VetBact

The disk diffusion method is one of the oldest and most commonly used methods in susceptibility testing. Mueller-Hinton or a modification of the medium is usually chosen for routine susceptibility testing. For weak and slow growing bacteria, modified Mueller-Hinton agar is used with an addition of blood and β-NAD. The plate is inoculated as a lawn of pure culture of the tested bacteria using standardised inoculum. Within 15 minutes of inoculating the agar surface paper discs impregnated with specific concentrations of antibiotics are placed on the agar. The concentrations of the antimicrobial agents in the discs have been chosen to give inhibition zone sizes that correlate with achievable serum levels in the patient.

The plate is incubated in an aerobic atmosphere with the exception of certain fastidious bacteria such as Haemophilus spp. which are incubated in increased CO2 content (5%). The antimicrobial agents immediately begin to diffuse into the medium and are thereby diluted - the further from the disc, the lower concentration.

At the same time, after a short lag phase, the bacteria begin to multiply. Depending on the bacteria´s growth rate, its sensitivity to the antibiotic in question and the antibiotic´s diffusion rate, inhibition zones of different sizes will be observed around the discs. The diameter of the zone is inversely proportional to the MIC value of the strain and with the help of breakpoint tables the bacteria can be categorizes as susceptible (S), intermediate (I) or resistant (R).

Updated: 2024-08-23.