Comparison of Disk Diffusion and Broth Microdilution for Antimicrobial Susceptibility Testing

Disk diffusion (Kirby-Bauer) and broth microdilution are the two foundational methods for antimicrobial susceptibility testing (AST) in microbiology laboratories. Disk diffusion provides qualitative susceptibility results (susceptible, intermediate, resistant) based on inhibition zone diameters, while broth microdilution yields quantitative minimum inhibitory concentration (MIC) values. Disk diffusion is best suited for high-throughput screening of rapidly growing bacteria when categorical results are sufficient, whereas broth microdilution is the reference standard for determining precise MIC values, particularly for slow-growing organisms, fastidious bacteria, and when monitoring resistance trends or guiding therapy for serious infections. The choice between methods depends on laboratory resources, throughput needs, organism characteristics, and whether quantitative MIC data are required for clinical or research decisions.

At a Glance

Scientific Principle

Both methods rely on exposing a standardized bacterial inoculum to antimicrobial agents under controlled conditions, but they differ fundamentally in how the antimicrobial is presented to the organism.

Disk diffusion uses filter paper disks impregnated with a defined concentration of antimicrobial agent. When placed on an agar plate inoculated with a standardized bacterial suspension, the antimicrobial diffuses radially into the agar, establishing a concentration gradient. After incubation, bacterial growth is inhibited where the antimicrobial concentration exceeds the minimum inhibitory concentration for that organism. The resulting zone of inhibition diameter correlates inversely with the MIC—larger zones indicate greater susceptibility. The relationship between zone diameter and MIC is not linear but follows a predictable pattern defined by CLSI or EUCAST breakpoints [1][4].

Broth microdilution involves preparing serial two-fold dilutions of antimicrobial agents in a liquid growth medium within microtiter plate wells. Each well receives a standardized bacterial inoculum. After incubation, the MIC is read as the lowest concentration of antimicrobial that visibly inhibits bacterial growth. This method directly measures the antimicrobial concentration required to suppress growth, providing quantitative data that can be compared across laboratories and over time [2][5].

The fundamental difference in data type—zone diameter versus MIC—has important implications. Zone diameters are influenced by the diffusion characteristics of the antimicrobial, the agar depth, and the inoculum density. MIC values are more directly interpretable but require precise preparation of antimicrobial dilutions and careful endpoint reading.

Materials and Instrumentation Choices

Disk Diffusion Materials

• Mueller-Hinton agar (MHA): The standard medium for disk diffusion. For fastidious organisms (e.g., Streptococcus pneumoniae, Haemophilus influenzae), supplemented MHA (e.g., with 5% sheep blood or 1% hemoglobin + IsoVitaleX) is required. The agar depth must be 4 ± 0.5 mm; deviations affect zone diameters.
• Antimicrobial disks: Commercially prepared disks with defined drug concentrations. Store at -20°C or -70°C in sealed desiccated containers. Allow to warm to room temperature before opening to prevent condensation.
• Inoculum preparation: 0.5 McFarland standard (approximately 1.5 × 10⁸ CFU/mL for bacteria). Use a spectrophotometer or nephelometer for accurate standardization.
• Swabs: Sterile cotton or synthetic swabs for lawn inoculation.
• Incubator: 35 ± 2°C, ambient air (or 5% CO₂ for fastidious organisms).
• Measuring device: Calipers or automated zone reader for zone diameter measurement.

Broth Microdilution Materials

• Mueller-Hinton broth (MHB): Cation-adjusted MHB (CAMHB) is recommended for most bacteria. For fastidious organisms, supplement with 2–5% lysed horse blood or other growth factors.
• Microtiter plates: Sterile, 96-well U-bottom or flat-bottom plates. U-bottom plates facilitate visual reading of growth.
• Antimicrobial stock solutions: Prepare from reference powders of known potency. Store at -70°C in small aliquots to avoid freeze-thaw cycles.
• Dilution equipment: Multichannel pipettes (8- or 12-channel), reagent reservoirs, and sterile tips. Automated liquid handlers improve reproducibility for high-throughput applications.
• Inoculum preparation: 0.5 McFarland standard, then dilute 1:100 in broth to achieve approximately 5 × 10⁵ CFU/mL in each well.
• Incubator: 35 ± 2°C, ambient air or appropriate atmosphere.
• Reading aid: Mirrors, inverted plate readers, or automated MIC readers.

Key Decision Points

Agar source and lot: Different manufacturers' MHA can produce variable zone diameters. Laboratories should validate each new lot against reference strains. CLSI recommends using MHA from a single manufacturer for consistency.

