Antimicrobial Susceptibility Testing: Interpreting MICs and Zone Diameters

Antimicrobial susceptibility testing (AST) determines whether a bacterial isolate is susceptible, intermediate, or resistant to specific antimicrobial agents. The two most common phenotypic methods are disk diffusion, which measures zone diameters in millimeters, and broth dilution, which measures the minimum inhibitory concentration (MIC) in micrograms per milliliter. Results are interpreted using breakpoints established by organizations such as the Clinical and Laboratory Standards Institute (CLSI) or the European Committee on Antimicrobial Susceptibility Testing (EUCAST). This article explains the principles of both methods, how to interpret results using standardized breakpoints, and common pitfalls that lead to inaccurate reporting. The content is intended for laboratory students, technicians, researchers, and diagnostic professionals who perform or interpret AST in clinical or research settings.

The Role of AST in Clinical and Veterinary Diagnostics

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 [6].

In veterinary medicine, AST is equally important for guiding treatment decisions in food animals and companion animals. A retrospective laboratory-based study of Staphylococcus spp. isolated from dogs and cats in Poland evaluated species distribution and antimicrobial susceptibility patterns with particular emphasis on MIC-based testing. The study included 281 records of Staphylococcus isolates obtained from routine diagnostic submissions to a commercial veterinary laboratory. Bacterial identification was performed using MALDI-TOF-MS, and antimicrobial susceptibility was assessed using a commercial MIC panel. After exclusion of nitrofurantoin from comparative analyses, the highest resistance rates were observed for penicillin (86.5%), trimethoprim-sulfamethoxazole (77.2%), and tetracycline (68.0%), whereas no resistant isolates were detected for teicoplanin or vancomycin. Direct MIC-based interpretation of ampicillin-sulbactam yielded no resistant isolates. Staphylococcus pseudintermedius was the predominant species in dogs, while Staphylococcus felis predominated in cats. Dog-derived isolates showed a significantly higher resistance burden than cat-derived isolates, and multidrug resistance was significantly more frequent in isolates obtained from dogs than cats (72.5% vs. 55.7%). Among the four dominant species, resistance burden and multidrug resistance rates were highest in S. pseudintermedius. Significant between-host differences after multiple-testing correction were retained for erythromycin and trimethoprim-sulfamethoxazole, both being more frequent in canine isolates [16].

Core Principles of Disk Diffusion Testing

Disk diffusion testing, also known as the Kirby-Bauer method, involves placing paper disks impregnated with defined concentrations of antimicrobial agents onto an agar plate that has been inoculated with a standardized bacterial suspension. After incubation, the antimicrobial agent diffuses through the agar, creating a concentration gradient. Bacterial growth is inhibited where the concentration exceeds the organism's MIC, producing a clear zone around the disk. The diameter of this zone is measured in millimeters and interpreted using breakpoint tables.

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 [7].

Zone diameter interpretation depends on several factors, including the antimicrobial agent, the disk potency, the organism being tested, and the breakpoint system being used. A larger zone diameter generally indicates greater susceptibility, while a smaller zone or no zone indicates resistance. However, the relationship between zone diameter and MIC is not linear across all antimicrobial-organism combinations, and breakpoints are established through rigorous testing and clinical correlation.

Standardization of Disk Diffusion Testing

Standardization is essential for reproducible disk diffusion results. Key parameters that must be controlled include the agar medium, the inoculum density, the incubation temperature and duration, and the measurement technique. Mueller-Hinton agar is the recommended medium for most non-fastidious organisms. The inoculum is typically prepared by suspending colonies from an overnight culture in sterile saline or broth to achieve a turbidity equivalent to a 0.5 McFarland standard. The inoculated plates should be used within a specified time after inoculation to prevent changes in inoculum density.

The World Health Organization's Laboratory Quality Management System Handbook provides guidance on quality practices in laboratory testing, including the importance of standardized procedures and quality control measures [1]. Laboratories performing AST should follow the procedures outlined in their chosen breakpoint system's documentation and participate in external quality assessment programs to verify the accuracy of their results.

