Antimicrobial Susceptibility Testing Methods: A Comparative Overview for Clinical Microbiology

Antimicrobial susceptibility testing (AST) determines whether a bacterial isolate is susceptible, intermediate, or resistant to specific antimicrobial agents. Clinical microbiology laboratories use AST to guide treatment decisions, monitor resistance trends, and support antimicrobial stewardship programs. The three primary phenotypic methods are disk diffusion, broth dilution, and gradient diffusion. Each method has distinct principles, workflows, cost structures, and interpretive frameworks. This article compares these methods for laboratory students, technicians, researchers, and diagnostic professionals, with emphasis on practical selection criteria, quality control requirements, and result interpretation.

AST is a critical function of the clinical microbiology laboratory and is essential for optimizing care of patients with infectious diseases, monitoring antimicrobial resistance trends, and informing public health initiatives. Several methods are available for performing AST including broth microdilution, agar dilution, and disk diffusion. Technological advances such as the development of commercial automated susceptibility testing platforms and the advent of rapid diagnostic tests have improved the rapidity, robustness, and clinical application of AST. Numerous accrediting and regulatory agencies are involved in the process of AST and setting and revising breakpoints, including the U.S. Food and Drug Administration and the Clinical and Laboratory Standards Institute. Challenges to optimizing AST include the emergence of new resistance mechanisms, the development of new antimicrobial agents, and generation of new data requiring updates and revisions to established methods and breakpoints. Together, the challenges in AST methods and their interpretation create important opportunities for well-informed clinicians to improve patient outcomes and provide value to antimicrobial stewardship programs, especially in the setting of rapidly changing and increasing antimicrobial resistance. Addressing AST challenges will involve continued development of new technologies along with collaboration between clinicians and the laboratory to facilitate optimal antimicrobial use, combat the increasing burden of antimicrobial resistance, and inform the development of novel antimicrobials. This updated primer serves to reinforce important principles of AST, and to provide guidance on their implementation and optimization.

At a Glance: Comparison of Core AST Methods

The table below summarizes the key operational characteristics of the three conventional phenotypic AST methods. Laboratories select methods based on workload, organism types, antimicrobial panels needed, available expertise, and budget.

The choice of method depends on laboratory resources and clinical needs. Disk diffusion is economical and flexible for routine work. Broth microdilution provides quantitative MIC values and serves as a reference method. Gradient diffusion offers a practical compromise when MIC values are needed without the labor of preparing dilution panels.

Core Principles of Antimicrobial Susceptibility Testing

AST measures the in vitro activity of an antimicrobial agent against a bacterial isolate under standardized conditions. The goal is to predict clinical efficacy by categorizing isolates based on established clinical breakpoints. Breakpoints are set by organizations such as the Clinical and Laboratory Standards Institute (CLSI) and the European Committee on Antimicrobial Susceptibility Testing (EUCAST). These breakpoints are periodically revised as new resistance mechanisms emerge, new antimicrobial agents are developed, and new clinical data become available.

The BSAC Standing Committee on Antimicrobial Susceptibility Testing is one of several European national breakpoint committees that agreed in 2002 to harmonize clinical MIC breakpoints. The process of harmonization has since been completed for commonly used agents, and breakpoints for new agents have been set by EUCAST in accordance with a procedure defined by the European Medicines Agency. EUCAST breakpoints have now been adopted by a large majority of laboratories in Europe. BSAC implemented the EUCAST breakpoints in its own disc diffusion susceptibility testing method as harmonized breakpoints were agreed to over the years. Since the development of the EUCAST disc diffusion method, several countries with their own disc diffusion methods have switched to the EUCAST method, and BSAC will replace support of its own disc diffusion method with support for the EUCAST method from January 2016. The EUCAST breakpoints are also available in automated systems. The harmonized breakpoints and methods will help to avoid different reports of susceptibility for the same isolate in different countries and enable more reliable comparison of resistance rates in surveillance studies in different countries.

Standardization is essential for reproducible AST results. Variables that must be controlled include inoculum density, culture medium composition and depth, incubation temperature and duration, and antimicrobial concentration or content. The Clinical and Laboratory Standards Institute and EUCAST publish detailed protocols for each method. Laboratories must follow these protocols exactly to produce clinically valid results.

The interpretive categories are susceptible, intermediate, and resistant. Susceptible means the organism is likely to respond to treatment with that antimicrobial at the recommended dosing regimen. Intermediate means the organism may respond if the antimicrobial is concentrated at the site of infection or if higher doses are used. Resistant means the organism is unlikely to respond to treatment with that antimicrobial regardless of dose. EUCAST has refined the intermediate category as "susceptible, increased exposure," which indicates that the organism is susceptible to the antimicrobial when exposure is increased through higher dosing or altered administration.

