HOPE Lab - Antimicrobial Susceptibility Testing (AST)

Antimicrobial Susceptibility Testing (AST) Protocol

Kirby‑Bauer Disk Diffusion Method

HOPE Lab Internal Training Resource

Introduction to Antimicrobial Susceptibility Testing (AST)

This laboratory protocol details the Kirby‑Bauer disk diffusion method for Antimicrobial Susceptibility Testing (AST), designed to help our students and researchers accurately assess antimicrobial resistance (AMR). Antimicrobial susceptibility testing is an important laboratory procedure used to determine the effectiveness of antimicrobial agents against isolated microorganisms. It plays an indispensable role in guiding both empirical and definitive therapy for infectious diseases. By identifying specific antimicrobial agents to which a microorganism is susceptible, AST aids clinicians in selecting appropriate treatments, optimizing patient outcomes, and preventing the emergence and spread of antimicrobial resistance (AMR). AMR occurs through mechanisms such as drug inactivation, target modification, reduced permeability, and active efflux.

AMR is a global health crisis that occurs when microorganisms evolve mechanisms to resist the effects of antimicrobial drugs. This phenomenon poses a significant challenge to public health, limiting therapeutic options and increasing morbidity and mortality rates. Consequently, understanding the mechanisms of antimicrobial resistance and implementing effective AST protocols are essential steps in combating this growing threat. The protocol detailed below is used in our lab and provided here for the convenience of our students to ensure consistency and accuracy in their results.

Learning Outcomes

Upon completing this training protocol, our lab students will be able to:

Standardize a bacterial suspension to a 0.5 McFarland standard
Inoculate a Mueller‑Hinton agar plate to obtain confluent growth
Perform the Kirby‑Bauer disk diffusion test
Measure zones of inhibition accurately
Interpret AST results using standardized breakpoints

Principle of Antimicrobial Susceptibility Testing

Antimicrobial susceptibility testing (AST) is based on determining the ability of a microorganism to grow in the presence of an antimicrobial agent under standardized laboratory conditions. The fundamental principle is that susceptible microorganisms show inhibited growth when exposed to an effective antimicrobial concentration, whereas resistant microorganisms continue to grow under the relevant test conditions.

In the Kirby-Bauer disk diffusion method, this principle is applied by placing paper disks containing defined amounts of antimicrobial agents onto the surface of an agar medium inoculated with a standardized bacterial suspension. The antimicrobial agent diffuses outward from each disk into the agar, producing a concentration gradient in which the concentration is highest near the disk and progressively decreases with distance. Where the antimicrobial concentration is sufficient to inhibit growth of the test organism, a clear zone of inhibition develops around the disk.

The diameter of the zone of inhibition reflects the interaction between the antimicrobial agent and the test microorganism under standardized test conditions. Zone size can be affected by factors such as inoculum density, agar characteristics, disk potency, incubation conditions, and the diffusion properties of the antimicrobial agent. Therefore, the measured zone diameter must be interpreted using established, organism-specific and antimicrobial-specific breakpoints rather than by zone size alone. Under standardized conditions, disk diffusion provides a reproducible measure that can be used to assign an appropriate antimicrobial susceptibility category.

Kirby-Bauer disk diffusion does not directly determine the minimum inhibitory concentration (MIC). MIC is the lowest concentration of an antimicrobial agent that prevents visible growth of a microorganism and is typically determined by dilution-based susceptibility testing. In contrast, disk diffusion measures the diameter of growth inhibition produced by antimicrobial diffusion through agar, and the measured zone diameter is interpreted against standardized breakpoints to determine the susceptibility category of the test organism.

Core Mechanisms of Antimicrobial Action

Antimicrobial agents exert their effects through diverse mechanisms, targeting different cellular components or processes of microorganisms. These mechanisms include:

1. Inhibition of cell wall synthesis: This mode of action is characteristic of many antibiotics, such as penicillins and cephalosporins. They interfere with the synthesis of peptidoglycan, an essential component of the bacterial cell wall, leading to osmotic lysis and cell death.

2. Disruption of cell membrane integrity: Agents like polymyxins target the bacterial cell membrane, leading to leakage of cellular contents and eventual cell death. This mechanism is often rapid and concentration‑dependent.

3. Inhibition of protein synthesis: Antibiotics such as aminoglycosides and tetracyclines bind to ribosomes and disrupt protein synthesis, hindering bacterial growth. This class of antibiotics is typically bacteriostatic but can be bactericidal at higher concentrations.

