Antimicrobial Susceptibility Testing

Antimicrobial susceptibility testing (AST) is the laboratory measurement of whether a microorganism's growth is inhibited by an antimicrobial agent, and at what concentration. By exposing an isolate to defined drug concentrations and reading the result against established breakpoints, the laboratory categorises the organism as susceptible, intermediate (or susceptible-dose-dependent), or resistant.

Definition

Antimicrobial susceptibility testing is the in vitro determination of the activity of antimicrobial agents against a microorganism, typically expressed as a minimum inhibitory concentration and interpreted against standardised breakpoints as susceptible, intermediate, or resistant.

Scope

The entry covers the principles of dilution and diffusion methods, the minimum inhibitory concentration, interpretive breakpoints and their categories, the detection of clinically important resistance mechanisms, and the role of susceptibility data in resistance surveillance. It is presented as laboratory methodology; it does not give drug choices or dosing for any patient.

Core questions

• Is this organism's growth inhibited by a given antimicrobial agent, and at what concentration?
• How is the minimum inhibitory concentration measured and interpreted against breakpoints?
• Which methods - broth or agar dilution, disk diffusion, gradient strips, automated systems - are used, and what are their trade-offs?
• How are specific resistance mechanisms detected and reported, and how do susceptibility data feed surveillance?

Key concepts

• Minimum inhibitory concentration (MIC)
• Interpretive breakpoints
• Susceptible / intermediate / resistant categories
• Broth and agar dilution methods
• Disk diffusion and gradient diffusion
• Automated susceptibility systems
• Resistance mechanisms and their detection
• Antimicrobial resistance surveillance

Mechanisms

Susceptibility testing exposes a standardised inoculum of the organism to a range of antimicrobial concentrations and reads inhibition of growth. Dilution methods determine the minimum inhibitory concentration (MIC), the lowest concentration that prevents visible growth; diffusion methods read a zone of growth inhibition around a drug-containing disk or gradient strip and relate its size to susceptibility. Results are interpreted against breakpoints - concentration thresholds set by standards bodies - to assign susceptible, intermediate, or resistant categories (Jorgensen & Ferraro, 2009). Some workflows additionally detect specific resistance determinants, because resistance can arise through mechanisms such as enzymatic drug modification, target alteration, efflux, or transferable plasmid-borne genes (Strahilevitz et al., 2009), and resistance traits interact in complex ways with bacterial virulence and fitness (Beceiro et al., 2013). Faster and more informative susceptibility testing is part of the broader agenda for improved infectious-disease diagnostics (Caliendo et al., 2013).

Clinical relevance

Susceptibility results are a key input to clinical reasoning about treating an infection and to antimicrobial stewardship, but their interpretation depends on the site of infection, the agent, and patient factors. This entry explains how susceptibility is measured and categorised; it is reference material and is not a source of drug selection or dosing recommendations for any individual.

Epidemiology

Aggregated susceptibility results are a primary data source for antimicrobial-resistance surveillance, tracking how resistance emerges and spreads in populations and healthcare settings. The recognition that resistance can be carried on mobile genetic elements (Strahilevitz et al., 2009) and can co-vary with virulence (Beceiro et al., 2013) underlies the public-health attention to laboratory resistance data.

History

Standardised susceptibility testing developed through the twentieth century from ad hoc growth-inhibition observations into defined dilution and diffusion methods with reference standards and interpretive breakpoints, later supplemented by automated systems and molecular detection of resistance genes. Reviews of general principles and contemporary practice describe how these methods and their interpretive frameworks became standardised (Jorgensen & Ferraro, 2009).

Seminal works

• jorgensen-2009
• strahilevitz-2009
• beceiro-2013

Frequently asked questions

What does a minimum inhibitory concentration (MIC) tell you?

The MIC is the lowest concentration of an antimicrobial that prevents visible growth of the organism in vitro. Compared against standardised breakpoints, it is used to categorise the organism as susceptible, intermediate, or resistant (Jorgensen & Ferraro, 2009).

Why are there different susceptibility testing methods?

Dilution methods give a numeric MIC, while diffusion methods (disk or gradient) infer susceptibility from a zone of inhibition; automated systems speed throughput. Laboratories choose among them based on the organism, the drugs, and the need for a numeric result versus speed.