Antibiotic Susceptibility Testing

Antibiotic susceptibility testing is the set of laboratory methods that measure whether a bacterial isolate is inhibited by a given antibacterial drug and at what concentration. Its central output is the minimum inhibitory concentration or an equivalent zone measurement, which is compared against standardised breakpoints to classify the isolate as susceptible, intermediate (or susceptible-dose-dependent), or resistant.

Definition

Antibiotic susceptibility testing is the in vitro determination of a bacterial isolate's response to antibacterial agents, quantified as a minimum inhibitory concentration or inhibition-zone diameter and interpreted against defined breakpoints to assign a susceptibility category.

Scope

The entry covers the main testing methods (broth and agar dilution, disk diffusion, gradient strips, and automated systems), the concept of the minimum inhibitory concentration, and how interpretive breakpoints from standards bodies translate a measurement into a category. It is a reference and educational topic on methodology and does not provide treatment selection or dosing guidance.

Core questions

• What is the minimum inhibitory concentration and how is it measured?
• How do dilution, disk-diffusion, gradient, and automated methods differ?
• How do breakpoints translate a measurement into susceptible, intermediate, or resistant?
• What roles do standards bodies such as CLSI and EUCAST play?

Key concepts

• Minimum inhibitory concentration (MIC)
• Broth and agar dilution
• Disk diffusion (Kirby-Bauer)
• Gradient diffusion (MIC test strips)
• Automated susceptibility systems
• Interpretive breakpoints
• Susceptible / intermediate / resistant categories
• CLSI and EUCAST standards and expert rules

Mechanisms

Susceptibility testing exposes a standardised inoculum of the isolate to defined antibiotic concentrations. Reference dilution methods (broth microdilution, agar dilution) determine the minimum inhibitory concentration directly as the lowest concentration that prevents visible growth. Disk diffusion places antibiotic-impregnated disks on inoculated agar and measures the zone of growth inhibition, which correlates with the MIC; gradient strips read an MIC where the inhibition ellipse crosses a calibrated scale; and automated instruments derive results from growth detection over time (Jorgensen & Ferraro, 2009). The raw measurement is then interpreted against breakpoints — concentrations that separate susceptible from resistant — set by standards organisations such as CLSI and EUCAST, whose expert rules also flag results that are inconsistent with known resistance mechanisms (Leclercq et al., 2013; Blair et al., 2015).

Clinical relevance

Susceptibility testing connects the microbiology laboratory to clinical decision-making by reporting, for an isolate, which agents are likely to be active. The category assignments and breakpoints described here underpin that reporting and inform stewardship and surveillance. This entry explains the methods for orientation and study; it is not a basis for choosing or dosing therapy.

Epidemiology

Aggregated susceptibility results form antibiograms and feed resistance-surveillance systems, and the harmonisation of breakpoints and expert rules by CLSI and EUCAST is what makes results comparable across laboratories and over time (Jorgensen & Ferraro, 2009; Leclercq et al., 2013).

History

Standardised susceptibility testing was consolidated in the 1960s, when the Kirby-Bauer disk-diffusion method established reproducible procedures and interpretive criteria. Subsequent decades brought commercial dilution panels, gradient-strip methods, and automated instruments, alongside the formalisation of breakpoints and expert rules by CLSI and EUCAST (Jorgensen & Ferraro, 2009; Leclercq et al., 2013).

Debates

How should breakpoints be set and harmonised?

Breakpoints define the susceptible-resistant boundary and differ in places between standards systems such as CLSI and EUCAST; their derivation from pharmacokinetic-pharmacodynamic data, MIC distributions, and resistance mechanisms is a continuing area of methodological refinement.

Key figures

• James H. Jorgensen
• Mary Jane Ferraro
• Roland Leclercq

Seminal works

• jorgensen-ferraro-2009
• leclercq-2013

Frequently asked questions

What does the minimum inhibitory concentration mean?

The MIC is the lowest concentration of an antibiotic that prevents visible bacterial growth in vitro; it is the core quantitative result of susceptibility testing and is interpreted against breakpoints.

What do 'susceptible', 'intermediate', and 'resistant' mean on a report?

They are interpretive categories assigned by comparing the measured MIC or inhibition zone to standardised breakpoints, indicating whether the isolate is likely inhibited by, of uncertain response to, or not inhibited by the drug under standard conditions.