4. Inhibition of nucleic acid synthesis: Quinolones and fluoroquinolones inhibit DNA replication by targeting DNA gyrase and topoisomerase IV, essential enzymes for bacterial DNA replication and transcription.

5. Metabolic inhibition: Sulfonamides and trimethoprim interfere with folic acid metabolism, which is vital for bacterial growth. This pathway is unique to bacteria, making these agents selectively toxic.

6. Disruption of membrane function: Some antimicrobial agents, such as daptomycin, insert into the bacterial cell membrane, causing rapid depolarisation and disruption of essential cellular functions.

Understanding these mechanisms is essential for accurately interpreting AST results and selecting appropriate therapeutic options.

Procedure for Antimicrobial Susceptibility Testing (AST)

Disk Diffusion Method (Kirby‑Bauer Test)

This protocol details the disk diffusion method for antimicrobial susceptibility testing (AST) used in our lab, and is intended primarily for our students to ensure accuracy and consistency. The disk diffusion method, commonly known as the Kirby‑Bauer test, is a standardized technique used to assess the susceptibility of bacteria to antimicrobial agents. This method involves inoculating a standardized bacterial suspension onto an agar plate and placing antibiotic‑impregnated disks on the surface. After incubation, the diameter of the zone of inhibition surrounding each disk is measured and compared to established interpretive standards to determine susceptibility.

Materials

Mueller‑Hinton agar plates
Antimicrobial susceptibility disks (containing known concentrations of antibiotics)
Sterile cotton swabs
Inoculation loop
Sterile saline solution (0.85%)
Graduated cylinder
Spectrophotometer or densitometer
Non‑selective culture medium
Incubator
Calipers or ruler

Step‑by‑Step Kirby‑Bauer Disk Diffusion Procedure

1. Preparation of bacterial suspension

Select a pure, well‑isolated colony of the test organism from a non‑selective culture medium.
Suspend the colony in sterile saline solution to match the turbidity of a 0.5 McFarland standard. This can be visually compared or measured using a spectrophotometer or densitometer.

2. Inoculation of agar plate

Dip a sterile cotton swab into the bacterial suspension and remove excess fluid by pressing the swab against the inside of the tube.
Inoculate the entire surface of the Mueller‑Hinton agar plate by streaking the swab back and forth in three directions, rotating the plate 60 degrees after each pass.

3. Application of antibiotic disks

Using sterile forceps, place the desired antimicrobial disks onto the inoculated agar plate at predetermined distances.
Ensure that the disks are evenly spaced and do not overlap.

4. Incubation

Invert the agar plate and incubate at 35 ± 2°C for 16‑18 hours.

5. Measurement of zone of inhibition

After incubation, measure the diameter of the zone of inhibition around each antibiotic disk in millimeters.
Use calipers or a ruler for accurate measurement.

6. Interpretation

Compare the measured zone diameters to established interpretive standards provided by clinical and laboratory standards institutes (e.g., CLSI, EUCAST). Results should be interpreted using the latest organism‑specific breakpoints published by CLSI or EUCAST.
Interpret the results as susceptible, intermediate, or resistant based on the zone diameter and the breakpoint for each antimicrobial agent:

Susceptible (S): The organism is inhibited by the standard therapeutic dose of the antimicrobial agent.

Intermediate (I): Susceptibility is uncertain; clinical efficacy may be variable. This category serves as a buffer zone to prevent technical errors.

Resistant (R): The organism is not inhibited by the standard therapeutic dose of the antimicrobial agent.

Infographic

Mueller‑Hinton agar plates displaying distinct zones of inhibition around various antibiotic susceptibility disks.

Caution

Biosafety: Handle microorganisms following appropriate biosafety level precautions to minimize risks.
Antibiotic resistance: Overuse of antibiotics can contribute to the development of antimicrobial resistance. Use antibiotics judiciously and in accordance with established guidelines.
Interpretation: Proper interpretation of results requires expertise in microbiology and antimicrobial therapy. Consult with a clinical microbiologist when necessary.
Quality control: Regular quality control is essential to ensure the accuracy and reliability of results.

Quality Control

Positive and negative controls: Include standard strains with known susceptibility patterns as positive and negative controls in each test run.
Disk potency: Ensure that antibiotic disks are within their expiration date and stored according to manufacturer specifications.
Media preparation: Prepare Mueller‑Hinton agar according to manufacturer's instructions and quality control specifications.
Inoculum preparation: Verify the accuracy of the McFarland standard for inoculum preparation.
Zone size interpretation: Use standardized guidelines (e.g., CLSI, EUCAST) for interpreting zone sizes.

