Physiology of Nitrogen Fixation by a Species of Achromobacter

The metabolic pathway by which certain microorganisms incorporate atmospheric nitrogen into their protoplasm is at the present time unknown. In an attempt to determine this pathway scientists are studying the physiology of various nitrogen fixing bacteria and are within reasonable limits extrapolating this information from one organism to another. Thus far they have found certain physiological characteristics which appear to be common to all nitrogen fixing bacteria. Several years ago a species of the genus Achromobacter was isolated and shown to fix atmospheric nitrogen. Thus far very little has been published concerning the physiology of nitrogen fixation by this organism. This study was carried out in an attempt to confirm some of the reported findings regarding the physiology of nitrogen fixation by this organism and to determine other physiological characteristics which may be of value in the elucidation of the mechanisms of biological nitrogen fixation. The organism used for this study was a nitrogen fixing species of the genus Achromobacter. This organism, referred to as strain N4, was isolated from a Danish water course and was shown to fix nitrogen by Vagn Jensen. It as yet bears no species name. Its physiological characteristics are described by Jensen. At the beginning of this study physiological tests were made to confirm the identity and purity of the culture, which was carried on nutrient agar containing 0.2 per cent yeast extract. Because of the fact that growth in most experiments took place under anaerobic conditions in a medium which was essentially free of combined nitrogen the problem of contamination was not great. Periodic checks were made, however, to detect possible contamination by A. acrogenes. This was done by inoculating from experimental cultures into tubes of lactose broth containing gas tubes. Preliminary experiments were carried out using the modification described by Pengra and Wilson of a medium proposed by Monad and Wollman for the cultivation of Escherichia coli. Results of these experiments indicated that Achromobacter N4 produces relatively large quantities of acid during its gasless fermentation of carbohydrate