Zone Edge Interpretation

For certain organism-antimicrobial combinations, the quality of the zone edge provides additional interpretive information. A sharp zone edge with no growth or a faint haze of growth may indicate beta-lactamase production, while a soft or fuzzy zone edge may indicate heteroresistance or slow growth. The evaluation of penicillin and oxacillin susceptibility testing conducted on 200 Staphylococcus lugdunensis isolates compared disc diffusion with penicillin 1 IU (P1, EUCAST) and penicillin 10 IU (P10, CLSI) with nitrocefin discs (Cefinase) and automated broth microdilution (Vitek 2). Oxacillin susceptibility was extrapolated from cefoxitin (FOX, 30 μg) disc diffusion and compared with Vitek 2 results. The reference methods were blaZ and mecA PCR. Penicillin zone diameter and zone edge correlated with blaZ PCR results in all except two P10-susceptible isolates (very major error) and one P1-resistant isolate (major error). A total of 148 isolates were blaZ negative, of which 146 and 149 isolates were susceptible by P1 and P10, respectively. A total of 127 were penicillin susceptible by Vitek 2. Vitek 2 overcalled resistance in 21 blaZ-negative, 20 P1-susceptible, and 22 P10-susceptible isolates (Vitek 2 major error rate, 14.2%). Two mecA-positive isolates were oxacillin resistant by FOX disc and Vitek 2 methods (categorical agreement). However, 18 FOX-susceptible mecA-negative isolates tested resistant by Vitek 2. In conclusion, Vitek 2 overestimated penicillin and oxacillin resistance compared with disc diffusion and PCR results. In this study, disc diffusion with zone edge interpretation was more accurate and specific than automated broth microdilution for S. lugdunensis [11].

Core Principles of Broth Dilution Testing

Broth dilution testing determines the MIC, which is the lowest concentration of an antimicrobial agent that inhibits visible growth of the organism. The method involves preparing a series of twofold dilutions of the antimicrobial agent in broth, inoculating each dilution with a standardized bacterial suspension, and incubating the tubes or microtiter plates under defined conditions. The MIC is read as the lowest concentration that shows no visible growth.

Broth microdilution is performed in microtiter plates with 96 wells, allowing multiple antimicrobial agents and dilutions to be tested simultaneously. This method is amenable to automation and is used by many commercial susceptibility testing systems. Agar dilution involves incorporating antimicrobial agents into agar plates at defined concentrations and inoculating the plates with a standardized bacterial suspension. Agar dilution is often used for research purposes and for testing organisms that grow poorly in broth.

The Assay Guidance Manual from the National Center for Advancing Translational Sciences provides general guidance on assay development and validation, including considerations for antimicrobial susceptibility testing in research settings [3]. The U.S. Food and Drug Administration's Bioanalytical Method Validation Guidance provides recommendations for validating analytical methods used in drug development, which may be relevant for laboratories developing or modifying AST methods [4].

MIC Interpretation and Breakpoints

MIC values are interpreted using breakpoints established by CLSI, EUCAST, or other recognized organizations. A breakpoint is the MIC concentration that separates susceptible from intermediate and resistant categories. The interpretation of an MIC value depends on the organism, the antimicrobial agent, the site of infection, and the dosing regimen used clinically.

For example, an MIC of 0.5 μg/mL for ciprofloxacin against Escherichia coli may be interpreted as susceptible, intermediate, or resistant depending on the breakpoints used. The same MIC value may have different interpretations for different organisms or for different antimicrobial agents. Laboratories must use the breakpoints specified by their chosen standard and must update their breakpoints when new versions are published.

The harmonization of breakpoints between BSAC and EUCAST has helped to avoid different reports of susceptibility for the same isolate in different countries and enables more reliable comparison of resistance rates in surveillance studies in different countries [7]. Laboratories should be aware of which breakpoint system they are using and should report results accordingly.