Disk Diffusion Method

The disk diffusion method, also known as the Kirby-Bauer method, is one of the most widely used AST techniques in clinical laboratories. A standardized inoculum of the test organism is spread evenly across the surface of Mueller-Hinton agar. Filter paper disks impregnated with defined concentrations of antimicrobial agents are placed on the agar surface. During incubation, the antimicrobial diffuses from the disk into the agar, creating a concentration gradient. The organism grows across the plate except in areas where the antimicrobial concentration exceeds the organism's minimum inhibitory concentration. The resulting zone of inhibition is measured in millimeters and interpreted using published breakpoint tables.

Procedure and Materials

The disk diffusion method requires Mueller-Hinton agar, antimicrobial disks, a standardized inoculum, and incubation conditions appropriate for the organism being tested. The inoculum is prepared by suspending isolated colonies in sterile saline or broth to a turbidity equivalent to a 0.5 McFarland standard. The suspension is inoculated onto the agar plate within 15 minutes of preparation to avoid changes in viable cell count. The disks are applied within 15 minutes of inoculation to allow the inoculum to dry before the antimicrobial begins to diffuse.

Plates are incubated at 35 to 37 degrees Celsius for 16 to 18 hours in ambient air for most organisms. Fastidious organisms require supplemented media and modified incubation conditions. For example, Haemophilus influenzae requires Haemophilus test medium and increased carbon dioxide, while Streptococcus pneumoniae requires Mueller-Hinton agar with 5 percent sheep blood.

Zone Measurement and Interpretation

Zone diameters are measured to the nearest millimeter using a ruler, caliper, or automated zone reader. The zone edge is read at the point of obvious inhibition of growth. For some organism-drug combinations, such as penicillin with staphylococci, the zone edge must be examined for sharpness. A sharp zone edge indicates beta-lactamase production, while a fuzzy zone edge indicates susceptibility. This interpretation is critical for accurate categorization.

Disc diffusion with penicillin 1 IU and penicillin 10 IU has been compared with nitrocefin discs and automated broth microdilution for Staphylococcus lugdunensis. Penicillin zone diameter and zone edge correlated with blaZ PCR results in all except two penicillin 10 IU susceptible isolates and one penicillin 1 IU resistant isolate. A total of 148 isolates were blaZ negative, of which 146 and 149 isolates were susceptible by penicillin 1 IU and penicillin 10 IU, respectively. A total of 127 were penicillin susceptible by automated broth microdilution. Automated broth microdilution overcalled resistance in 21 blaZ negative, 20 penicillin 1 IU susceptible, and 22 penicillin 10 IU susceptible isolates, with a major error rate of 14.2 percent. Two mecA positive isolates were oxacillin resistant by cefoxitin disc and automated broth microdilution methods, showing categorical agreement. However, 18 cefoxitin susceptible mecA negative isolates tested resistant by automated broth microdilution. In this study, disc diffusion with zone edge interpretation was more accurate and specific than automated broth microdilution for Staphylococcus lugdunensis.

Advantages and Limitations

Disk diffusion is inexpensive, flexible, and well suited for testing large numbers of isolates. Custom panels can be assembled to match the clinical scenario or formulary. The method is widely standardized and does not require specialized equipment beyond an incubator, ruler, and McFarland turbidity standard.

The primary limitation is that disk diffusion provides a qualitative or semi-quantitative result. Zone diameters are not equivalent to MIC values, although they correlate with MIC for many organism-drug combinations. The method is not suitable for slow-growing organisms, anaerobes, or organisms that require prolonged incubation. Factors affecting the disk diffusion method include medium composition, agar depth, inoculum density, disk content, incubation conditions, and the timing of disk application. Each of these factors must be controlled to produce reliable results.

Broth Dilution Method

Broth dilution determines the minimum inhibitory concentration (MIC), which is the lowest concentration of an antimicrobial that inhibits visible growth of the organism. The method is performed in either macrodilution format using test tubes or microdilution format using microtiter plates. Broth microdilution is the most common format and serves as a reference method for evaluating other AST techniques.

Broth Microdilution Procedure

In broth microdilution, serial twofold dilutions of the antimicrobial are prepared in Mueller-Hinton broth within the wells of a microtiter plate. Each well contains a defined antimicrobial concentration, typically ranging from 0.015 to 256 µg/mL depending on the agent and organism. A standardized inoculum is added to each well to achieve a final concentration of approximately 5 x 10^5 colony-forming units per milliliter. The plates are incubated at 35 to 37 degrees Celsius for 16 to 20 hours.

