Antimicrobial susceptibility testing, also known as antibiotic susceptibility testing, is an essential laboratory procedure used to determine the sensitivity of bacteria or other microbes to antibiotics or antimicrobial drugs. By identifying which antibiotics a microbe is sensitive or resistant to, antimicrobial susceptibility testing helps health care providers determine the most appropriate treatment for infections. This is crucial for effective treatment of infectious diseases and slowing the development of antimicrobial resistance.
What is antimicrobial susceptibility testing?
Antimicrobial susceptibility testing involves exposing microbes, such as bacteria, viruses, fungi or parasites that cause infections, to antimicrobial drugs under controlled laboratory conditions. The microbes are grown in the presence of specific concentrations of antibiotics. This allows technicians to see if the microbes can grow or if they are inhibited by the antibiotics. Growth of the microbes indicates resistance to that antibiotic, while inhibition or absence of growth indicates the microbe is susceptible or sensitive to that antibiotic.
Susceptibility to a drug is described as either sensitive, intermediate or resistant. Sensitive indicates the microbe is inhibited by usual concentrations of the drug. Resistant means the microbe is not affected by usual concentrations of that antibiotic. An intermediate result means that higher concentrations of the drug may inhibit the microbe.
Types of tests used
There are different methods used for performing antimicrobial susceptibility testing, with variations depending on the specific microbe being tested. Some common types include:
- Disk diffusion tests - Paper disks impregnated with specific antibiotics are placed on agar plates inoculated with the microbe. After incubation, the diameter of any growth inhibition zones around the disks are measured to determine sensitivity.
- Broth microdilution - Serial dilutions of antibiotics in broth cultures are inoculated with standardized amounts of bacteria. After incubation, the minimum inhibitory concentration (MIC) of each drug is determined, indicating sensitivity.
- Etest - Premeasured concentration gradients of antibiotics are applied to agar plates using plastic test strips. After incubation, MIC values are read directly off the strip.
- Automated systems - Robotics allows high volume laboratories to perform susceptibility testing faster using standardized broth microdilution methods in sealed microplates. Results are reported directly from the instrument.
Factors impacting the tests
Proper execution of Antimicrobial Susceptibility Testing is crucial for accurate and meaningful results. Risk of contamination must be minimized. Standardized procedures are followed to ensure reproducibility, such as incubating at proper temperatures, using pure isolated cultures of a single microbial species, and interpreting zone diameters or MIC values based on established clinical breakpoints. The health and growth conditions of the tested microbes as well as consistency of antibiotic concentrations can impact test outcomes if not standardized. Proper quality control also monitors for test variability and reliability over time.
Clinical significance and applications
The results of antimicrobial susceptibility testing directly impact patient care by aiding selection of the most effective antibiotic for treating specific infections. Tests can detect emerging resistance in common community and hospital-acquired pathogens to guide empirical therapy and infection control. Ongoing monitoring reveals shifts in local patterns of resistance which may require changes to clinical practice guidelines and formularies.
Susceptibility testing is routinely performed on bacterial isolates from infections such as pneumonia, bloodstream infections, surgical site infections and UTIs. It is especially important for organisms causing serious diseases like tuberculosis or invasive Salmonellosis, and to detect multidrug-resistant organisms. Testing critical bacterial pathogens helps optimize treatment outcomes while limiting unnecessary broad-spectrum antibiotics and development of further resistance.
Importantly, susceptibility tests are not definitive for predicting response to treatment in an individual patient. Other host and disease factors like severity of illness, site of infection, and pharmacokinetics of the drug also influence clinical effectiveness. But judicious use of results from standardized, accurate susceptibility testing remains a cornerstone of antimicrobial stewardship and management of infectious diseases.
Antimicrobial susceptibility testing is a valuable laboratory procedure that identifies which antibiotic drugs are likely to be effective for treating infections caused by specific bacteria or other microbes. By determining sensitivity patterns, these tests aid clinicians in selecting optimal antibiotic regimens while helping curb the rise of antimicrobial resistance. Ongoing monitoring through standardized and quality controlled methods ensures susceptibility data remains meaningful to guide clinical practice and public health efforts against infectious diseases.
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