The method is elegant in its simplicity. You grow a standardised density of bacteria on an agar plate, place antibiotic-impregnated discs on the surface, allow the antibiotic to diffuse outward and the bacteria to grow overnight, and measure the diameter of the clear area that forms around each disc where the antibiotic concentration exceeded the minimum inhibitory concentration. From that diameter, using a published table of breakpoints, you classify the organism as susceptible, susceptible at increased exposure, or resistant to each antibiotic tested.
For all its simplicity, disc diffusion has a significant number of variables that can compromise its reliability, and the interpretation requires understanding the breakpoint system being used. This page covers the method in full, the critical quality control steps, the EUCAST and CLSI breakpoint systems and how they differ, reading special tests such as the double disc synergy test for ESBL detection and the D-zone test for inducible clindamycin resistance, and the common sources of error.
What Happens When an Antibiotic Disc Meets an Agar Plate
When an antibiotic disc is placed on an agar plate that has been inoculated with bacteria, the antibiotic begins to diffuse outward from the disc into the agar. The diffusion follows a concentration gradient: highest immediately around the disc, decreasing with distance. As the agar warms during incubation and bacteria begin to grow, there is a point at which the antibiotic concentration falls below the minimum inhibitory concentration for the organism being tested. At that point, bacteria can grow. The result, after overnight incubation, is a clear zone immediately around the disc (where drug concentration exceeded the MIC) surrounded by a lawn of bacterial growth (where drug concentration fell below the MIC). The boundary of that clear zone, measured across its diameter in millimetres, is the zone of inhibition.
The zone diameter and the MIC are inversely related: a larger zone means that the antibiotic was more potent against this organism (or the MIC is lower), a smaller zone means lower potency (higher MIC). The relationship is consistent enough, for a given organism-antibiotic combination tested under standardised conditions, that zone diameters can be used to infer whether the MIC falls above or below the breakpoint concentration that defines susceptibility.
Performing the Test: The Details That Matter
Preparing Mueller Hinton Agar
Mueller Hinton Agar (MHA) is the standard medium for disc diffusion because its composition produces consistent, reproducible results. The agar depth must be 4 mm in circular plates because deeper agar affects the diffusion rate and produces artificially smaller zones. Plates must be poured to a consistent depth and allowed to gel and dry before use. MHA contains no added blood for standard testing, though blood-supplemented MHA (Mueller Hinton blood agar) is used for fastidious organisms like Streptococcus pneumoniae and Haemophilus influenzae.
Preparing the Inoculum
The organism is suspended in saline or broth and the turbidity is compared to a 0.5 McFarland turbidity standard (which corresponds to approximately 1 to 2 x 10^8 CFU/mL for E. coli). Achieving the correct inoculum density is critical. Too heavy an inoculum compresses zone diameters (the organism overcomes low antibiotic concentrations more easily). Too light an inoculum produces excessively large zones. The comparison to the 0.5 McFarland standard must be done by eye against a white background with black lines or using a nephelometer/densitometer for more precision.
Inoculating the Plate
A sterile swab is dipped into the suspension, excess moisture removed by pressing it against the tube wall, and the swab is streaked across the plate in three directions rotating 60 degrees between each set of streaks. This produces a uniform bacterial lawn across the entire agar surface. The plate should be completely covered with no gaps in the lawn when viewed after incubation.
Applying the Discs
Antibiotic discs are applied within 15 minutes of inoculating the plate, before the bacteria have started to grow. Discs are applied using a multi-disc applicator or individually with forceps. Each disc must make complete contact with the agar surface. Discs should not overlap or be placed too close to the plate edge (within 15 mm of the edge) or too close to each other (minimum 24 mm centre to centre for most combinations) because overlapping diffusion fields distort zone sizes.