After incubation, the MIC is read as the lowest antimicrobial concentration that prevents visible growth. Growth is indicated by turbidity or a pellet of cells at the bottom of the well. The MIC value is interpreted using clinical breakpoints to categorize the isolate as susceptible, intermediate, or resistant.

Agar Dilution

Agar dilution is an alternative dilution method in which antimicrobial agents are incorporated into agar plates at defined concentrations. A replicator device delivers a standardized inoculum to the surface of each plate. The MIC is the lowest antimicrobial concentration that prevents visible colony growth. Agar dilution is useful for testing multiple isolates simultaneously against a single antimicrobial and is often used for surveillance studies and reference testing. The method is labor-intensive to prepare but efficient for batch testing.

Automated Broth Microdilution Systems

Commercial automated systems perform broth microdilution in a closed format with integrated incubation, reading, and interpretation. These systems provide results within 6 to 16 hours and can test multiple antimicrobials simultaneously. The BD Phoenix NMIC-413 panel has been evaluated for carbapenem-resistant Enterobacteriaceae and carbapenem-resistant Pseudomonas aeruginosa. In a study of 314 archived Gram-negative clinical isolates, the NMIC-413 panel and the disk diffusion method were employed for antimicrobial susceptibility testing of meropenem, imipenem, cefepime, and cefoperazone/sulbactam. Broth microdilution was used as the reference method. For carbapenem-resistant Enterobacteriaceae, the NMIC-413 panel met the acceptable standards and demonstrated higher categorical agreement values than the disk diffusion method for all four antibiotics, with values of 99.3, 96.6, 98.0, and 98.7 percent versus 98.7, 96.0, 96.0, and 97.3 percent, respectively. For carbapenem-resistant Pseudomonas aeruginosa, the NMIC-413 panel also met the acceptable standards and showed superior categorical agreement values for meropenem and cefepime compared to the disk diffusion method, with values of 98.2 and 96.4 percent versus 96.4 and 92.9 percent, respectively. Categorical agreement values for imipenem and cefoperazone/sulbactam were similar between the two methods.

Automated systems reduce hands-on time and improve workflow efficiency. However, they require significant capital investment and ongoing maintenance costs. The performance of automated systems must be validated against reference methods, and categorical agreement, essential agreement, very major error rates, and major error rates must meet established thresholds.

Advantages and Limitations

Broth dilution provides quantitative MIC values that are essential for managing infections caused by organisms with borderline susceptibility or emerging resistance. The method is flexible and can be adapted for fastidious organisms, anaerobes, and antimicrobial agents that are not available as disks or gradient strips. Broth microdilution is the reference method for many organism-drug combinations.

The limitations include the labor and cost of preparing dilution panels, the potential for errors in antimicrobial preparation, and the need for careful quality control. Manual broth microdilution is time-consuming for large workloads. Automated systems address this limitation but introduce their own challenges, including the need for instrument maintenance, software updates, and validation of new antimicrobial panels.

Gradient Diffusion Method

Gradient diffusion uses a plastic strip impregnated with a continuous gradient of antimicrobial concentrations. The strip is placed on an inoculated agar plate, and the antimicrobial diffuses into the agar, creating a gradient that mimics the concentration gradient of disk diffusion but with a continuous range of concentrations. After incubation, an elliptical zone of inhibition forms around the strip. The MIC is read at the point where the zone edge intersects the strip.

Procedure and Interpretation

The gradient diffusion method uses the same agar medium and inoculum preparation as disk diffusion. A 0.5 McFarland standard inoculum is spread evenly across the agar surface. The strip is applied to the dried agar surface using an applicator. Plates are incubated under the same conditions as disk diffusion. After incubation, the MIC is read at the intersection of the zone edge with the graduated scale on the strip.

For some organism-drug combinations, the zone edge must be examined for the presence of microcolonies or a trailing edge. These findings can indicate heteroresistance or partial resistance and may require confirmation with another method. The manufacturer's instructions must be followed for each strip type because the reading conventions vary by organism and antimicrobial.

Advantages and Limitations

Gradient diffusion provides a quantitative MIC value with the simplicity of a disk diffusion setup. The method is useful for confirming MIC values for isolates that test intermediate or resistant by disk diffusion, for testing antimicrobials that are not available in disk form, and for determining MIC values for fastidious organisms when appropriate media and strips are available.

The limitations include the high cost per strip, the limited range of antimicrobials available, and the need for careful interpretation of zone edges. Gradient diffusion is not suitable for all organism-drug combinations, and some combinations require specialized media or incubation conditions. The method is less practical than disk diffusion for high-throughput testing of large numbers of isolates.

