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Microbiology

Susceptibility testing

This section discusses details of three antimicrobial susceptibility testing (AST) methods: broth microdilution, critical concentration, and direct detection of resistance markers

The Goal: Guiding Effective Antimicrobial Therapy

Antimicrobial susceptibility testing is essential for guiding the selection of appropriate antimicrobial agents to treat infections caused by Mycobacteria and Nocardia. These organisms often exhibit intrinsic resistance to many commonly used antibiotics, making accurate and reliable AST critical for successful treatment outcomes

Broth Microdilution

  • Principle: A quantitative method that determines the Minimum Inhibitory Concentration (MIC) of an antimicrobial agent, or the lowest concentration that inhibits visible growth
  • Procedure: Serial dilutions of antibiotics are prepared in broth, inoculated with the organism, and incubated. The MIC is determined by visually inspecting the wells for growth
  • Results: Provides quantitative MIC values, which are interpreted using established breakpoints to categorize the organism as susceptible, intermediate, or resistant
  • Advantages: Provides quantitative data, can be automated for high-throughput testing, versatile for testing various agents and organisms
  • Limitations: Can be labor-intensive, subjective interpretation, requires specialized media, may not accurately predict activity against biofilms

Critical Concentration

  • Principle: A qualitative or semi-quantitative method primarily used for Mycobacterium tuberculosis. Tests the organism’s ability to grow in the presence of a single, predetermined concentration of an antimicrobial agent
  • Procedure: Organisms are inoculated onto solid media containing a specific “critical concentration” of the drug. Growth or no growth is assessed after incubation
  • Results: Qualitative (susceptible or resistant) based on growth at the critical concentration
  • Advantages: Simple to perform, cost-effective, provides relatively rapid results
  • Limitations: Qualitative, limited scope (tests only a few drugs), may not detect low-level resistance, subjective interpretation, does not detect heteroresistance

Direct Detection of Resistance Markers

  • Principle: Molecular methods used to detect specific genetic mutations associated with resistance to antimicrobial agents, directly from clinical specimens or cultures
  • Procedure: Nucleic acid amplification tests (NAATs), DNA sequencing, or hybridization assays are used to identify specific resistance genes or mutations
  • Results: Detection of specific resistance markers indicates resistance to the corresponding antimicrobial agent
  • Advantages: Rapid results, detects specific resistance mechanisms, can be performed directly on clinical specimens
  • Limitations: Limited scope (detects only known resistance mechanisms), can be more expensive, requires specialized equipment and expertise, doesn’t replace phenotypic methods

Choosing the Right Method

The choice of AST method depends on several factors, including:

  • The organism being tested
  • The available resources and expertise
  • The clinical context
  • The need for quantitative versus qualitative results

Direct detection of resistance markers is increasingly used for rapid detection of resistance in M. tuberculosis, while broth microdilution remains the gold standard for many other organisms. The critical concentration method may be useful in resource-limited settings