April 22, 2024

Antimicrobial Susceptibility Testing: An Important Tool to Combat Antimicrobial Resistance

With antibiotic resistant infections spreading worldwide, it has become crucial for clinicians to identify effective treatment options for infections. Antimicrobial susceptibility testing plays a key role in guiding appropriate antibiotic usage and slowing the development of resistance. This article discusses the importance and process of antimicrobial susceptibility testing.

What is Antimicrobial Susceptibility Testing?

Antimicrobial susceptibility testing, also known as antibiotic sensitivity testing, determines which antibiotics or antimicrobial drugs are likely to work best to treat an infection caused by a particular microbe. A sample containing the infecting microorganism is exposed to various antibiotics to see which, if any, can effectively inhibit or kill the microbe. This helps clinicians narrow down treatment options to antibiotics that have the best chance of curing the infection.

Susceptibility testing provides important guidance when choosing empiric antibiotic therapy but does not guarantee clinical outcome, as factors like drug dosing, patient compliance, and immune status also impact treatment success. Testing should be performed on clinically relevant isolates from normally sterile body sites like blood, cerebrospinal fluid or joint fluid. Results help clinicians avoid ineffective antibiotics and reduce selective pressure that drives resistance development.

Importance of Testing

With the rise of multidrug-resistant organisms, susceptibility testing plays a crucial role in guiding appropriate antimicrobial use. When resistant organisms infect patients, ineffective antibiotics provide no clinical benefit while still selecting for further resistance. This testing helps clinicians choose targeted therapy and avoid broad-spectrum antibiotics when narrower-spectrum drugs may work.

It is especially important for infections caused by organisms already known to have high resistance rates like methicillin-resistant Staphylococcus aureus (MRSA), vancomycin-resistant enterococci (VRE), multidrug-resistant gram-negatives and resistant tuberculosis. Testing also benefits public health by providing resistance surveillance data to track emerging threats.

The Testing Process

Several standardized methods can be used to perform antimicrobial susceptibility testing in the microbiology laboratory. The most common techniques are discussed here.

Disk Diffusion Method

Disk diffusion uses antibiotic-impregnated paper disks placed onto inoculated agar plates. As the antibiotic diffuses radially outward, a concentration gradient is established. Microbes are incubated and any zone of growth inhibition around a disk indicates susceptibility to that antibiotic. The size of the zone is interpreted using clinical breakpoints.

Automated broth microdilution systems

Automated systems like VITEK or BD Phoenix use microwell plates with serial antibiotic dilutions. Inoculated wells are incubated and monitored by automated readings. The minimum inhibitory concentration (MIC), or lowest concentration that prevents visible growth, is determined and matched to clinical breakpoints. This method is considered the reference standard by organizations like CLSI.

Gradient strip method

Gradient strips contain an antibiotic gradient established along a narrow plastic strip. Inoculated agar plates are overlayed with the strip and incubated. Growth is assessed along the inhibitory gradient to determine the MIC in a manner similar to broth microdilution.

Reporting and Interpretation

After 18-24 hours of incubation, test results are reported as susceptible, intermediate or resistant categories based on predefined clinical breakpoints. These interpretive standards are periodically re-evaluated and updated by expert organizations like CLSI based on current pharmacokinetic/pharmacodynamic and resistance data. Reporting standardized results helps clinicians across different institutions make consistent treatment decisions.

Quality Control

To assure accuracy and reproducibility, each Antimicrobial Susceptibility Testing  test run undergoes quality control testing. Reference organisms with known susceptibility profiles are tested in parallel to make sure systems are performing within acceptable performance ranges established by standards organizations. Good laboratory practices like technologist competency and equipment calibration also play a vital role in generating reliable susceptibility data.

Applications in the Clinical Setting

With standardized, well-controlled testing, laboratory antimicrobial susceptibility results provide significant clinical utility. Some important applications of these results include:

– Guiding selection of initial empiric therapy before final identification/susceptibility results are available based on the likely pathogens, their usual susceptibility patterns and resistance trends in the facility or community.

– Fine-tuning or changing therapy as needed based on final identification and individual patient’s susceptibility report.

– Helping decide whether to continue, narrow or broaden antibiotic treatment based on progress of the infection.

– Informing decisions about streamlining or de-escalation from intravenous to oral therapy.

– Identifying multidrug-resistant organisms to guide appropriate isolation precautions and infection control measures.

– Providing data for antimicrobial stewardship programs to analyze antibiotic use, resistance patterns and make formulary, guideline or policy changes as needed.

– Aiding epidemiological investigations of institutional outbreaks by comparing resistance mechanisms and molecular fingerprints of implicated isolates.

High quality, standardized antimicrobial susceptibility testing coupled with judicious use of its results remains one of the cornerstones of modern infectious disease management. With the continued spread of resistance, this testing will grow in importance to help combat a looming post-antibiotic era. Continued monitoring of resistance mechanisms and clinical validation of interpretive criteria ensures this valuable tool maintains relevance in battling one of the greatest public health challenges.

1. Source: Coherent Market Insights, Public sources, Desk research
2. We have leveraged AI tools to mine information and compile it