Quantitative amino acid assimilation in homofermentative metabolism

Abstract

Summary The question has been examined whether, in a microorganism which covers its energy requirements from the fermentation of hexose to lactic acid, amino acids serve solely as protein building blocks without being involved in degradative energy metabolism. The present evidence, pertaining to S. faecalis 9790 and the amino acids histidine, isoleucine, leucine, lysine, threonine, and valine suggests the following conclusions: 1. During the logarithmic growth phase the total concentration of each amino acid in the organism is constant and independent of the concentration in the medium. 2. In an adequately buffered medium, logarithmic growth continues until the limiting amino acid is used up. 3. Each amino acid is characterized by a specific assimilation factor which expresses the amount of amino acid required for the production of a unit quantity of logarithmic phase bacteria. 4. There is no measurable metabolic degradation of the limiting amino acid during the phase of logarithmic growth, i.e., all of the initial supply is present either in the organism or in the medium. 5. After the limiting amino acid has been taken up, substantial additional growth, as measured by optical density, occurs. An exception is the case of lysine, where bacterial lysis takes place after depletion of the medium. 6. During the phase of postlogarithmic growth the total quantities of the amino acids present intracellularly remain constant, but their concentrations, with respect to bacterial substance, decrease. The postlogarithmic metabolism needs further study. This process of relative amino acid dilution probably explains the previous observation that less amino acid is needed to produce a given level of (postlogarithmic) growth than corresponds to the cellular amino acid content, if the latter is determined on a logarithmic phase or early postlogarithmic phase cells. 7. During the logarithmic growth phase, but not during the postlogarithmic phase, the bacterial cells may be considered as being in a standard state.