At a Glance: Comparing Disk Diffusion and Broth Dilution Methods

The following table summarizes the key characteristics of the two primary phenotypic AST methods.

Both methods are phenotypic, meaning they measure the actual growth response of the organism to the antimicrobial agent. Phenotypic AST is universal, mechanism-independent and allows exact categorization, but it demands time for the microorganisms to start the 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 AST include inoculum effect, delayed expression of resistance, lag phase and initial biomass increase in susceptible isolates. Criteria for a successful rapid AST 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 (MALDI-TOF MS)-based AST seems to be particularly promising, as it can optimally be combined with MALDI-TOF MS 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 AST from other clinical specimens [9].

Practical Workflow for Performing AST

The following workflow applies to both disk diffusion and broth dilution methods. Laboratories should follow the detailed procedures provided by their chosen breakpoint system and should document any deviations from the standard protocol.

Step 1: Verify Organism Identity

AST should only be performed on pure cultures of identified organisms. Mixed cultures can produce misleading results because the growth of one organism may obscure the inhibition zone of another. Confirm the organism identity using appropriate methods such as Gram stain, biochemical tests, or MALDI-TOF MS before proceeding with AST.

Step 2: Prepare the Inoculum

Prepare a standardized bacterial suspension from an overnight culture. The suspension should be adjusted to a turbidity equivalent to a 0.5 McFarland standard, which corresponds to approximately 1 to 2 x 10^8 colony-forming units per milliliter for most organisms. Use a spectrophotometer or a McFarland turbidity standard to verify the density.

Step 3: Inoculate the Medium

For disk diffusion, inoculate the surface of a Mueller-Hinton agar plate by dipping a sterile swab into the standardized suspension, removing excess liquid, and streaking the swab across the entire agar surface in three directions to ensure even distribution. For broth dilution, add the standardized suspension to the broth medium containing the antimicrobial dilutions according to the manufacturer's instructions.

Step 4: Apply Disks or Antimicrobial Dilutions

For disk diffusion, apply the antimicrobial disks to the inoculated agar surface using a dispenser or sterile forceps. Press each disk gently to ensure contact with the agar. For broth dilution, the antimicrobial dilutions are already present in the microtiter plate or tubes.

Step 5: Incubate Under Standardized Conditions

Incubate the plates or microtiter plates at the recommended temperature (typically 35 to 37 degrees Celsius) for 16 to 24 hours. The incubation atmosphere may need to be adjusted for organisms that require carbon dioxide or anaerobic conditions. The World Health Organization's Laboratory Biosafety Manual provides guidance on safe handling of microorganisms and appropriate containment practices [2].

Step 6: Measure and Interpret Results

For disk diffusion, measure the diameter of each inhibition zone to the nearest millimeter using a ruler, caliper, or automated zone reader. For broth dilution, read the MIC as the lowest concentration that shows no visible growth. Interpret the results using the breakpoints specified by the chosen standard.

Step 7: Perform Quality Control

Quality control organisms with known susceptibility patterns should be tested in parallel with clinical isolates. The results for quality control organisms must fall within the acceptable ranges specified by the breakpoint system. If quality control results are out of range, the test results are invalid and must be repeated.

Options and Tradeoffs in AST Methods

The choice of AST method depends on the laboratory's resources, workload, and the organisms being tested. Disk diffusion is simple, inexpensive, and suitable for routine testing of fast-growing organisms. Broth dilution provides quantitative MIC results that are useful for guiding dosing decisions and for detecting low-level resistance. Automated systems offer convenience and standardization but may have limitations for certain organism-antimicrobial combinations.