Method Selection and Workflow Decisions

Laboratories must select AST methods based on their clinical workload, organism types, antimicrobial panels, available expertise, and budget. The selection process should consider the following factors.

Workload and Throughput

Laboratories processing large numbers of isolates daily benefit from automated broth microdilution systems or disk diffusion with automated zone reading. Manual broth microdilution and gradient diffusion are better suited for lower workloads or confirmatory testing. The turnaround time for results is a critical consideration, particularly for patients with sepsis or other severe infections.

Rapid antimicrobial susceptibility testing from positive blood culture bottles can reduce turnaround time from days to hours. A study of 144 positively flagged semiautomated blood culture bottles showed that rapid antimicrobial susceptibility testing with disk diffusion at 4, 6, and 8 hours correlated with standard antimicrobial susceptibility testing using CLSI M100 as the gold standard. Among 144 positively flagged semiautomated blood culture bottles, 53 showed monomicrobial growth. Escherichia coli, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Staphylococcus aureus were isolated. Categorical agreement for rapid antimicrobial susceptibility testing at 4, 6, and 8 hour readings met the equivalence criteria of categorical agreement of at least 90 percent, very major errors of no more than 1.5 percent, and major errors of no more than 3 percent.

A separate study of 95 positive blood culture isolates compared rapid antimicrobial susceptibility testing by disk diffusion and automated broth microdilution with standard methods. For Gram-negative bacteria, rapid antimicrobial susceptibility testing versus standard disk diffusion showed 91 percent categorical agreement with overall very major error and major error rates of 0.7 and 2.2 percent, respectively. Rapid antimicrobial susceptibility testing versus standard broth microdilution demonstrated a categorical agreement of 97 percent and essential agreement of 97.7 percent, with very major error and major error rates of 1.7 and 0.1 percent, respectively. These findings suggest that rapid antimicrobial susceptibility testing is a reliable tool for Gram-negative bacteria from direct blood culture broths and can support early initiation of targeted antimicrobial therapy in patients with sepsis.

Organism Types

Fastidious organisms, anaerobes, and organisms with unusual growth requirements may not grow under standard AST conditions. Broth microdilution with supplemented media is often required for these organisms. Antimicrobial susceptibility testing of anaerobic bacteria is not routinely performed in many laboratories, and literature on the topic remains scarce. Anaerobes are potential human pathogens and predominate the human microbiota. Despite their significant role in human health and disease, they are not paid enough attention. Educated experience-based treatment has often been instituted for anaerobic infections due to the challenging nature of antimicrobial susceptibility testing and relatively predictable patterns of antimicrobial resistance. However, antimicrobial resistance in anaerobes remains no more predictable, especially in Gram-negative anaerobes like Bacteroides species, where multi-drug resistance is also emerging. The changing antibiograms and the emergence of resistance determinants in anaerobes necessitate routine antimicrobial susceptibility testing, periodic monitoring, and literature review of these organisms. The present scenario dictates limiting the empirical management of anaerobic infections and encouraging consistent antimicrobial susceptibility testing practice in routine by employing economical, faster, and more pragmatic approaches, especially in tertiary care setups that cater to a vast patient population and are held accountable for producing accurate clinical and surveillance data.

Antimicrobial Panel Design

The antimicrobial panel should reflect the organisms being tested, the clinical formulary, and local resistance patterns. Disk diffusion allows flexible panel design, while commercial broth microdilution panels are fixed by the manufacturer. Gradient diffusion strips are purchased individually, which allows selective testing of specific antimicrobials.

Cost Considerations

Disk diffusion is the most economical method for routine testing. Broth microdilution costs are moderate for manual methods and higher for automated systems due to instrument and reagent costs. Gradient diffusion is the most expensive per test but may be cost-effective when MIC values are needed for a limited number of isolates.

Quality Control and Validation

Quality control is essential for producing reliable AST results. Laboratories must implement a quality control program that includes testing reference strains with known susceptibility patterns, monitoring media and reagent performance, and participating in external quality assessment programs.

Quality Control Strains

Reference strains with established susceptibility ranges are tested with each batch of AST to verify that the method, media, reagents, and interpretation are performing correctly. Common quality control strains include Escherichia coli ATCC 25922, Pseudomonas aeruginosa ATCC 27853, Staphylococcus aureus ATCC 29213, and Enterococcus faecalis ATCC 29212. The quality control results must fall within published acceptable ranges for each antimicrobial and method.

