First in a series of three articles on Curvature of the Line Response of Microbiological Assays starting with The Agar Diffusion Assay

In the agar diffusion assay of antibiotics, curvature of the response line is a recurrent problem. The purpose of this review is to consider three questions:

• Why do we have this problem ?
• How serious is the problem ?
• What should we do about it ?

There are essentially two mathematical relationships in microbiological assays:

1. the response [or function of response] is related to the dose of the preparation, this is the basis of the 'slope ratio assay';
2. the response [or function of response] is related to the logarithm of the dose of the preparation, this is the basis of the 'parallel line assay'.

The former is applicable to the assay of growth promoting substances such as vitamins and amino acids. The latter relationship has two applications, - the agar diffusion assay of antibiotics or vitamins and the turbidimetric assay of antibiotics.