A study evaluating the effect of biochanin A on the antibiotic susceptibilities of Escherichia coli grown under different respiration and metabolic conditions on agar media investigated the synergistic effects of 120 ppm biochanin A with five classes of antibiotics on two strains of E. coli (25922 and 51659, an O157:H7 strain) on two agar culture media using commercially available ETEST strips and the FDA/CLSI breakpoint interpretation criteria. The effect of 120 ppm biochanin A on MIC across the antibiotic classes was relatively consistent where it had largely no effect, an occasionally negative effect, or a neutral effect. Biochanin A addition yielded negligible changes to ciprofloxacin (susceptible) for either strain under the redox and metabolic conditions tested. The culture conditions seemed more influential over MIC than biochanin A. The absence of oxygen generally conferred clinical resistance or a higher MIC, and at times media composition altered the MICs as well. For example, neither macrolide inhibited either strain (greater than 256 μg/mL) under anaerobic conditions but generally had measurable MICs aerobically. Additionally, the MICs for streptomycin were consistently higher across each strain when cultured on tryptic soy agar instead of Mueller Hinton broth. Despite previous studies in which biochanin A decreased MICs in other bacterial species, neither strain of E. coli was consistently impacted by the presence of 120 ppm biochanin A in the culture medium. The data indicate that biochanin A is likely not a reliable potentiator for these organisms under the tested conditions [14].

This study illustrates that culture conditions, including oxygen availability and media composition, can significantly influence MIC results. Laboratories must follow standardized protocols to ensure that results are comparable across tests and across laboratories.

Automated Susceptibility Testing Systems

Automated systems such as Vitek 2 and MicroScan provide rapid and standardized AST results. These systems use broth microdilution or other methods to determine MICs and interpret results using built-in breakpoint tables. However, automated systems may have limitations for certain organisms or antimicrobial agents. The study of S. lugdunensis found that Vitek 2 overestimated penicillin and oxacillin resistance compared with disc diffusion and PCR results, with a major error rate of 14.2% for penicillin [11]. Laboratories using automated systems should be aware of these limitations and should confirm unexpected results with alternative methods.

A combined lysis-filtration method with MALDI-TOF VITEK mass spectrometry and the VITEK2 system has been described for same-day identification and full panel antimicrobial susceptibility testing of bacteria from positive blood culture bottles [22]. This approach can significantly reduce the time to results for bloodstream infections, which is critical for patient management.

Rapid AST Methods

Rapid AST methods are being developed to reduce the time to results and to combat the spread of antimicrobial resistance. 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 a focus of current research [8].

Human health is threatened by the spread of antimicrobial resistance and resulting infections. One reason for the resistance spread is the treatment with inappropriate and ineffective antibiotics because standard antimicrobial susceptibility testing methods are time-consuming and laborious. 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 [8].

Records and Measurements in AST

Accurate record keeping is essential for AST quality assurance and for monitoring antimicrobial resistance trends. The following records should be maintained for each AST test:

Essential Records

• Patient or animal identification and specimen source
• Organism identification and relevant phenotypic characteristics
• Date and time of specimen collection and testing
• AST method used and breakpoint system applied
• Zone diameters or MIC values for each antimicrobial agent tested
• Interpretation (susceptible, intermediate, or resistant) for each antimicrobial agent
• Quality control results for the test run
• Any deviations from standard procedures and their impact on results

Quality Control Records

Quality control testing should be performed daily or as specified by the breakpoint system. Records should include the quality control organism used, the expected and observed results, and any corrective actions taken when results are out of range. The World Health Organization's Laboratory Quality Management System Handbook provides guidance on quality control practices and documentation requirements [1].

Surveillance Records

Aggregate AST data can be used to monitor antimicrobial resistance trends over time and to inform empirical treatment guidelines. Laboratories should maintain databases of AST results and should periodically analyze the data to identify emerging resistance patterns. The harmonization of breakpoints and methods between countries enables more reliable comparison of resistance rates in surveillance studies [7].

Common Failure Patterns in AST

Several common errors can lead to inaccurate AST results. Recognizing these failure patterns is essential for troubleshooting and for maintaining the reliability of AST results.