Antimicrobial disk storage: Improper storage (temperature fluctuations, moisture exposure) degrades disks. Always check expiration dates and perform quality control with reference strains weekly.

Microtiter plate format: Pre-prepared frozen or lyophilized panels are commercially available but expensive. In-house preparation allows customization but requires rigorous quality control for drug stability and well-to-well consistency.

Inoculum standardization: Both methods require precise inoculum preparation. Over-inoculation reduces zone diameters in disk diffusion and elevates MICs in broth microdilution. Under-inoculation produces the opposite effects. Use a calibrated nephelometer rather than visual comparison to the McFarland standard.

Controls

Disk Diffusion Controls

• Positive growth control: Inoculated plate without disks to confirm adequate growth.
• Negative control: Uninoculated agar to verify sterility.
• Reference strains: Escherichia coli ATCC 25922, Staphylococcus aureus ATCC 25923, Pseudomonas aeruginosa ATCC 27853. Test weekly and whenever new lot of disks or agar is introduced. Zone diameters must fall within CLSI- or EUCAST-defined acceptable ranges.
• Blank disk control: Place a blank disk on the inoculated plate to verify no inhibition from the disk itself.

Broth Microdilution Controls

• Growth control well: Inoculated broth without antimicrobial to confirm adequate growth (turbidity or pellet).
• Sterility control well: Uninoculated broth to verify medium sterility.
• Reference strains: E. coli ATCC 25922, S. aureus ATCC 29213, P. aeruginosa ATCC 27853. MIC values must fall within CLSI- or EUCAST-defined QC ranges.
• Drug control: Test a well containing a known concentration of antimicrobial to verify drug stability and activity.

Importance of Controls

Controls serve multiple critical functions. They validate that the medium supports growth, that antimicrobial disks or solutions retain potency, that the inoculum is correct, and that incubation conditions are appropriate. Without proper controls, erroneous results—false susceptibility or false resistance—may go undetected. Reference strains provide a benchmark for inter-laboratory comparison and longitudinal quality monitoring [1][4].

Conceptual Workflow

Disk Diffusion Workflow

1. Prepare inoculum: Adjust bacterial suspension to 0.5 McFarland standard in sterile saline or broth. Use within 15 minutes.
2. Inoculate plate: Dip sterile swab into suspension, rotate against tube wall to remove excess. Streak entire agar surface in three directions (horizontal, vertical, diagonal) to ensure confluent growth.
3. Apply disks: Within 15 minutes of inoculation, place disks onto agar using sterile forceps or disk dispenser. Press gently to ensure contact. Do not move disks after placement.
4. Incubate: Invert plates and incubate at 35 ± 2°C for 16–18 hours (24 hours for some organisms).
5. Read zones: Measure zone diameters (including disk) to nearest millimeter using calipers or automated reader. Read at the point of complete inhibition as judged by the naked eye. For sulfonamides and trimethoprim, read at 80% inhibition (slight haze allowed).
6. Interpret: Compare zone diameters to CLSI or EUCAST breakpoint tables. Report as susceptible, intermediate, or resistant.

Broth Microdilution Workflow

1. Prepare antimicrobial dilutions: Prepare two-fold serial dilutions in CAMHB at 2× final concentration. Dispense 100 µL per well.
2. Prepare inoculum: Adjust bacterial suspension to 0.5 McFarland, then dilute 1:100 in CAMHB to achieve approximately 5 × 10⁵ CFU/mL.
3. Inoculate plate: Add 100 µL of diluted inoculum to each well (except sterility control). Final inoculum: approximately 5 × 10⁵ CFU/mL. Final antimicrobial concentration: 1×.
4. Incubate: Cover plate with lid or seal, incubate at 35 ± 2°C for 16–20 hours.
5. Read MIC: Determine the lowest concentration showing no visible growth (clear well or pellet < 2 mm). Use a reading mirror or inverted plate reader.
6. Interpret: Compare MIC to CLSI or EUCAST breakpoints. Report as susceptible, intermediate, or resistant.

Critical Decision Points

Inoculum timing: For disk diffusion, the interval between inoculation and disk application should not exceed 15 minutes. Prolonged delay allows bacteria to begin growing before antimicrobial exposure, reducing zone sizes.

Agar drying: After inoculation, allow the plate surface to dry for 3–5 minutes (lid ajar) before applying disks. Excess moisture causes disks to float and produces irregular zones.