Validation of New Methods

New AST methods, including commercial systems and laboratory-developed tests, must be validated before implementation. Validation studies compare the new method with a reference method and calculate categorical agreement, essential agreement, very major error rate, and major error rate. The updated ISO 20776-2 standard removed categorical agreement as a core metric for validation of antimicrobial susceptibility testing devices based on MIC determination, prioritizing essential agreement and bias due to categorical agreement's dependence on evolving clinical breakpoints. This change impacts external quality assessment and proposes essential agreement and bias as primary metrics.

A study validating a novel susceptibility assay for complicated and recurrent urinary tract infections compared the assay to broth microdilution and disk diffusion following CLSI guidelines for assessment of error rates and agreement. The performance measures without correction for heteroresistance showed essential agreement of at least 90 percent, very major errors of less than 1.5 percent, and major errors of less than 3.0 percent, all meeting the threshold guidelines established by CLSI for antimicrobial susceptibility testing. The categorical agreement also met acceptable criteria of greater than 88 percent, as the majority of the errors were minor with essential agreement. The very major and major error rates decreased to less than 1.0 percent when heteroresistance was accounted for.

External Quality Assessment

External quality assessment programs provide blinded specimens to laboratories for testing. The results are compared with those of other laboratories and with reference results. Participation in external quality assessment is required for laboratory accreditation and helps identify systematic errors in methods, interpretation, or reporting.

Interpretation and Reporting

AST results must be interpreted using the appropriate breakpoints for the organism, antimicrobial, and method. The interpretive categories are reported to clinicians along with the organism identification. The report should include the MIC value or zone diameter, the interpretive category, and any comments that are relevant to clinical management.

Breakpoint Selection

Laboratories must select the breakpoint standard that is appropriate for their region and clinical setting. CLSI breakpoints are widely used in the United States and other countries, while EUCAST breakpoints are used in Europe and many other regions. The two standards may produce different interpretive categories for the same organism-drug combination.

A study evaluating fosfomycin susceptibility using CLSI versus EUCAST criteria among multidrug-resistant uropathogens found significant differences in interpretation. Among 251 samples that grew multidrug-resistant organisms, only 57 percent of the Gram-negative isolates were sensitive according to EUCAST guidelines, while 87.6 percent of all isolates were sensitive by CLSI criteria. Among the 161 carbapenem-resistant isolates, 135 were fosfomycin-susceptible and 18 were resistant according to CLSI. In contrast, by EUCAST criteria, only 40 isolates were fosfomycin-susceptible, and the remaining 121 were resistant. Although fosfomycin disk diffusion criteria according to CLSI and EUCAST are only validated for Escherichia coli, susceptibility among other Gram-negative bacteria was also interpreted using the same criteria, which represents a major limitation of the study.

Reporting of Results

The laboratory report should clearly state the organism identification, the antimicrobial agents tested, the MIC value or zone diameter, and the interpretive category. Results should be reported as soon as they are available, with preliminary reports for rapid testing and final reports after confirmation. The laboratory should flag results that indicate resistance to critical antimicrobial agents or that suggest the presence of emerging resistance mechanisms.

Common Failure Patterns and Troubleshooting

Several common problems can compromise AST results. Recognizing these patterns and implementing corrective actions is essential for laboratory quality.

Inoculum Errors

Inoculum density that is too high or too low can shift zone diameters and MIC values. A 0.5 McFarland standard must be prepared accurately using a calibrated turbidity meter or by visual comparison with a standard. The inoculum should be used within 15 minutes of preparation to avoid changes in viable cell count.

Medium Problems

Mueller-Hinton agar must be prepared according to the manufacturer's instructions and checked for proper depth, pH, and sterility. Agar that is too thick or too thin can affect antimicrobial diffusion and zone sizes. The presence of antimicrobial antagonists, such as thymidine or thymine, can cause false resistance for sulfonamides and trimethoprim. The agar should be tested with quality control strains to verify performance.

Incubation Errors

Incubation temperature, duration, and atmosphere must match the requirements of the organism and method. Extended incubation can cause zone diameters to shrink as the organism continues to grow. Incubation in carbon dioxide can lower the pH of the medium and affect the activity of some antimicrobials.

Interpretation Errors

Zone edges must be read at the correct point of inhibition. For some organism-drug combinations, the zone edge is not sharp, and microcolonies or a trailing edge may be present. These findings can indicate heteroresistance or partial resistance and may require confirmation with another method. The interpretive breakpoints must be current and appropriate for the organism and antimicrobial being tested.

