Antimicrobial susceptibility testing

Introduction

Microbiology laboratories have an important role in tackling antimicrobial resistance (AMR), largely through routinely performed antimicrobial susceptibility testing (AST). AST is used to detect resistance in clinically important bacteria and provide information about which antibiotic therapeutic options will be effective against a given bacterial pathogen. Standardised AST data are also used to track the emergence and spread of antibiotic resistance in human and animal populations, and are an integral part of a One Health approach to AMR surveillance.

This module focuses on phenotypic methods used for AST. These rely on observable characteristics of a bacterial strain, such as whether it can grow at particular concentrations of the antibiotic being tested. Phenotypic methods for AST are based on measuring the minimum inhibitory concentration (MIC) of an antibiotic agent, and the availability of a clinical breakpoint. These terms are explained in the following sections.

It is also possible to use genotypic methods to find out whether an antibiotic will work against a pathogen. Genotypic methods detect the presence of resistance genes or mutations conferring resistance in a bacterial isolate. To get the full resistance profile for an organism, however, would require multiple different commercial test kits or whole genome sequencing; at the moment, phenotypic methods remains more widely used in clinical laboratories. Genotypic methods are covered in the Testing for methods of resistance module.

After completing this module, you will be able to:

• describe the principles of AST
• understand the concepts of phenotypic methods for AST
• understand the process of disk diffusion testing and the relevant quality measures needed
• understand the principles of other phenotypic methods, including automated methods
• understand that the method used may depend on the organism being tested
• understand the concept of MICs and breakpoints
• recognise relevant international guidelines, and know where to find them and when to use them
• recognise that additional testing may be needed to identify/confirm resistance mechanisms
• recognise the importance of procedures designed to ensure the quality of AST testing in your workplace.