KINETICS OF THE SIMULTANEOUS UTILIZATION OF SUGAR MIXTURES BY ESCHERICHIA-COLI IN CONTINUOUS-CULTURE

In natural environments heterotrophic microorganisms encounter complex mixtures of carbon sources, each of which is present at a concentrati on of a few micrograms per liter or even less, Under such conditions n o significant growth would be expected if cells utilized only one of t he available carbon compounds, as suggested by the principle of diauxi c growth, Indeed, there is much evidence that microbial cells utilize many carbon compounds simultaneously. Whereas the kinetics of single-s ubstrate and diauxic growth are well understood, little is known about how microbial growth rates depend on the concentrations of several si multaneously utilized carbon sources. In this study this question was answered for carbon-limited chemostat growth of Escherichia coli fed w ith mixtures of up to six sugars; the sugars used were glucose, galact ose, maltose, ribose, arabinose, and fructose. Independent of the mixt ure composition and dilution rate tested, E. coli utilized all sugars simultaneously. Compared with growth with a single sugar at a particul ar growth rate, the steady-state concentrations were consistently lowe r during simultaneous utilization of mixtures of sugars, The steady-st ate concentrations of particular sugars depended approximately linearl y on their contributions to the total carbon consumption rate of the c ulture. Our experimental data demonstrate that the simultaneous utiliz ation of mixtures of carbon sources enables heterotrophic microbes to grow relatively fast even in the presence of low environmental substra te concentrations. We propose that the observed reductions in the stea dy-state concentrations of individual carbon sources during simultaneo us utilization of mixtures of carbon sources by heterotrophic microorg anisms reflect a general kinetic principle.