standardized laboratory methods used to determine whether a microorganism is inhibited or killed by an antimicrobial agent and at what concentration. Its outputs, typically a minimum inhibitory concentration (MIC) or an inhibition zone diameter, are interpreted against published breakpoints to classify the organism as susceptible, intermediate, or resistant.

Definition

Antimicrobial susceptibility testing is the standardized measurement of a microorganism's in vitro response to antimicrobial agents, expressed as an MIC or zone diameter and interpreted against breakpoints to assign a susceptible, intermediate, or resistant category.

Scope

This entry covers the principal AST methods, the quantities they measure, and how results are interpreted. It addresses dilution methods (broth microdilution, agar dilution), diffusion methods (disk diffusion, gradient strip), and automated systems, together with the breakpoint systems of CLSI and EUCAST. It is methodological and does not provide dosing or treatment recommendations.

Core questions

What concentration of an antimicrobial inhibits the growth of this organism?
How do dilution and diffusion methods differ in what they measure and how they are read?
How is a measured MIC or zone diameter translated into a clinical category?

Key concepts

Minimum inhibitory concentration (MIC)
Broth microdilution (reference method)
Agar dilution
Disk diffusion (Kirby-Bauer)
Gradient diffusion (MIC gradient strip)
Inhibition zone diameter
Clinical breakpoints (CLSI, EUCAST)
Susceptible-intermediate-resistant categories
Quality control strains and standardized inoculum

Mechanisms

AST methods share a common logic: a standardized inoculum of the organism is exposed to defined concentrations of an antimicrobial under controlled conditions, and growth inhibition is read. Dilution methods (broth microdilution, regarded as a reference method, and agar dilution) determine the lowest concentration that inhibits visible growth, the MIC. Diffusion methods place the drug into agar so that it diffuses to form a concentration gradient: disk diffusion reads the diameter of the zone of inhibition, while gradient strips read the MIC where the inhibition ellipse intersects a calibrated scale. Automated commercial systems generate MICs from miniaturized broth dilution and optical or turbidimetric growth detection. Results are interpreted against breakpoints from CLSI or EUCAST, and expert rules cross-check results for consistency with known resistance phenotypes (jorgensen-2009; leclercq-2013; clsi-m100; eucast-breakpoints).

Clinical relevance

Susceptibility results inform infectious-disease management, antimicrobial stewardship, and surveillance, and they populate the cumulative antibiograms laboratories report. This entry explains how those results are produced and interpreted as reference material; it is not a source of individual diagnostic or prescribing decisions.

Epidemiology

Because AST is standardized, results are comparable across laboratories and over time, enabling aggregation into local antibiograms and into national and international resistance surveillance. Method choice, breakpoint version, and quality control all influence the comparability of these data (clsi-m100; eucast-breakpoints).

History

Susceptibility testing developed in the decades after the introduction of antibiotics. The disk diffusion method was standardized in the 1960s and became widely used for its simplicity, while broth and agar dilution provided quantitative MICs. Over time, standards bodies, notably CLSI in the United States and EUCAST in Europe, formalized methods, breakpoints, and quality control, and automated systems and gradient strips broadened routine practice (jorgensen-2009).

Debates

Differences between CLSI and EUCAST breakpoints: CLSI and EUCAST set breakpoints using related but distinct processes, and their categorizations and the definition of the intermediate category can differ; harmonization and the interpretation of discrepant results are ongoing methodological issues.
Reproducibility near breakpoints: Measured MICs carry inherent variability, so results close to a breakpoint may be categorized differently across replicates or methods, which complicates interpretation of borderline organisms.

Seminal works

jorgensen-2009
leclercq-2013
clsi-m100

Frequently asked questions

What is a minimum inhibitory concentration (MIC)?: The MIC is the lowest concentration of an antimicrobial that prevents visible growth of an organism under standardized conditions; it is the quantitative result that dilution methods aim to determine.
Why are there different AST methods?: Dilution methods give a numeric MIC, diffusion methods such as disk diffusion are simple and inexpensive, and automated systems offer speed and throughput; laboratories choose among them based on the organism, the drugs tested, and available resources.
What do susceptible, intermediate, and resistant mean?: They are interpretive categories assigned by comparing a measured MIC or zone diameter to breakpoints set by standards bodies; they summarize the likelihood that an organism will respond to an agent under usual conditions and are defined methodologically rather than as treatment advice.