Effect of organic acids on the growth and fermentation of ethanologenic Escherichia coli LY01

Abstract

Hemicellulose residues can be hydrolyzed into a sugar syrup using dilute mineral acids. Although this syrup represents a potential feedstock for biofuel production, toxic compounds generated during hydrolysis limit microbial metabolism. Escherichia coli LY01, an ethanologenic biocatalyst engineered to ferment the mixed sugars in hemicellulose syrups, has been tested for resistance to selected organic acids that are present in hemicellulose hydrolysates. Compounds tested include aromatic acids derived from lignin (ferulic, gallic, 4‐hydroxybenzoic, syringic, and vanillic acids), acetic acid from the hydrolysis of acetylxylan, and others derived from sugar destruction (furoic, formic, levulinic, and caproic acids). Toxicity was related to hydrophobicity. Combinations of acids were roughly additive as inhibitors of cell growth. When tested at concentrations that inhibited growth by 80%, none appeared to strongly inhibit glycolysis and energy generation, or to disrupt membrane integrity. Toxicity was not markedly affected by inoculum size or incubation temperature. The toxicity of all acids except gallic acid was reduced by an increase in initial pH (from pH 6.0 to pH 7.0 to pH 8.0). Together, these results are consistent with the hypothesis that both aliphatic and mononuclear organic acids inhibit growth and ethanol production in LY01 by collapsing ion gradients and increasing internal anion concentrations. © 1999 John Wiley & Sons, Inc. Biotechnol Bioeng 66: 203–210, 1999.

Engineering the metabolism of Escherichia coli W3110 for the conversion of sugar to redox-neutral and oxidized products: Homoacetate production

“…Concentrations as low as 50 mM acetate have been shown to induce a stress response in E. coli (44). The minimal inhibitory concentration for growth has been previously reported as 300-400 mM acetate at neutral pH (45,46). Oxygen transfer often becomes limiting during aerobic bioconversion processes, promoting the accumulation of reduced products (31,32).…”

Screening of lactic acid bacteria for their potential as microbial cell factories for bioconversion of lignocellulosic feedstocks

“…The method can be chosen so that it is optimal for a given plant substrate; however, it is always a matter of a compromise between the inhibitors and the amount of released sugars available for the microorganisms. Usually, a mixture of different inhibitors is formed during pretreatment, and since some of them were previously shown to have additive or synergistic effects [[19],[22],[23]], we chose to investigate strain performance on a mixture of different inhibitors. MRS medium with combination of inhibitors was used as representatives of three different types of lignocellulosic biomass and was used to test for any additive or synergistic effects between different inhibitors and to simulate the strains’ performance on real-life feedstocks.…”

Physico-Chemical Alternatives in Lignocellulosic Materials in Relation to the Kind of Component for Fermenting Purposes

“…Furan inhibitors are considered particularly undesirable due to their relative abundance and toxic effect [41,68,69,70]. The inhibitor and toxic effects appear to be caused by the aldehyde functional group rather than the furan ring [59].…”

“…Many references point out the synergistic effect of different inhibitors in lignocellulosic fermentation [22,41,44,45,47,48,49,94]. Furan inhibitors in combination with acids, especially acetic acid, have been demonstrated to have synergistic effects [44,47,48,49].…”

“…The type of lignocellulosic material (grasses, hardwoods, softwoods, etc. ), the cell wall composition, and the severity of the thermochemical conditions employed for hydrolysis (defined as the combination of time, high temperature, and low pH used) mostly determine the nature of the inhibitors and the concentration can vary greatly [8,22,35,36,37,38,39,40,41,42,43]. Moreover, individual inhibitors may not have a strong effect on fermenting microorganisms, but combinations of them can drastically hamper fermentation reactions [41,44,45,46,47,48,49].…”