Antibiotic susceptibility testing is a laboratory procedure that determines how well specific antibiotics can stop the growth of bacteria or fungi causing an infection. This testing identifies which medications are effective against a particular microorganism, as not all antibiotics work against all types of bacteria or fungi.
The Purpose of Susceptibility Testing
Healthcare professionals request antibiotic susceptibility testing to guide the selection of appropriate treatment for bacterial or fungal infections. When the specific pathogen is identified, testing allows for a shift to targeted therapy, ensuring an antibiotic known to be effective against that pathogen is chosen. This approach leads to more effective treatment outcomes and faster patient recovery.
The testing also plays a role in antibiotic stewardship by preventing the use of ineffective drugs. By providing precise information on drug effectiveness, susceptibility testing helps ensure patients receive the most suitable medication, minimizing unnecessary antibiotic exposure and preserving the effectiveness of these medications over time.
Methods for Determining Susceptibility
Laboratories employ several standardized methods to determine antibiotic susceptibility, with two common approaches being the disk diffusion test and dilution tests.
Disk Diffusion Test
The disk diffusion method, also known as the Kirby-Bauer test, involves growing a uniform layer of bacteria on a specialized agar plate. Small paper disks, each infused with a specific antibiotic, are then placed onto the bacterial lawn. The antibiotic diffuses outwards from the disk into the agar, creating a concentration gradient.
After an incubation period, a clear circular area, known as the “zone of inhibition,” may appear around the disks. This clear zone indicates that the antibiotic has inhibited or killed the bacterial growth in that area. The diameter of this zone is measured, and its size directly correlates with the bacteria’s susceptibility to that antibiotic; a larger zone means the bacteria are more susceptible.
Dilution Tests
Dilution tests, such as broth or agar dilution, provide a more quantitative measure by determining the Minimum Inhibitory Concentration (MIC). The MIC is the lowest concentration of an antibiotic that visibly prevents the growth of a microorganism after overnight incubation.
In broth dilution, a series of test tubes or wells contain a liquid growth medium with progressively decreasing concentrations of a specific antibiotic. A standardized amount of the bacterial suspension is added to each. After incubation, the tubes or wells are examined for visible bacterial growth. The MIC is identified as the lowest antibiotic concentration where no visible growth occurs, providing a precise numerical value of effectiveness.
Understanding the Results
Once susceptibility testing is complete, the results are reported in one of three categories: Susceptible (S), Intermediate (I), or Resistant (R). These classifications help clinicians understand the likely effectiveness of an antibiotic against the identified pathogen.
Susceptible (S)
This designation means there is a high probability of therapeutic success if the antibiotic is administered using a standard dosing regimen. It indicates that the bacteria are inhibited by drug concentrations typically achieved in the body.
Intermediate (I)
This category suggests that the antibiotic might be effective but only under specific conditions, such as a higher dosage, more frequent administration, or if the drug concentrates well at the infection site.
Resistant (R)
This result indicates a high likelihood of therapeutic failure, even with increased dosage. The antibiotic is not expected to inhibit the growth of the bacteria at levels achievable in the patient.
These interpretations are based on established “breakpoints” set by organizations like the Clinical and Laboratory Standards Institute (CLSI) or the European Committee on Antimicrobial Susceptibility Testing (EUCAST).
The Relationship Between Susceptibility and Resistance
A “Resistant” result on an antibiotic susceptibility test means that the particular bacteria have developed mechanisms to overcome the effects of a specific antibiotic. This finding is directly linked to the broader global public health concern of antibiotic resistance, also known as antimicrobial resistance (AMR). AMR is a significant threat, contributing to an estimated 1.27 million deaths annually worldwide, with projections indicating a substantial increase if current trends continue.
Antibiotic susceptibility testing serves as a surveillance tool, providing data that extends beyond individual patient treatment. By tracking the patterns of susceptibility and resistance across different bacterial strains within a hospital or region, aggregated data, often compiled into “antibiograms,” can be created. These antibiograms offer insights into local resistance trends, which are then used to inform hospital-wide treatment guidelines and help doctors make initial treatment decisions when the specific susceptibility of an infection is not yet known. Such data also contribute to national and international surveillance systems, like the World Health Organization’s Global Antimicrobial Resistance Surveillance System (GLASS), which monitor the emergence and spread of resistant strains. This collective effort aids public health officials in understanding the scope of the resistance problem and developing strategies to preserve the effectiveness of existing antibiotics for future use.