Method-Specific Limitations

Standard methods for antimicrobial susceptibility testing, developed almost 60 years ago for testing conventional antibiotics, are not necessarily fit for purpose when it comes to determining the susceptibility of microorganisms to antimicrobial peptides. Without careful consideration of the parameters comprising antimicrobial susceptibility testing, there is a risk of failing to identify novel antimicrobials at a time when antimicrobial resistance is leading the planet toward a post-antibiotic era. More physiologically and clinically relevant antimicrobial susceptibility testing will allow better determination of the preclinical activity of drug candidates and allow the identification of lead compounds. An important consideration is the efficacy of antimicrobial peptides in biological matrices replicating sites of infection, such as blood, plasma, serum, lung bronchiolar lavage fluid, sputum, urine, and biofilms, as this will likely be more predictive of clinical efficacy. Additionally, specific antimicrobial susceptibility testing for different target microorganisms may help to better predict efficacy of antimicrobial peptides in specific infections.

A study comparing reference methods in evaluating different compounds with unique physicochemical characteristics demonstrated that combining different susceptibility tests is mandatory for a successful preclinical screening of antimicrobial compounds. A selection of substances including natural extracts, both free and in the form of nanocomposites with fumed silica, ionic liquids, ozonated oils, commercial and pure antibiotics, was tested using broth microdilution, disk diffusion, and agar dilution. These methods were chosen following EUCAST and CLSI guidelines, and comparisons were made to evaluate their applicability and limitations for non-conventional substances. The study highlighted significant variability in the outcomes depending on the method used, especially for substances with intrinsic properties such as high viscosity, poor solubility, or specific interactions with the testing medium. In several cases, the use of a single standard method failed to accurately evaluate antimicrobial activity.

Biosafety and Regulatory Considerations

AST procedures involve the manipulation of potentially pathogenic microorganisms. Laboratories must follow biosafety guidelines to protect laboratory workers and prevent environmental contamination. The World Health Organization Laboratory Biosafety Manual provides guidance on risk assessment, containment levels, and safe handling of microorganisms. The World Health Organization Laboratory Quality Management System Handbook provides guidance on quality management, including document control, internal audits, and corrective actions.

Standard microbiological practices should be followed at all times. These practices include hand hygiene, use of personal protective equipment, restriction of access to laboratory areas, and proper disposal of contaminated materials. Work with organisms that require biosafety level 2 or higher containment must be performed in appropriate facilities with engineering controls such as biological safety cabinets.

The Assay Guidance Manual from the National Center for Advancing Translational Sciences provides guidance on assay development and validation, including considerations for antimicrobial susceptibility testing in research settings. The U.S. Food and Drug Administration Bioanalytical Method Validation Guidance provides recommendations for validating analytical methods used in drug development, including methods for measuring antimicrobial activity.

Emerging Technologies and Future Directions

Standard antimicrobial susceptibility testing methods are time-consuming and laborious, which can delay appropriate treatment and contribute to the spread of antimicrobial resistance. To reduce the antimicrobial susceptibility detection time, minimize treatments with empirical broad-spectrum antibiotics, and thereby combat the further spread of antimicrobial resistance, faster and point-of-care methods are needed. This requires many different research approaches.

Microfluidic systems for antimicrobial susceptibility testing offer the possibility to reduce the detection time, as small sample and reagent volumes can be used and the detection of single cells is possible. In some cases, the aim is to use human samples without pretreatment or pre-cultivation. Microfluidic methods for phenotypic antimicrobial susceptibility testing are an active area of research.

Antimicrobial susceptibility testing results are crucial for timely administration of effective antimicrobial treatment and should be made available to clinicians as fast as possible. Increasing rates of multidrug-resistant organisms emphasize the need for rapid antimicrobial susceptibility testing. Phenotypic rapid antimicrobial susceptibility testing is universal, mechanism-independent, and allows exact categorization, but it demands time for the microorganisms to start growth and to express the response to antibiotics. Detection of selected resistance mechanisms is more rapid, but the interpretation of its clinical impact is limited. Technical challenges of phenotypic rapid antimicrobial susceptibility testing include inoculum effect, delayed expression of resistance, lag phase, and initial biomass increase in susceptible isolates. Criteria for a successful rapid antimicrobial susceptibility testing assay are ease of use, random access, capacity for simultaneous testing of multiple specimens, affordability, and financial attractiveness for industry. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry based antimicrobial susceptibility testing seems to be particularly promising, as it can optimally be combined with MALDI-TOF mass spectrometry identification. Direct testing from clinical specimens provides particularly early findings, with positive blood cultures being the most suitable specimen type. Polymicrobial samples and inoculum effect are serious obstacles for direct antimicrobial susceptibility testing from other clinical specimens.