Incubation and Reading
Most organisms are incubated at 35 to 37 degrees Celsius for 16 to 18 hours in ambient air. Campylobacter requires microaerophilic conditions. Anaerobes require anaerobic jars or chambers. After incubation, zone diameters are measured to the nearest millimetre using a ruler, calliper, or automated zone reader, measuring across the full diameter of the clear zone including the area underneath the disc. The zone edge is defined as the point where growth starts: for most organisms this is a sharp, well-defined edge. Swarming organisms like Proteus mirabilis produce a thin veil of swarming growth inside the main zone which is ignored when reading the zone edge.
EUCAST vs CLSI: Reading the Breakpoints
Both EUCAST and CLSI publish annual breakpoint tables specifying the zone diameter (in mm) above which an organism is susceptible and below which it is resistant, for each organism-antibiotic combination.
EUCAST zone diameter breakpoints are derived from the same EUCAST MIC breakpoints used for broth dilution, using a regression-based method called the zone diameter-MIC correlation. EUCAST uses a three-category system: S (susceptible at standard dosing), I (susceptible at increased exposure: higher dose, continuous infusion, or concentrated infection site), and R (resistant).
CLSI zone diameter breakpoints are published in documents M100 and M02. They use a similar derivation process but from different datasets. The key difference for users is that CLSI uses "Susceptible" (S), "Intermediate" (I, meaning uncertain clinical significance), and "Resistant" (R). The CLSI "I" category has a different meaning from EUCAST "I": in CLSI it means the zone is in a buffer zone of uncertain clinical relevance rather than specifically actionable with increased dosing.
All UK clinical laboratories use EUCAST. European laboratories predominantly use EUCAST. North American, Latin American, and many Asian laboratories use CLSI. When interpreting a report, confirm which guideline the reporting laboratory uses.
Special Tests Using Disc Diffusion
Double Disc Synergy Test for ESBL Detection
The DDST detects ESBL-producing organisms by demonstrating that beta-lactamase inhibitors (clavulanic acid) synergistically enhance the activity of cephalosporins against the isolate. An amoxicillin-clavulanate disc is placed in the centre of the inoculated plate. Cefotaxime and ceftazidime discs are placed 20 mm away from the central disc on either side. If the organism produces an ESBL, the zones around the cephalosporin discs distort toward the amoxicillin-clavulanate disc, showing a classic keyhole or champagne cork shape. This distortion indicates that clavulanate is inhibiting the ESBL when it diffuses toward the cephalosporin disc, enhancing the cephalosporin's zone.
D-Zone Test for Inducible Clindamycin Resistance
Some Staphylococcus aureus and coagulase-negative staphylococci carry inducible clindamycin resistance (encoded by erm genes). These isolates appear susceptible to clindamycin on routine disc testing (large zone) but resistant to erythromycin (small zone). In a patient treated with clindamycin, the erm gene can be induced, causing treatment failure. The D-zone test places an erythromycin disc and a clindamycin disc 15 to 25 mm apart on the same plate. If the clindamycin zone is flattened on the side facing the erythromycin disc (creating a D shape), inducible resistance is present and the organism should be reported as resistant to clindamycin.
Common Sources of Error
Inoculum too heavy: Zones are smaller than they should be. All results trend toward resistance. Often caused by inadequate comparison to the McFarland standard or using colonies from the wrong growth phase.
Inoculum too light: Zones are larger than they should be. All results trend toward susceptibility. Often caused by using a diluted suspension, not picking enough colonies, or comparing to the McFarland standard in a tube with a different diameter.
Agar too deep or too thin: Affects diffusion rates and produces systematic errors in zone diameters. Plates must be poured to a consistent 4 mm depth.
Discs applied too late: If discs are placed more than 15 minutes after inoculation, the bacteria have already begun to grow when the antibiotic starts diffusing, and zones are smaller than expected.
Reading zones at wrong incubation time: Reading too early (12 hours) may not allow complete zone formation. Reading too late (24+ hours) can reduce zone sizes as slow-growing subpopulations appear.
