Food accessory substances and the nitrite bacteria

Abstract

In the isolation of nitrifying bacteria, soil is inoculated into a synthetic solution containing (NH₄)₂ SO₄ as a source of nitrogen for the “nitrite” bacteria and into one containing NaNO₂ for the “nitrate” bacteria. The solutions are incubated and when a test for nitrites is found in one case or a test for nitrates in the other, a small amount of the liquid is transferred to a new synthetic solution. This transferring is continued for some time. Then the material is plated out on silica jelly containing suitable inorganic salts. From the colonies developing, inoculations are made into synthetic solution and if nitrites are formed in the one containing (NH₄)₂ SO₄ a “nitrite”bacteria has been isolated or if nitrates are formed where NaNO₂ is the source of nitrogen a“nitrate”bacteria has been isolated.

Great difficulty was experienced in isolating cultures by the above procedure. At the beginning, in no case where inoculations from the colonies on silica jelly were made into the synthetic solution, were positive tests subsequently obtained. If, however, a solution giving a positive test were centrifuged and the sediment inoculated on silica jelly by means of a loop and from the colonies that developed, inoculations were made into synthetic solutions, positive tests would be obtained. This applied to both the“nitrite”and the“nitrate”organisms. Now if these positive cultures were inoculated on nutrient agar, growth would appear. This growth would not cause nitrification, showing the cultures in question were not pure. In one experiment the synthetic solution for nitrate bacteria which gave a positive test was centrifuged and inoculated on a gypsum block partly immersed in the synthetic solution. From the growth later obtained, dilutions were made and plated on silica jelly.