Same day identification and full panel antimicrobial susceptibility testing of bacteria from positive blood culture bottles has been made possible by a combined lysis-filtration method with MALDI-TOF VITEK mass spectrometry and the VITEK2 system. This approach combines rapid identification with phenotypic susceptibility testing to provide comprehensive results within a single day.

Records and Documentation

Accurate records are essential for quality assurance, regulatory compliance, and clinical decision-making. The laboratory should maintain records of all AST results, including the organism identification, the method used, the antimicrobial agents tested, the zone diameters or MIC values, and the interpretive categories. Quality control records should document the results of reference strain testing, media and reagent lot numbers, and any corrective actions taken.

The World Health Organization Laboratory Quality Management System Handbook emphasizes the importance of document control, record keeping, and internal audits. Laboratories should establish procedures for reviewing AST results before release, investigating discrepancies, and documenting corrective actions.

Professional Escalation Criteria

Laboratory personnel should escalate AST results that require clinical attention or that suggest the presence of unusual or emerging resistance mechanisms. Examples include results indicating resistance to last-line antimicrobial agents, results that are inconsistent with the organism identification, and results that suggest the presence of a notifiable or epidemiologically significant organism.

The laboratory should have a protocol for communicating critical results to clinicians and public health authorities. This protocol should define the criteria for escalation, the responsible personnel, and the communication methods. The laboratory should also participate in antimicrobial resistance surveillance programs and share data with local, national, and international databases.

Frequently Asked Questions

What is the difference between disk diffusion and broth dilution?

Disk diffusion measures the zone of inhibition around an antimicrobial disk on an agar plate and provides a qualitative or semi-quantitative result. Broth dilution measures the minimum inhibitory concentration, which is the lowest antimicrobial concentration that prevents visible growth in broth. Broth dilution provides a quantitative MIC value that can be interpreted using clinical breakpoints.

Why are MIC values preferred over zone diameters for some clinical decisions?

MIC values provide a quantitative measure of antimicrobial activity that can be compared across laboratories and over time. MIC values are essential for managing infections caused by organisms with borderline susceptibility, for selecting optimal dosing regimens, and for detecting emerging resistance. Zone diameters are semi-quantitative and are more difficult to compare across laboratories.

How are antimicrobial susceptibility testing breakpoints determined?

Breakpoints are established by organizations such as the Clinical and Laboratory Standards Institute and the European Committee on Antimicrobial Susceptibility Testing. Breakpoints are based on the distribution of MIC values in wild-type populations, pharmacokinetic and pharmacodynamic data, clinical outcome data, and resistance mechanisms. Breakpoints are periodically revised as new data become available.

What is the role of quality control strains in antimicrobial susceptibility testing?

Quality control strains are reference organisms with known susceptibility patterns that are tested with each batch of AST to verify that the method, media, reagents, and interpretation are performing correctly. Quality control results must fall within published acceptable ranges for each antimicrobial and method. Out-of-range results indicate a problem that must be investigated before patient results are released.

Can antimicrobial susceptibility testing be performed directly from clinical specimens?

Rapid antimicrobial susceptibility testing from positive blood culture bottles can provide results within 4 to 8 hours, compared with 5 to 9 days for conventional methods. Studies have shown that rapid antimicrobial susceptibility testing from blood culture broths is reliable for Gram-negative bacteria and can support early initiation of targeted antimicrobial therapy. Direct testing from other clinical specimens is more challenging due to polymicrobial samples and inoculum effects.

How do CLSI and EUCAST breakpoints differ?

CLSI and EUCAST are two major organizations that set antimicrobial susceptibility testing breakpoints. The two standards may produce different interpretive categories for the same organism-drug combination. Laboratories must select the breakpoint standard that is appropriate for their region and clinical setting and must use the corresponding method and interpretive criteria.

What are the limitations of antimicrobial susceptibility testing for antimicrobial peptides?

Standard antimicrobial susceptibility testing methods were developed for conventional antibiotics and are not necessarily fit for purpose for antimicrobial peptides. Factors such as high viscosity, poor solubility, and interactions with the testing medium can cause variability in outcomes depending on the method used. Combining different susceptibility tests is mandatory for successful preclinical screening of antimicrobial compounds.

When should gradient diffusion be used instead of disk diffusion or broth dilution?

Gradient diffusion is useful when MIC values are needed for a limited number of isolates or antimicrobials. The method is simpler than broth microdilution and provides a quantitative result. Gradient diffusion is more expensive than disk diffusion and is not suitable for high-throughput testing. The method is often used for confirmatory testing of isolates that test intermediate or resistant by disk diffusion.

