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1. The rigidity of agar gel in poured agar plates can be tested by a comparatively simple and easily repeatable method.
2. Commercial agar from different sources varies in the rigidity of gel it produces at a given concentration.
3. The rigidity of gel produced by any agar at a given concentration is affected by processing and by the mode of production of poured plates and to some extent by the diluent used.
4. The appearance of a spurious S-R change in colonies of Salmonella and Shigella on media with an increasing agar content is a measure of the rigidity of the agar gel used and not directly of the agar concentration of the medium.
5. The appearance of this spurious S-R change induced by rigid agar bears no relation to the agglutinability of the organism.
6. Both increased or decreased agglutinability of cultures of Salmonella and Shigella is associated with the appearance of colonies of an apparently rough morphology at a lower level of agar rigidity than that at which spurious S-R change develops in a normally agglutinable culture.
7. Without some standardization of the rigidity of the gel used in agar plates, erroneous conclusions may be drawn regarding the antigenic state of the organisms under test.
8. Samples of agar of differing gel rigidity can be blended to form a product producing a gel of a calculated 12 sec. value.
I am indebted to the Director-General, Army Medical Services, for permission to publish this article.
