It was shown by MacLeod and his co-workers that among bacteria only the true marine forms have a specific sodium requirement. These all require this ion in concentrations of the order of a few tenths of 1%. The non-marine bacteria studied by MacLeod were distinguished from marine forms by their lack of any requirement for sodium (MacLeod, Onofrey & Norris, 1954; MacLeod & Onofrey, 1956). In view of these facts, the following observation, which I made in the course of studying the amino acid nutrition of seemed significant. This bacterium can grow very well in media of ordinary ionic strength and thus is not obviously marine. I observed that it did not grow in a medium in which an artificial mixture of amino acids had replaced an acid hydrolysate of casein. Since almost 40% of the hydrolysate used was sodium chloride, I added a little NaCl to the medium containing the artificial mixture of amino acids. I then found that the growth was as rapid as in the medium made with casein hydrolysate. It was easily shown that a very much simpler mixture of amino acids permitted rapid growth in the presence of as little as 100 μg. NaCl/ml. This requirement for a low concentration of sodium seemed to set this organism apart both from marine and from terrestrial bacteria and thus to merit a more detailed analysis, the first results of which are reported here.


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