Acetogens utilize the acetyl-CoA Wood-Ljungdahl pathway as a terminal electron-accepting, energy-conserving, CO(2)-fixing process. The decades of research to resolve the enzymology of this pathway (1) preceded studies demonstrating that acetogens not only harbor a novel CO(2)-fixing pathway, but are also ecologically important, and (2) overshadowed the novel microbiological discoveries of acetogens and acetogenesis. The first acetogen to be isolated, Clostridium aceticum, was reported by Klaas Tammo Wieringa in 1936, but was subsequently lost. The second acetogen to be isolated, Clostridium thermoaceticum, was isolated by Francis Ephraim Fontaine and co-workers in 1942. C. thermoaceticum became the most extensively studied acetogen and was used to resolve the enzymology of the acetyl-CoA pathway in the laboratories of Harland Goff Wood and Lars Gerhard Ljungdahl. Although acetogenesis initially intrigued few scientists, this novel process fostered several scientific milestones, including the first (14)C-tracer studies in biology and the discovery that tungsten is a biologically active metal. The acetyl-CoA pathway is now recognized as a fundamental component of the global carbon cycle and essential to the metabolic potentials of many different prokaryotes. The acetyl-CoA pathway and variants thereof appear to be important to primary production in certain habitats and may have been the first autotrophic process on earth and important to the evolution of life. The purpose of this article is to (1) pay tribute to those who discovered acetogens and acetogenesis, and to those who resolved the acetyl-CoA pathway, and (2) highlight the ecology and physiology of acetogens within the framework of their scientific roots.

Strains of Clostidum thermoaceticum were tested for H2-and CO-dependent growth in a defined medium containing metals, minerals, vitamins, cysteine-sulfide, C02-bicarbonate, and H2 or CO. Ten of the thirteen strains tested grew at the expense of H2 and CO, and C. thermoaceticum ATCC 39073 was chosen for further study. The doubling times for H2-and CO-dependent growth under chemolithotrophic conditiods (the defined medium with nicotinic acid as sole essential vitamin and sulfide as sole reducer) were 25 and 10 h, respectively.Product stoichiometries for chemolithotrophic cultures approximated: 4.1H2 + 2.4CO2-+CH3COOH + 0.1 cell C + 0.3 unrecovered C and 6.8CO-*CH3COOH + 3.5CO2 + 0.4 cell C + 0.9 unrecovered C. H2-dependent growth produced signifiantly higher acetate concentrations per unit of biomass synthesized than did CO-or glucose-dependent growth. In dated the minimal nutritional requirements of this acetogen (37) so that a definitive assessment could be made of its heterotrophic and chemolithotrophic potentials. In the study presented here, numerous strains of C. thermoaceticum were obtained from various sources and evaluated. In addition, Acetogenium kivui (33, 34), a thermophilic nonclostridial acetogen which is capable of H2-dependent chemolithotrophic growth, was also included in this evaluation and used for comparative purposes. In this report, we demonstrate for the first time that certain strains of C. thermoaceticum grow chemolithotrophically (requiring only trace levels of nicotinic acid as the sole vitamin) at the expense of H2 or CO. Besides the overall metabolic properties exhibited by C. thermoaceticum and A. kivui under chemolithotrophic conditions, evidence is also presented which suggests that the type of energy source used during growth (e.g., H2 versus glucose) influences the expression or activity of hydrogenase and CO dehydrogenase in both of these acetogens. MATERIALS AND METHODSBacterial strains and cultivation. C. thermoaceticum (see Table 1 for strains used in this study) and A.

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Bile acids are important cholesterol-derived nutrient signaling hormones, synthesized in the liver, that act as detergents to solubilize dietary lipids. Bile acid 7α-dehydroxylating gut bacteria generate the toxic bile acids deoxycholic acid and lithocholic acid from host bile acids. The ability of these bacteria to remove the 7-hydroxyl group is partially dependent on 7α-hydroxysteroid dehydrogenase (HSDH) activity, which reduces 7-oxo-bile acids generated by other gut bacteria. 3α-HSDH has an important enzymatic activity in the bile acid 7α-dehydroxylation pathway. 12α-HSDH activity has been reported for the low-activity bile acid 7α-dehydroxylating bacterium ; however, this activity has not been reported for high-activity bile acid 7α-dehydroxylating bacteria, such as, , and Here, we demonstrate that these strains express bile acid 12α-HSDH. The recombinant enzymes were characterized from each species and shown to preferentially reduce 12-oxolithocholic acid to deoxycholic acid, with low activity against 12-oxochenodeoxycholic acid and reduced activity when bile acids were conjugated to taurine or glycine. Phylogenetic analysis suggests that 12α-HSDH is widespread among , in the family, and human gut 12α-HSDH activity has been established in the medically important bile acid 7α-dehydroxylating bacteria ,, and Experiments with recombinant 12α-HSDHs from these strains are consistent with culture-based experiments that show a robust preference for 12-oxolithocholic acid over 12-oxochenodeoxycholic acid. Phylogenetic analysis identified novel members of the gut microbiome encoding 12α-HSDH. Future reengineering of 12α-HSDH enzymes to preferentially oxidize cholic acid may provide a means to industrially produce the therapeutic bile acid ursodeoxycholic acid. In addition, a cholic acid-specific 12α-HSDH expressed in the gut may be useful for the reduction in deoxycholic acid concentration, a bile acid implicated in cancers of the gastrointestinal (GI) tract.

In the human gut, Clostridium scindens ATCC 35704 is a predominant bacterium and one of the major bile acid 7␣-dehydroxylating anaerobes. While this organism is well-studied relative to bile acid metabolism, little is known about the basic nutrition and physiology of C. scindens ATCC 35704. To determine the amino acid and vitamin requirements of C. scindens, the leave-one-out (one amino acid group or vitamin) technique was used to eliminate the nonessential amino acids and vitamins. With this approach, the amino acid tryptophan and three vitamins (riboflavin, pantothenate, and pyridoxal) were found to be required for the growth of C. scindens. In the newly developed defined medium, C. scindens fermented glucose mainly to ethanol, acetate, formate, and H 2. The genome of C. scindens ATCC 35704 was completed through PacBio sequencing. Pathway analysis of the genome sequence coupled with transcriptome sequencing (RNA-Seq) under defined culture conditions revealed consistency with the growth requirements and end products of glucose metabolism. Induction with bile acids revealed complex and differential responses to cholic acid and deoxycholic acid, including the expression of potentially novel bile acid-inducible genes involved in cholic acid metabolism. Responses to toxic deoxycholic acid included expression of genes predicted to be involved in DNA repair, oxidative stress, cell wall maintenance/metabolism, chaperone synthesis, and downregulation of one-third of the genome. These analyses provide valuable insight into the overall biology of C. scindens which may be important in treatment of disease associated with increased colonic secondary bile acids.