The growth of microbial cultures complies with the laws of thermodynamics

Abstract

The paper is the translation of the previously proposed growth model in a thermodynamic balance of the Gibbs free energy of the system (medium + microbes), based on a simple scheme of the cell duplication. In each duplication step, the cells garner a small extra Gibbs energy from the surrounding medium that loses also some energy through an exothermic effect. It turns out that the each duplication step implies an increase of the entropy of the system, but a decrease of the entropy of the involved cells. The overall number of duplication steps therefore determines the energy balance of the whole growth process. The growth model implies a relationship that links this number with the maximum specific growth rate and the no-growth latency that precedes the growth onset, namely, two parameters that reflect the biological efficiency of the cells. For this reason, the overall number of duplication steps, determined according to this model, seems the best proxy of the fitness of the microbial culture. In a Long Term Evolution Experiment (LTEE), the increasing fitness would therefore correspond to larger growth extent and specific rate, as well as to shorter pre-growth latency. This suggests that the gain of Gibbs free energy accumulated through the LTEE several-thousand generations leads to a faster attainment of the eventual steady state of the growth and a faster increase of the entropy of the system. If applied to a continuous LTEE carried out with a chemostat, this trend should reveal that the evolution of the culture (medium + cells) is an irreversible process.