Nutritional Factors of S. Lactis

Abstract

Whenever one is dealing with 3 variables, the trilinear chart is often found to be useful. Such a chart is based upon the thesis that in an equilateral triangle the sum of the perpendiculars from any point to the sides is equal to the altitude of the triangle. If this altitude is designated as 100%, then the position of the point within can be expressed in terms of per cent since × + y + z = 100.∗

In our studies with S. lactis we have used 1% Difco peptone, 1% Difco yeast extract and water as variables. These solutions are prepared and adjusted electrometrically to pH 6.5. Media 1, 21, and 16 (see Fig.) are these 3 solutions. Each of these media contained 0.5% lactose. By properly mixing these solutions, 18 other media are made which are represented in the figure by the various numbers. Each medium varies from the other in some small degree. Thus the media in the base line (16 to 21) have no peptone. This is the zero line for peptone.

If the altitude of the triangle is now divided into 5 equal parts and lines drawn parallel to the base through these points a series of triangles is obtained. The media on line 11–15 will all have 0.2% peptone (20% of the original 1% solution). Also, all media on line 7–10 will have 0.4% peptone. Medium 1, of course, has 1% peptone (100% of the 1% solution). The same reasoning can be used for the extract. In the case of the water, it is merely a diluent of the media in which it appears.

With this method, some interesting results have been obtained with S. lactis.