Wrong incubation conditions: Using ambient air for microaerophilic or anaerobic organisms produces no growth or abnormal colony density.
Frequently Asked Questions
What is the zone of inhibition?
The zone of inhibition is the clear area around an antibiotic disc on an inoculated agar plate where bacterial growth has been suppressed because the antibiotic concentration exceeds the organism's MIC. The diameter of this zone is measured in millimetres and compared to published breakpoints to determine susceptibility.
What is Mueller Hinton Agar?
Mueller Hinton Agar is the standard medium for disc diffusion testing, specified by both EUCAST and CLSI. Its low-sulfonamide-antagonist content, low thymine/thymidine content, and consistent composition produce reliable, reproducible zone diameters. It is made to a specific agar depth of 4 mm for disc diffusion.
What is the 0.5 McFarland standard?
The 0.5 McFarland standard is a turbidity reference corresponding to approximately 1 to 2 x 10^8 CFU/mL for a bacterial suspension. It is used to standardise the inoculum density for disc diffusion. The bacterial suspension is diluted or concentrated until its turbidity visually matches the 0.5 McFarland standard before swabbing the plate.
What is a DDST?
The Double Disc Synergy Test detects ESBL production in gram-negative bacteria. An amoxicillin-clavulanate disc is placed centrally on a plate with cephalosporin discs on either side. ESBL-producing isolates show zone distortion toward the clavulanate disc (keyhole or champagne cork shape), confirming the clavulanate is inhibiting the ESBL.
What does a D-shaped zone mean?
A D-shaped clindamycin zone in the D-zone test indicates inducible clindamycin resistance (iMLSB phenotype) mediated by erm genes. The zone of inhibition around the clindamycin disc is flattened on the side nearest the erythromycin disc. These isolates should be reported as resistant to clindamycin because erm gene induction during treatment can lead to clinical failure.
Why are zone sizes smaller with a heavy inoculum?
A heavier bacterial inoculum contains more bacteria to overcome the antibiotic, so the MIC of the collective culture is effectively higher. The antibiotic must diffuse further before falling below the effective inhibitory concentration for the dense population, but in practice the zone edge is reached at a shorter distance, producing a smaller zone. Standardising the inoculum to 0.5 McFarland prevents this.
What is cefoxitin disc testing used for in Staphylococcus?
Cefoxitin disc testing is the recommended disc diffusion surrogate test for detecting mecA-mediated resistance (MRSA). Cefoxitin is a better inducer of mecA expression than oxacillin, producing clearer zone size differences between MRSA and MSSA. A cefoxitin zone diameter of less than or equal to 19 mm (EUCAST) indicates MRSA.
Can disc diffusion detect all resistance mechanisms?
No. Disc diffusion detects resistance that results in a measurable reduction in zone size. Inducible resistance (like inducible clindamycin resistance) may not be apparent from the zone size alone without the D-zone test. Heteroresistance, where only a subpopulation of cells is resistant, can also be missed. The D-zone test and DDST are supplementary tests that detect specific mechanisms not reliably shown by routine zone reading alone.
What is swarming and how does it affect zone reading in Proteus?
Proteus mirabilis is a highly motile organism that swarms across agar surfaces, producing a thin veil of growth that extends into zones of inhibition. This swarming veil is ignored when reading the zone edge: the reader looks for the sharp boundary of the main lawn of growth beneath the swarming veil. Using Mueller Hinton Agar with 5 per cent blood, or reducing agar surface moisture, can partially suppress swarming.
What organisms require supplemented Mueller Hinton for disc diffusion?
Fastidious organisms that do not grow satisfactorily on plain Mueller Hinton agar require supplemented media. Streptococcus pneumoniae and other streptococci require Mueller Hinton blood agar (MHBA) with 5 per cent horse blood. Haemophilus influenzae requires Mueller Hinton Fastidious Agar or Haemophilus Test Medium (HTM) supplemented with NAD and haemin.