Inoculum Density Errors

An inoculum that is too heavy or too light can significantly affect zone diameters and MIC values. A heavy inoculum may produce smaller zones or higher MICs, leading to false resistance. A light inoculum may produce larger zones or lower MICs, leading to false susceptibility. Always verify the inoculum density using a McFarland standard or spectrophotometer.

Medium Problems

The quality and composition of the agar medium can affect AST results. Mueller-Hinton agar is recommended for most organisms because it provides consistent results across laboratories. Variations in medium composition, pH, or thickness can affect antimicrobial diffusion and bacterial growth. The study of E. coli found that MICs for streptomycin were consistently higher when cultured on tryptic soy agar instead of Mueller Hinton broth, demonstrating that media composition can alter MICs [14].

Incubation Conditions

Incorrect incubation temperature, duration, or atmosphere can affect AST results. Most organisms should be incubated at 35 to 37 degrees Celsius for 16 to 24 hours in ambient air. Organisms that require carbon dioxide or anaerobic conditions must be incubated accordingly. The absence of oxygen generally conferred clinical resistance or a higher MIC for macrolides against E. coli in the biochanin A study [14].

Disk Problems

Antimicrobial disks can lose potency if stored improperly or used after their expiration date. Disks should be stored according to the manufacturer's instructions and should be allowed to reach room temperature before use. Moisture can degrade disks, so they should be kept in a desiccator or sealed container.

Measurement Errors

Zone diameters should be measured to the nearest millimeter using a ruler, caliper, or automated zone reader. Measurements should be taken at the point of complete inhibition, which may require careful examination of the plate against a dark background. Automated zone readers can reduce measurement variability but should be calibrated regularly.

Breakpoint Misapplication

Using outdated or incorrect breakpoints can lead to misinterpretation of results. Laboratories must use the current breakpoints specified by their chosen standard and must update their breakpoints when new versions are published. The harmonization of breakpoints between BSAC and EUCAST has helped to avoid different reports of susceptibility for the same isolate in different countries [7].

Special Considerations for Anaerobic Bacteria

Antimicrobial susceptibility testing in anaerobes is highly disarrayed. The testing is not routinely performed in many laboratories to direct appropriate antimicrobial therapy, and literature on AST 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 heed. The educated experience-based treatment has often been instituted with anaerobic infections due to the challenging AST and relatively predictable patterns of antimicrobial resistance. However, the antimicrobial resistance in anaerobes remains no more predictable, especially in Gram-negative anaerobes like Bacteroides spp., where multi-drug resistance is also emerging. The changing antibiograms and the emergence of resistance determinants in anaerobes necessitate routine AST, periodic monitoring, and literature review of these organisms. The present scenario dictates to limit the empirical management of anaerobic infections and encourage consistent AST 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 [12].

Anaerobic AST requires specialized media, incubation conditions, and breakpoints. Laboratories that test anaerobes should have the appropriate infrastructure and expertise to ensure reliable results.

Special Considerations for Antimicrobial Peptides

During the development of antimicrobial peptides as potential therapeutics, antimicrobial susceptibility testing stands as an essential part of the process in identification and optimisation of candidate antimicrobial peptides. Standard methods for AST, 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 AST 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 AST 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 AST for different target microorganisms may help to better predict efficacy of antimicrobial peptides in specific infections [13].

Researchers developing antimicrobial peptides should be aware that standard AST methods may not accurately predict the activity of these compounds in biological matrices. Testing in physiologically relevant conditions may provide more useful information for preclinical development.

Biosafety and Regulatory Context

AST involves handling live microorganisms, including potentially pathogenic bacteria. Laboratories must follow appropriate biosafety practices to protect personnel and the environment. The World Health Organization's Laboratory Biosafety Manual provides guidance on risk assessment, containment measures, and safe handling of microorganisms [2]. The Laboratory Quality Management System Handbook provides guidance on quality practices in laboratory testing, including the importance of standardized procedures and quality control measures [1].