Reading endpoint: For broth microdilution, trailing growth (a faint haze beyond the MIC endpoint) can complicate reading. CLSI guidelines define the MIC as the lowest concentration that inhibits visible growth. For trailing endpoints, record the concentration where growth is completely inhibited.

Quality Checks

Pre-Analytical Quality Checks

• Verify expiration dates of disks, media, and antimicrobial powders.
• Check agar plates for contamination, cracks, or excessive moisture.
• Confirm McFarland standard calibration using a spectrophotometer or certified standard.
• Document lot numbers of all reagents.

Analytical Quality Checks

• Disk diffusion: Measure reference strain zones weekly. If any zone falls outside the acceptable range, investigate and repeat testing. Common causes include incorrect agar depth, expired disks, or inoculum errors.
• Broth microdilution: Verify reference strain MICs with each batch. MICs should fall within ±1 two-fold dilution of the expected value. Document all QC results.
• Inoculum verification: Periodically perform colony counts to confirm that the inoculum delivers 5 × 10⁵ CFU/mL (broth microdilution) or produces semi-confluent growth (disk diffusion).

Post-Analytical Quality Checks

• Review results for consistency: unexpected resistance patterns should prompt repeat testing.
• Check that all controls produced expected results.
• Document any deviations from standard procedures.

Documentation Requirements

Maintain records of:

• Date, technician, organism identification
• Method used (disk diffusion or broth microdilution)
• Antimicrobial agents tested
• Zone diameters or MIC values
• Interpretation (S/I/R)
• QC results for reference strains
• Lot numbers of media, disks, and antimicrobials
• Any procedural deviations

Result Interpretation

Disk Diffusion Interpretation

Zone diameters are interpreted using CLSI M100 or EUCAST breakpoint tables. These tables provide specific zone diameter ranges for each antimicrobial-organism combination. For example, for ciprofloxacin against Enterobacteriaceae, CLSI breakpoints might define ≥ 21 mm as susceptible, 16–20 mm as intermediate, and ≤ 15 mm as resistant (actual values vary by guideline version).

The relationship between zone diameter and MIC is inverse but not linear. A larger zone indicates lower MIC (greater susceptibility), but the exact MIC cannot be calculated from the zone diameter alone. However, studies have shown that disk diffusion can achieve high categorical agreement with broth microdilution when appropriate breakpoints are used. For example, one study found categorical agreement between disk diffusion and broth microdilution for cefiderocol ranging from 90.0% to 100% across species when using CLSI breakpoints [5].

Broth Microdilution Interpretation

MIC values are interpreted using the same CLSI or EUCAST breakpoint tables. The MIC is the lowest concentration that inhibits visible growth. For example, if an isolate grows in wells containing ≤ 2 µg/mL of an antimicrobial but not in the 4 µg/mL well, the MIC is 4 µg/mL.

MIC values provide quantitative data that can be used for:

• Resistance surveillance: Tracking MIC shifts over time (MIC creep)
• Therapeutic drug monitoring: Guiding dose adjustments
• Comparative studies: Evaluating new antimicrobial agents
• Epidemiological cutoff values (ECOFFs): Distinguishing wild-type from non-wild-type populations

Categorical Agreement and Errors

When comparing disk diffusion to broth microdilution, three types of errors can occur:

• Very major error (VME): Disk diffusion reports susceptible, broth microdilution reports resistant (false susceptibility)
• Major error (ME): Disk diffusion reports resistant, broth microdilution reports susceptible (false resistance)
• Minor error (mE): One method reports intermediate, the other reports susceptible or resistant

Acceptable error rates are typically < 1.5% for VME, < 3% for ME, and < 10% for mE. Studies have shown that disk diffusion can achieve acceptable categorical agreement for many antimicrobial-organism combinations, but performance varies by drug and species [1][5].

Breakpoint Updates

Breakpoints are periodically revised by CLSI and EUCAST based on new pharmacokinetic/pharmacodynamic data, clinical outcomes, and resistance mechanisms. For example, the 2025 CLSI breakpoints for minocycline against Acinetobacter baumannii complex reduced the susceptibility rate from 73.9% to 46.4% and increased categorical agreement between disk diffusion and broth microdilution from 64.1% to 90.9% [4]. Laboratories must stay current with breakpoint revisions and update their interpretive criteria accordingly.

