One aspect of the invention relates to a genetically modified thermophilic or mesophilic microorganism, wherein a first native gene is partially, substantially, or completely deleted, silenced, inactivated, or down-regulated, which first native gene encodes a first native enzyme involved in the metabolic production of an organic acid or a salt thereof, thereby increasing the native ability of said thermophilic or mesophilic microorganism to produce ethanol as a fermentation product. In certain embodiments, the aforementioned microorganism further comprises a first non-native gene, which first non- native gene encodes a first non-native enzyme involved in the metabolic production of ethanol.; Another aspect of the invention relates to a process for converting lignocellulosic biomass to ethanol, comprising contacting lignocellulosic biomass with a genetically modified thermophilic or mesophilic microorganism.