Laboratories performing AST should be aware of applicable regulations and accreditation requirements. In the United States, the Clinical Laboratory Improvement Amendments regulate laboratory testing and require laboratories to meet quality standards. The U.S. Food and Drug Administration is involved in the process of AST and setting and revising breakpoints [6]. Laboratories should ensure that their AST methods are validated and that they participate in external quality assessment programs.

Professional Escalation Criteria

Laboratory personnel should escalate unexpected or clinically significant AST results to the appropriate professionals. The following situations warrant escalation:

Unexpected Resistance Patterns

If an isolate shows resistance to an antimicrobial agent that is unexpected for the species, the result should be confirmed by repeat testing or by an alternative method. For example, the study of S. lugdunensis found that Vitek 2 overestimated penicillin and oxacillin resistance compared with disc diffusion and PCR results [11]. Confirming unexpected results can prevent inappropriate treatment decisions.

Quality Control Failures

If quality control results are out of range, the test results are invalid and must be repeated. Quality control failures should be documented and investigated to identify the cause. The World Health Organization's Laboratory Quality Management System Handbook provides guidance on corrective actions for quality control failures [1].

Outbreak or Surveillance Concerns

If AST results indicate an unusual cluster of resistant isolates, the results should be reported to the appropriate public health or infection control authorities. Monitoring antimicrobial resistance trends is essential for informing public health initiatives [6].

Clinical Impact

If AST results have a significant impact on patient or animal treatment decisions, the results should be communicated promptly to the clinician or veterinarian. AST results are crucial for timely administration of effective antimicrobial treatment, and should be made available to clinicians as fast as possible [9].

Common Failure Patterns and Troubleshooting

The following table summarizes common failure patterns in AST and recommended troubleshooting steps.

Limitations of AST

AST has several limitations that should be considered when interpreting results. Phenotypic AST requires time for the microorganisms to start the 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 AST include inoculum effect, delayed expression of resistance, lag phase and initial biomass increase in susceptible isolates [9].

AST results may not always predict clinical outcomes. The relationship between in vitro susceptibility and clinical efficacy depends on many factors, including the pharmacokinetics and pharmacodynamics of the antimicrobial agent, the site of infection, and the host immune response. Clinicians should use AST results in conjunction with clinical judgment and other laboratory data.

AST methods developed for conventional antibiotics are not necessarily fit for purpose when it comes to determining the susceptibility of microorganisms to antimicrobial peptides [13]. Researchers should consider the limitations of standard AST methods when testing novel antimicrobial agents.

Frequently Asked Questions

What is the difference between MIC and zone diameter?

The minimum inhibitory concentration (MIC) is the lowest concentration of an antimicrobial agent that inhibits visible growth of the organism, expressed in micrograms per milliliter. Zone diameter is the measurement in millimeters of the clear zone of growth inhibition around an antimicrobial disk on an agar plate. Both are phenotypic measurements of susceptibility, but they are measured using different methods. MIC is determined by broth dilution or agar dilution, while zone diameter is determined by disk diffusion. Both are interpreted using breakpoints established by CLSI or EUCAST.

How do I choose between CLSI and EUCAST breakpoints?

The choice between CLSI and EUCAST breakpoints depends on the laboratory's geographic location, regulatory requirements, and the preferences of the clinicians or veterinarians who will use the results. EUCAST breakpoints have been adopted by a large majority of laboratories in Europe, while CLSI breakpoints are commonly used in the United States and other regions. The harmonization of breakpoints between BSAC and EUCAST has helped to avoid different reports of susceptibility for the same isolate in different countries [7]. Laboratories should use one breakpoint system consistently and should document which system they are using.

Why do I need to perform quality control with every AST run?

Quality control organisms with known susceptibility patterns are tested in parallel with clinical isolates to verify that the test system is working correctly. If quality control results are out of range, the test results are invalid and must be repeated. Quality control testing is a fundamental component of laboratory quality assurance and is required by accreditation bodies. The World Health Organization's Laboratory Quality Management System Handbook provides guidance on quality control practices [1].