Troubleshooting

Limitations

Disk Diffusion Limitations

• No quantitative MIC: Cannot provide precise MIC values needed for resistance surveillance or therapeutic drug monitoring.
• Limited to rapidly growing bacteria: Slow-growing organisms (e.g., Mycobacterium spp., anaerobes) may not produce readable zones within standard incubation times.
• Diffusion-dependent: Antimicrobials with poor diffusion through agar (e.g., some lipophilic drugs) produce unreliable zones.
• pH sensitivity: The modified Kirby-Bauer test with buffer solution has been shown to improve detection performance by addressing pH variation that affects accuracy [1].
• Subjective reading: Zone measurement can vary between technicians, especially for drugs with trailing endpoints (sulfonamides, trimethoprim).
• Limited antimicrobial panel: Commercial disks are available only for commonly tested drugs; custom disks are not standardized.

Broth Microdilution Limitations

• Labor-intensive: Preparing serial dilutions and inoculating 96-well plates is time-consuming without automation.
• Cost: Commercial panels are expensive; in-house preparation requires reference powders and rigorous QC.
• Reading challenges: Trailing growth, skipped wells, and contamination can complicate endpoint determination.
• Drug stability: Some antimicrobials degrade in solution, requiring fresh preparation or validated storage conditions.
• Limited throughput: While 96-well plates allow testing multiple isolates, the workflow is less efficient than disk diffusion for large numbers of isolates.
• Methodological constraints: Certain compounds with unique physicochemical characteristics may interfere with standard broth microdilution, resulting in underestimated activity [2].

General Limitations

• Breakpoint differences: CLSI and EUCAST breakpoints often differ, leading to discordant interpretations. Laboratories must specify which guidelines they follow [3][5].
• Not all organisms are testable: Some bacteria require specialized media or conditions that are not compatible with standard methods.
• Time to result: Both methods require overnight incubation, which may delay clinical decision-making.

Documentation

Essential Records

Maintain the following documentation for each AST run:

1. Test request form: Organism identification, source, requested antimicrobials
2. Method documentation: Disk diffusion or broth microdilution, guideline version (CLSI M100 year, EUCAST version)
3. Raw data: Zone diameters (mm) or MIC values (µg/mL)
4. Interpretation: S/I/R for each antimicrobial
5. QC results: Reference strain zone diameters or MICs, acceptance criteria
6. Reagent lot numbers: Agar, disks, antimicrobial powders, media
7. Incubation conditions: Temperature, atmosphere, duration
8. Technician identification: Name or initials
9. Deviations: Any procedural modifications and rationale

Data Management

• Store raw data in laboratory information management system (LIMS) or paper records for at least the period required by institutional policy (typically 2–5 years).
• For research studies, maintain electronic databases with MIC values (not just categorical interpretations) to enable future analysis.
• Document any breakpoint changes and the date of implementation.

Reporting

• Report categorical results (S/I/R) for clinical use.
• Include MIC values when requested or when they affect clinical decision-making (e.g., for drugs with narrow therapeutic windows).
• Note the guideline version used (e.g., "Interpreted per CLSI M100-ED34").
• Flag any results that required repeat testing or confirmation.

Biosafety Considerations

Risk Assessment

Disk diffusion and broth microdilution are performed with bacterial cultures that may include pathogens. Even when working with BSL-1 organisms (e.g., E. coli K-12, non-pathogenic Bacillus species), standard microbiological practices apply. For clinical isolates, assume BSL-2 precautions unless the organism is known to be BSL-1 [6].

Standard Practices

• Perform all manipulations of bacterial cultures in a Class II biological safety cabinet (BSC).
• Wear laboratory coat, gloves, and eye protection.
• Decontaminate work surfaces before and after procedures with appropriate disinfectant (e.g., 10% bleach or 70% ethanol).
• Use leak-proof containers for waste disposal.
• Autoclave all contaminated materials before disposal.

Specific Precautions

• Disk diffusion: Avoid aerosol generation when streaking plates. Use a BSC for inoculum preparation and plate inoculation.
• Broth microdilution: Use barrier tips to prevent cross-contamination. Seal plates with adhesive film or lids to prevent leakage during incubation.
• Reading plates: Open plates only within a BSC. Use a mirror or automated reader to minimize plate manipulation.

Waste Management

• Dispose of used disks, swabs, and pipette tips in biohazard waste.
• Autoclave all agar plates and microtiter plates before disposal.
• Decontaminate reusable equipment (e.g., disk dispensers, forceps) with appropriate disinfectant.

Training

All personnel must receive training in:

• Standard microbiological practices
• Proper use of BSCs
• Emergency procedures for spills and exposures
• Waste disposal protocols

Refer to the CDC/NIH BMBL 6th Edition for comprehensive biosafety guidance [6] and the NIH Guidelines for research involving recombinant or synthetic nucleic acid molecules if applicable [7].