Related Diagnostic Guides

• Comparison of Disk Diffusion and Broth Microdilution for Antimicrobial Susceptibility Testing
• Disk Diffusion Method for Antimicrobial Susceptibility Testing: Protocol and Interpretation
• How to Interpret Zone Diameter Measurements in Disk Diffusion Susceptibility Testing
• Common Sources of Error in Disk Diffusion Susceptibility Testing and How to Avoid Them
• Antimicrobial Susceptibility Testing in Secondary Viral Co-infections

References and Further Reading

• Laboratory Quality Management System Handbook. World Health Organization.
• Laboratory Biosafety Manual. World Health Organization.
• Assay Guidance Manual. National Center for Advancing Translational Sciences.
• Bioanalytical Method Validation Guidance. U.S. Food and Drug Administration.
• NCBI Literature Resources. National Center for Biotechnology Information.
• Antimicrobial susceptibility testing: An updated primer for clinicians in the era of antimicrobial resistance: Insights from the Society of Infectious Diseases Pharmacists.. Pharmacotherapy, 2023.
• Antimicrobial susceptibility testing breakpoints and methods from BSAC to EUCAST.. The Journal of antimicrobial chemotherapy, 2016.
• Microfluidic Systems for Antimicrobial Susceptibility Testing.. Advances in biochemical engineering/biotechnology, 2022.
• How to accelerate antimicrobial susceptibility testing.. Clinical microbiology and infection : the official publication of the European Society of Clinical Microbiology and Infectious Diseases, 2019.
• Cytomegalovirus.. Microbiology spectrum, 2016.
• Antimicrobial Susceptibility Testing for Staphylococcus lugdunensis.. Journal of clinical microbiology, 2022.
• Antimicrobial susceptibility testing of anaerobic bacteria: In routine and research.. Anaerobe, 2022.
• Antimicrobial Susceptibility Testing of Antimicrobial Peptides to Better Predict Efficacy.. Frontiers in cellular and infection microbiology, 2020.
• Ciprofloxacin-Based Ionic Liquids Increase Mutation Frequency in Escherichia coli. 2026.
• Comparison of the Antimicrobial Properties of Nanoemulsions Made from Phytosphingosine and Chitosan with a Triple Antibiotic Paste Against &lt,i&gt,Enterococcus faecalis&lt,/i&gt, in Endodontics: An &lt,i&gt,in-vitro&lt,/i&gt, Study.. 2026.
• Beyond One-Size-Fits-All: Addressing Methodological Constraints in Novel Antimicrobials Discovery.. 2025.
• Evaluation of fosfomycin susceptibility using CLSI vs EUCAST criteria among multi drug resistant uropathogens in a tertiary care Hospital.. 2026.
• CRISPRi-mediated silencing of the ABC transporter &lt,i&gt,Ms_5779&lt,/i&gt, compromises cell envelope integrity and impairs intracellular host lipid exploitation.. 2026.
• Performance evaluation of four antibiotics using the BD Phoenix™ NMIC-413 antimicrobial susceptibility testing panel for carbapenem-resistant &lt,i&gt,Enterobacteriaceae&lt,/i&gt, and carbapenem-resistant &lt,i&gt,Pseudomonas aeruginosa&lt,/i&gt,.. 2025.
• Performance of Rapid Antibiotic Susceptibility Testing from Semiautomated Blood Culture Bottles with Disk Diffusion using CLSI Guidelines: A Cross-sectional Study. Journal of Clinical and Diagnostic Research, 2025.
• Comparison of Rapid Antimicrobial Susceptibility Testing of Flash Positive Blood Culture Bottles using Disk Diffusion and Automated Broth Microdilution on VITEK 2 Compact with standard methods: Clinical Laboratory Standards Institute Guidelines (CLSI) protocol.. Indian Journal of Medical Microbiology, 2025.
• Beyond Categorical Agreement: Embracing Essential Agreement for Robust AST Proficiency Testing in CLSI-Dominant India.. Indian Journal of Medical Microbiology, 2026.
• Pooled Antibiotic Susceptibility Testing Performs Within CLSI Standards for Validation When Measured Against Broth Microdilution and Disk Diffusion Antibiotic Susceptibility Testing of Cultured Isolates. Antibiotics, 2024.
• Advanced Phenotypic Antimicrobial Susceptibility Testing Methods. Advanced Techniques in Diagnostic Microbiology Volume 1 Techniques Third Edition, 2018.
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• Same day identification and full panel antimicrobial susceptibility testing of bacteria from positive blood culture bottles made possible by a combined lysis-filtration method with MALDI-TOF VITEK mass spectrometry and the VITEK2 system. Plos One, 2014.

This article is educational and does not replace validated laboratory procedures, institutional biosafety review, manufacturer instructions, or professional interpretation.