Control of the galactose-to-glucose consumption ratio in co-fermentation using engineered Escherichia coli strains

Co-utilization of carbon sources in microorganisms for the bioproduction of chemicals

Carbon source is crucial for the cell growth and metabolism in microorganisms, and its utilization significantly affects the synthesis efficiency of target products in microbial cell factories. Compared with a single carbon source, co-utilizing carbon sources provide an alternative approach to optimize the utilization of different carbon sources for efficient biosynthesis of many chemicals with higher titer/yield/productivity. However, the efficiency of bioproduction is significantly limited by the sequential utilization of a preferred carbon source and secondary carbon sources, attributed to carbon catabolite repression (CCR). This review aimed to introduce the mechanisms of CCR and further focus on the summary of the strategies for co-utilization of carbon sources, including alleviation of CCR, engineering of the transport and metabolism of secondary carbon sources, compulsive co-utilization in single culture, co-utilization of carbon sources via co-culture, and evolutionary approaches. The findings of representative studies with a significant improvement in the bioproduction of chemicals via the co-utilization of carbon sources were discussed in this review. It suggested that by combining rational metabolic engineering and irrational evolutionary approaches, co-utilizing carbon sources can significantly contribute to the bioproduction of chemicals.

Growth productivity as a determinant of the inoculum effect for bactericidal antibiotics

Understanding the mechanisms by which populations of bacteria resist antibiotics has implications in evolution, microbial ecology, and public health. The inoculum effect (IE), where antibiotic efficacy declines as the density of a bacterial population increases, has been observed for multiple bacterial species and antibiotics. Several mechanisms to account for IE have been proposed, but most lack experimental evidence or cannot explain IE for multiple antibiotics. We show that growth productivity, the combined effect of growth and metabolism, can account for IE for multiple bactericidal antibiotics and bacterial species. Guided by flux balance analysis and whole-genome modeling, we show that the carbon source supplied in the growth medium determines growth productivity. If growth productivity is sufficiently high, IE is eliminated. Our results may lead to approaches to reduce IE in the clinic, help standardize the analysis of antibiotics, and further our understanding of how bacteria evolve resistance.

HyfF subunit of hydrogenase 4 is crucial for regulating FOF1 dependent proton/potassium fluxes during fermentation of various concentrations of glucose

Escherichia coli anaerobically ferment glucose and perform proton/potassium exchange at pH 7.5. The role of hyf (hydrogenase 4) subunits (HyfBDF) in sensing different concentrations of glucose (2 g L^-1 or 8 g L^-1) via regulating H⁺/K⁺ exchange was studied. HyfB, HyfD and HyfF part of a protein family of NADH-ubiquinone oxidoreductase ND2, ND4 and ND5 subunits is predicted to operate as proton pump. Specific growth rate was optimal in wild type and mutants grown on 2 g L^-1 glucose reaching ~ 0.8 h^-1. It was shown that in wild type cells proton but not potassium fluxes were stimulated ~ 1.7 fold reaching up to 1.95 mmol/min when cells were grown in the presence of 8 g L^-1 glucose. Interestingly, cells grown on peptone only had similar proton/potassium fluxes as grown on 2 g L^-1glucose. H⁺/K⁺ fluxes of the cells grown on 2 g L^-1 but not 8 g L^-1 glucose depend on externally added glucose concentration in the assays. DCCD-sensitive H⁺ fluxes were tripled and K⁺ fluxes doubled in wild type cells grown on 8 g L^-1 glucose compared to 2 g L^-1 when in the assays 2 g L^-1glucose was added. Interestingly, in hyfF mutant when cells were grown on 2 g L^-1glucose and in 2 g L^-1 assays DCCD-sensitive fluxes were not determined compared to wild type while in hyfD mutant it was doubled reaching up to 0.657 mmol/min. In hyf mutants DCCD-sensitive K⁺ fluxes were stimulated in hyfD and hyfF mutants compared to wild type but depend on external glucose concentration. DCCD-sensitive H⁺/K⁺ ratio was equal to ~ 2 except hyfF mutant grown and assayed on 2 g L^-1glucose while in 8 g L^-1 conditions role of hyfB and hyfD is considered. Taken together it can be concluded that Hyd-4 subunits (HyfBDF) play key role in sensing glucose concentration for regulation of DCCD-sensitive H⁺/K⁺ fluxes for maintaining proton motive force generation.