SUMMARY: The influence of amino acids on respiration of was studied over a range of water activity (a) values. Decrease of the a value by adding NaCl or sucrose decreased the rate of glucose oxidation and induced a lag. At relatively low values (0.970 a) but not at high values (0.998 a), additions of certain amino acids (proline, aspartic acid, asparagine, glutamic acid, glutamine, cysteine) caused an appreciable synergistic increase in respiration rate. Proline was the most stimulatory amino acid tested. At 0–960 a, only proline and its analogue azetidine-2-carboxylic acid gave appreciable stimulation. These were the only amino acids which decreased the lag, and lags were not affected by the presence of chloramphenicol. Proline was also stimulatory when glucose was replaced by pyruvate or succinate. The respiration of organisms grown at low a values was not stimulated by proline unless the organisms were first subjected to osmotic shock. The proline effect was also observed with bacteria of four other genera. The data are discussed in relation to the stimulation by proline of bacterial growth at low water activities.


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