Antimicrobial susceptibility testing is a laboratory procedure that determines how effectively an antimicrobial drug, such as an antibiotic, can stop the growth of a microorganism. This testing guides the selection of appropriate treatments for infections. Understanding antimicrobial susceptibility is fundamental for managing infectious diseases and ensuring patient recovery.
The Rise of Antimicrobial Resistance
Antimicrobial resistance (AMR) occurs when microbes, including bacteria, viruses, fungi, and parasites, evolve and no longer respond to the drugs designed to kill them. This makes common infections difficult, or sometimes impossible, to treat. The misuse and overuse of antimicrobial agents in human and animal health are key drivers of this resistance.
AMR is a significant global public health threat, contributing to prolonged illnesses, increased hospital stays, and higher mortality rates. In 2019, bacterial AMR was directly responsible for an estimated 1.27 million global deaths and contributed to 4.95 million deaths.
Determining Susceptibility
Laboratories employ several methods to determine antimicrobial susceptibility. One widely used technique is the Minimum Inhibitory Concentration (MIC) test. MIC measures the lowest concentration of an antimicrobial drug that prevents the visible growth of a microorganism after a standardized incubation period, typically expressed in micrograms per milliliter (µg/mL) or milligrams per liter (mg/L). This quantitative method involves preparing a series of decreasing drug concentrations in a broth, then inoculating each with a standardized bacterial culture. The lowest concentration tube that remains clear, indicating no visible growth, is the MIC.
Another common method is the Disk Diffusion test, also known as the Kirby-Bauer test. This qualitative procedure involves uniformly spreading the isolated microorganism across the surface of an agar plate. Small paper disks, each impregnated with a specific antimicrobial agent, are then placed on the inoculated agar. As the plate incubates, the antimicrobial diffuses from the disk into the agar, creating a concentration gradient.
If the antimicrobial inhibits the microorganism’s growth, a clear circular area, called a “zone of inhibition,” forms around the disk. The size of this zone is measured in millimeters, with larger zones indicating greater susceptibility to that particular drug. Automated systems are also available that can streamline both MIC and disk diffusion testing, offering efficient and standardized results.
Understanding Your Test Results
After susceptibility testing, results are categorized to guide treatment options. The most common categories are Susceptible (S), Intermediate (I), and Resistant (R). A “Susceptible” (S) result indicates the microorganism is likely inhibited by the drug at the usual recommended dosage, indicating likely treatment success.
An “Intermediate” (I) result implies the drug may be effective under certain conditions, such as when a higher dosage is used, or if the drug concentrates well at the infection site. Response rates for intermediate isolates may be lower than for susceptible ones. A “Resistant” (R) result signifies the drug is unlikely to inhibit the microorganism at achievable concentrations, meaning treatment will likely fail.
These interpretations are based on established clinical breakpoints, which are specific values that classify an isolate into one of these categories. Organizations like the Clinical and Laboratory Standards Institute (CLSI) and the European Committee on Antimicrobial Susceptibility Testing (EUCAST) develop and regularly update these breakpoints. These breakpoints correlate laboratory test results with expected clinical outcomes, ensuring the reported categories are clinically meaningful.
Tailoring Treatment
Antimicrobial susceptibility results are important in patient care, enabling healthcare providers to select the most effective treatment for an infection. Doctors use these results to choose a specific antimicrobial drug that is likely to work against the identified pathogen. This personalized approach helps avoid ineffective drugs, which can prolong illness, worsen patient outcomes, and contribute further to the development of drug resistance.
The concept of personalized medicine in infectious diseases emphasizes tailoring treatment based on the specific characteristics of the infecting microbe and the patient. Susceptibility testing is central to this approach, providing precise data that guides the choice of antimicrobial and its dosage. Beyond individual patient care, susceptibility testing plays a broader role in antibiotic stewardship programs. These programs aim to optimize antimicrobial use, minimize patient harm, and control the spread of resistance by ensuring appropriate selection, dosing, and duration of therapy. The data from susceptibility testing also contributes to public health surveillance, helping to track resistance trends and inform treatment guidelines on a larger scale.