Can I use AST results to guide treatment decisions in animals?

Yes, AST results can guide treatment decisions in animals, but they should be used in conjunction with clinical judgment and knowledge of the antimicrobial agent's pharmacokinetics and pharmacodynamics. A study of Staphylococcus spp. isolated from dogs and cats in Poland found high resistance rates for penicillin, trimethoprim-sulfamethoxazole, and tetracycline, and no resistant isolates for teicoplanin or vancomycin [16]. These findings illustrate the importance of AST for guiding treatment decisions in veterinary medicine.

What should I do if my automated AST system gives an unexpected result?

If an automated AST system gives an unexpected result, the result should be confirmed by an alternative method, such as disk diffusion or a reference method. The study of S. lugdunensis found that Vitek 2 overestimated penicillin and oxacillin resistance compared with disc diffusion and PCR results [11]. Confirming unexpected results can prevent inappropriate treatment decisions.

How do culture conditions affect AST results?

Culture conditions, including oxygen availability and media composition, can significantly influence MIC results. A study of E. coli found that the absence of oxygen generally conferred clinical resistance or a higher MIC for macrolides, and that MICs for streptomycin were consistently higher when cultured on tryptic soy agar instead of Mueller Hinton broth [14]. Laboratories must follow standardized protocols to ensure that results are comparable across tests and across laboratories.

What is the role of rapid AST in clinical practice?

Rapid AST methods are being developed to reduce the time to results and to combat the spread of antimicrobial resistance. AST results are crucial for timely administration of effective antimicrobial treatment, and should be made available to clinicians as fast as possible [9]. 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 [8].

How should I document AST results for surveillance purposes?

AST results should be documented in a standardized format that includes the organism identification, the antimicrobial agents tested, the zone diameters or MIC values, and the interpretation. Aggregate AST data can be used to monitor antimicrobial resistance trends over time and to inform empirical treatment guidelines. The harmonization of breakpoints and methods between countries enables more reliable comparison of resistance rates in surveillance studies [7].

Related Diagnostic Guides

• Comparison of Disk Diffusion and Broth Microdilution for Antimicrobial Susceptibility Testing
• How to Interpret a Kirby-Bauer Disk Diffusion Test: Zone Diameter Measurement and CLSI Breakpoints
• How to Interpret Kirby-Bauer Zone Diameters Using CLSI Breakpoints
• How to Interpret Zone Diameter Measurements in Disk Diffusion Susceptibility Testing
• How to Interpret Kirby-Bauer Disk Diffusion Results Using CLSI Breakpoints

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.
• The effect of biochanin A on the antibiotic susceptibilities of Escherichia coli grown under different respiration and metabolic conditions on agar media.. 2026.
• Phenotypic and genomic characterization of linezolid non-susceptibility in poultry-derived multidrug-resistant Enterococcus spp. from Hungary
• Species distribution and antimicrobial resistance of &lt,i&gt,Staphylococcus&lt,/i&gt, spp. isolated from dogs and cats in Poland: a retrospective laboratory-based study.. 2026.
• Preliminary study on the effects of sub-MIC concentrations of octenidine and polyhexanidine on biofilms produced by animal isolates of Pseudomonas aeruginosa.. 2026.
• Problems in interpretation of piperacillin susceptibility of TEM-1 producingEscherichia coli in the disk diffusion test. European Journal of Clinical Microbiology and Infectious Diseases, 1991.
• PalAST: A Cross-Platform Mobile Application for Automated Disk Diffusion Antimicrobial Susceptibility Testing. International Journal of Interactive Mobile Technologies, 2023.
• Advanced Phenotypic Antimicrobial Susceptibility Testing Methods. Advanced Techniques in Diagnostic Microbiology Volume 1 Techniques Third Edition, 2018.
• Factors affecting the antimicrobial susceptibility testing of bacteria by disc diffusion method. Medicina, 2008.
• 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.