Frequently Asked Questions

1. Can I use disk diffusion results to calculate an MIC value? No. While there is an inverse relationship between zone diameter and MIC, the correlation is not linear or precise enough to calculate an exact MIC from a zone measurement. Disk diffusion provides categorical results (S/I/R) only. For quantitative MIC data, broth microdilution or another MIC method must be used.

2. Why do CLSI and EUCAST breakpoints sometimes give different interpretations for the same zone diameter? CLSI and EUCAST use different pharmacokinetic/pharmacodynamic data, clinical outcome studies, and resistance mechanism considerations to set breakpoints. They also use different disk contents for some drugs. Laboratories must choose one guideline and apply it consistently. When comparing methods, it is essential to specify which breakpoints were used [3][5].

3. How do I choose between disk diffusion and broth microdilution for my laboratory? Consider your primary needs: if you test large numbers of isolates for routine susceptibility screening and only need categorical results, disk diffusion is more efficient and cost-effective. If you need precise MIC values for resistance surveillance, therapeutic drug monitoring, or testing fastidious organisms, broth microdilution is the better choice. Many laboratories use both methods, employing disk diffusion for initial screening and broth microdilution for confirmation or when MIC data are required.

4. What should I do if my disk diffusion and broth microdilution results disagree? First, confirm that both tests were performed correctly and that the same breakpoint version was used. Check QC results for both methods. If the discrepancy persists, consider whether the organism has a known resistance mechanism that might affect one method more than the other (e.g., inducible resistance, biofilm formation). Repeat both tests, and if disagreement continues, use broth microdilution as the reference method, as it is the gold standard for MIC determination [1][5].

References and Further Reading

1. Shi Y, Li S, He L, Wang H, Shang Y, Zhang K, Han X, Wei Y, Chu W, Xiong Y. Comparative evaluation of tigecycline susceptibility testing methods for carbapenem-resistant Klebsiella pneumoniae and Acinetobacter baumannii. 2026. PubMed ID: 42226269. https://pubmed.ncbi.nlm.nih.gov/42226269/

2. Puxeddu S, Canton S, Scano A, Delogu I, Pibiri A, Cabriolu C, Vascellari S, Pettinau F, Pivetta T, Ennas G, Manzin A, Angius F. Beyond One-Size-Fits-All: Addressing Methodological Constraints in Novel Antimicrobials Discovery. 2025. PubMed ID: 40868042. https://pubmed.ncbi.nlm.nih.gov/40868042/

3. Duggan C, Cantillon D, Lawrie D, Neal T, Cruise J, Graf FE, Owen V, Fraser AJ, Lewis JM, Brookfield C, Heinz E, Edwards T. Comparison of multiple cefiderocol susceptibility testing methods against genomic determinants of resistance in blaNDM carbapenemase producing Enterobacterales. 2026. https://doi.org/10.64898/2026.01.27.701980

4. Yu X, Liu Y, Du J, Hu F, Yin D. Assessment of the revision of the 2025 CLSI breakpoints for the interpretation of minocycline susceptibility for Acinetobacter baumannii complex. 2026. PubMed ID: 41685899. https://pubmed.ncbi.nlm.nih.gov/41685899/

5. Lin Y-T, Lin H-H, Chen C-H, Tseng K-H, Ho M-W, Hsueh P-R. Interpretive agreement of susceptibility between broth microdilution and disk diffusion methods for cefiderocol, using criteria from the Clinical and Laboratory Standards Institute, European Committee on Antimicrobial Susceptibility Testing, and the Food and Drug Administration. 2026. PubMed ID: 41369533. https://pubmed.ncbi.nlm.nih.gov/41369533/

6. CDC and NIH. Biosafety in Microbiological and Biomedical Laboratories (BMBL), 6th Edition. U.S. Department of Health and Human Services, 2020. https://www.cdc.gov/labs/bmbl/index.html

7. National Institutes of Health. NIH Guidelines for Research Involving Recombinant or Synthetic Nucleic Acid Molecules. https://osp.od.nih.gov/policies/biosafety-and-biosecurity-policy/nih-guidelines-for-research-involving-recombinant-or-synthetic-nucleic-acid-molecules/

8. National Center for Biotechnology Information. NCBI Bookshelf: Molecular Biology and Laboratory Methods. https://www.ncbi.nlm.nih.gov/books/

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