Introduction to Antimicrobial Susceptibility Testing
Antimicrobial susceptibility testing, also known as antibiotic sensitivity testing, is a laboratory procedure that determines the sensitivity of bacteria, fungi, or other microbes to specific antibiotics or antimicrobial agents. This test helps clinicians determine the most appropriate antibiotic to prescribe for a particular infection based on which antimicrobial agents the microbe causing the infection is sensitive or resistant to. By matching the infecting microbe to an antibiotic it is susceptible to, Antimicrobial Susceptibility Testing helps maximize the effectiveness of treatment and reduce the development of antimicrobial resistance.
How Antimicrobial Susceptibility Testing Works
Susceptibility testing involves first isolating and identifying the microbe causing an infection from a clinical sample such as blood, urine, sputum, or wound culture. Once isolated and identified, the microbe is tested against a panel of antibiotics that may potentially treat the infection. There are two main methods used for susceptibility testing - disk diffusion testing and minimum inhibitory concentration (MIC) determination.
In disk diffusion testing, filter paper disks impregnated with specific antibiotics are placed on culture plates inoculated with the isolated microbe. As the antibiotics diffuse out of the disks and into the surrounding agar, a gradient is created. The microbe is then incubated to allow growth. The diameter of any resultant zone of inhibition surrounding each antibiotic disk indicates whether the microbe is susceptible, intermediate, or resistant to that antibiotic.
MIC determination quantifies the concentration of antibiotic needed to inhibit visible growth of the microbe. Broth or agar is inoculated with the microbe and serial dilutions of the antibiotic being tested. Tubes or wells containing the varying antibiotic concentrations are then incubated and observed for growth. The lowest concentration that inhibits visible growth is reported as the MIC value. MICs can be determined manually or using automated systems.
Using Susceptibility Results to Improve Patient Care
Once tested, susceptibility results report which antibiotics the isolated microbe is sensitive or resistant to. For bacterial infections, therapies are chosen based on their likelihood to effectively treat the infection at typical achievable concentrations, presence of resistance mechanisms, and the patient's treatment history and risk factors. Choosing antibiotics the microbe has proven susceptible to based on test results maximizes the treatment's chances of success and clinical effectiveness.
For patients with serious or refractory infections, susceptibility testing also helps determine whether multidrug-resistant organisms may be involved that require alternative treatment regimens. Monitoring susceptibility patterns over time further aids clinicians by revealing emerging resistance in a community or institution. Overall, the test optimizes antibiotic selection and dosing for each individual patient's infection based on science rather than guesswork to improve outcomes.
Limitations and Challenges of Susceptibility Testing
While susceptibility testing provides valuable clinical guidance, some limitations and challenges exist. Results depend on correct microbial identification, as certain organisms like yeasts require specialized testing and breakpoints differ between species. Mixed infections complicate analysis if more than one microbe is present in a sample. There are also issues with microbes having an incorrect or evolving phenotype versus genotype that may affect accuracy.
Additionally, some microbes inherently demonstrate lower reliability in testing. Fungi and certain anaerobes can be difficult to culture, impairing the ability to perform susceptibility testing. Rapidlygrowing bacteria like enterococci and nonfermenting gram-negatives may exhibit variable results upon repeat testing. Lastly, new antimicrobial agents still lack defined clinical and laboratory breakpoints before being incorporated into routine testing panels. Constant practice and quality control is thus needed to optimize testing's utility.
Extended-Spectrum Beta-Lactamases and Susceptibility Testing
A major challenge clinically is carbapenem-resistant Enterobacteriaceae producing carbapenemases like the New Delhi Metallo-β-lactamase (NDM) or Klebsiella pneumoniae carbapenemase (KPC). Carbapenems have historically been reserved for treating multidrug-resistant gram-negative infections. However, carbapenemases hydrolyze and confer resistance even to this "last resort" class of antibiotics.
Carbapenemase production is consequently identified through phenotypic tests like the Modified Hodge Test or Carba NP test that detect decreased susceptibility to carbapenems. Genotypic analysis helps characterize the specific resistance gene. While genetic methods are more reliable for detection, susceptibility tests remain valuable for tracking the spread and changing resistance patterns conveyed by these clinically-important beta-lactamase enzymes. Continued surveillance mitigates their threat and informs optimal treatment options.
Antimicrobial susceptibility testing is a cornerstone practice of clinical microbiology that directly supports clinicians in selecting effective antibiotic therapies. By matching the causative microbe to antibiotics it demonstrates sensitivity towards, the tests help maximize clinical cure and minimize unnecessary exposure to broad-spectrum agents to curb resistance development. Ongoing quality control and surveillance further refines test reliability and captures emerging resistance to optimize patient outcomes through improved antimicrobial stewardship now and in the future.
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