Factors Affecting Results

Several technical factors can influence zone sizes and interpretation. Standardization of these variables is essential for reproducible AST results.

Inoculum density

Effect: Too heavy reduces zone size; too light increases zone size
Control measure: Standardize to 0.5 McFarland using a spectrophotometer or densitometer

Agar depth

Effect: Thicker agar reduces diffusion, leading to smaller zones
Control measure: Use 4 mm depth (approximately 25 mL per 150 mm plate)

Disk potency

Effect: Expired or improperly stored disks may have reduced activity
Control measure: Check expiration dates; store disks as per manufacturer instructions

Incubation time and temperature

Effect: Extended incubation may allow regrowth; incorrect temperature affects bacterial growth rate
Control measure: Maintain 35 ± 2°C for 16‑18 hours

Plate dryness

Effect: Excessively dry plates reduce diffusion and zone size
Control measure: Use freshly prepared plates; avoid over‑drying

Spacing between disks

Effect: Overlapping zones can lead to misinterpretation
Control measure: Space disks at least 24 mm apart (centre to centre)

pH of medium

Effect: pH variation can affect antibiotic activity and diffusion
Control measure: Verify pH (7.2–7.4 at 25°C) before use

Cation content

Effect: Variation in divalent cations (Mg²⁺, Ca²⁺) affects certain antibiotics like aminoglycosides
Control measure: Use commercially prepared Mueller‑Hinton agar from reputable suppliers

Troubleshooting

Problem: No growth or poor growth

Possible cause: Incorrect inoculum preparation, improper incubation conditions, contaminated media
Solution: Prepare inoculum correctly, check incubator temperature and CO₂ levels, use fresh media.

Problem: Excessive growth

Possible cause: Over‑inoculation, contaminated media, improper incubation conditions
Solution: Adjust inoculum density, use fresh media, check incubator temperature and CO₂ levels.

Problem: Inconsistent zone sizes

Possible cause: Variation in inoculum density, improper disk application, disk deterioration, media variation
Solution: Prepare inoculum consistently, apply disks correctly, use fresh disks, standardize media preparation.

Problem: No zones of inhibition

Possible cause: Resistant organism, disk placement error, media problems
Solution: Confirm organism identification, check disk placement, and use fresh media.

Problem: Discrepancies with reference laboratory

Possible cause: Technical errors, different interpretive criteria, different antimicrobial agents
Solution: Review test procedures, compare interpretive criteria, and check antimicrobial agent specifications.

Additional Considerations

Organism identification: Accurate identification of the microorganism is critical for appropriate antimicrobial selection.
Patient factors: Consider the patient's clinical condition, immune status, and potential allergies when selecting antimicrobial therapy.
Combination therapy: In some cases, combination therapy with multiple antimicrobial agents may be necessary.
Susceptibility testing methods: Other susceptibility testing methods, such as broth microdilution and E‑test, may be used for specific purposes or organisms.
Emerging resistance mechanisms: Stay updated on emerging antimicrobial resistance mechanisms to guide therapeutic decisions.

By following these guidelines and implementing appropriate quality control measures, laboratories can ensure the accuracy and reliability of antimicrobial susceptibility testing results, which are essential for effective antimicrobial therapy.

References

Ali F, Silvy TN, Hossain TJ, Uddin MK, Uddin MS. 2021. Prevalence and antimicrobial resistance phenotypes of Salmonella species recovered at various stages of broiler operations in Hathazari, Bangladesh. Int J One Health, 7, 158–164. (doi:10.14202/IJOH.2021)
Hossain TJ. 2024. Methods for screening and evaluation of antimicrobial activity: A review of protocols, advantages, and limitations. European Journal of Microbiology and Immunology, 14(2), 97–115. (doi:10.1556/1886.2024.00035)
Paul S, Hossain TJ, Ali F, Hossain ME, Chowdhury T, Faisal IK, Ferdouse J. 2024. Assessment of the in‑vitro probiotic efficacy and safety of Pediococcus pentosaceus L1 and Streptococcus thermophilus L3 isolated from Laban, a popular fermented milk product. Arch Microbiol, 206, 82. (doi:10.1007/s00203-023-03812-5)

HOPE Lab Internal Training Notice

This document is a localized laboratory training protocol provided exclusively for students and researchers at the HOPE Lab. It is designed to align with internal laboratory training benchmarks. For definitive clinical diagnostic reporting, users must consult the most recent official updates from standard reference frameworks such as the CLSI or EUCAST guidelines.

HOPE Lab Internal Training Document
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