JAC Advance Access published online on March 12, 2008
Journal of Antimicrobial Chemotherapy, doi:10.1093/jac/dkn090
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Original research |
Principles of assessing bacterial susceptibility to antibiotics using the agar diffusion method
School of Biomedical Sciences and Institute of Infection and Immunity, University of Nottingham, Nottingham NG7 2UH, UK
Received 8 January 2008; returned 26 January 2008; revised 13 February 2008; accepted 14 February 2008
* Corresponding author. Tel: +44-115-823-0177; Fax: +44-115-823-0142; E-mail: boyan.bonev{at}nottingham.ac.uk
Objectives: The agar diffusion assay is one method for quantifying the ability of antibiotics to inhibit bacterial growth. Interpretation of results from this assay relies on model-dependent analysis, which is based on the assumption that antibiotics diffuse freely in the solid nutrient medium. In many cases, this assumption may be incorrect, which leads to significant deviations of the predicted behaviour from the experiment and to inaccurate assessment of bacterial susceptibility to antibiotics. We sought a theoretical description of the agar diffusion assay that takes into consideration loss of antibiotic during diffusion and provides higher accuracy of the MIC determined from the assay.
Methods: We propose a new theoretical framework for analysis of agar diffusion assays. MIC was determined by this technique for a number of antibiotics and analysis was carried out using both the existing free diffusion and the new dissipative diffusion models.
Results: A theory for analysis of antibiotic diffusion in solid media is described, in which we consider possible interactions of the test antibiotic with the solid medium or partial antibiotic inactivation during diffusion. This is particularly relevant to the analysis of diffusion of hydrophobic or amphipathic compounds. The model is based on a generalized diffusion equation, which includes the existing theory as a special case and contains an additional, dissipative term.
Conclusions: Analysis of agar diffusion experiments using the new model allows significantly more accurate interpretation of experimental results and determination of MICs. The model has more general validity and is applicable to analysis of other dissipative processes, for example to antigen diffusion and to calculations of substrate load in affinity purification.
Key Words: agar diffusion assay , antibiotic activity testing , antibiotic resistance