Formate-Dependent Acetogenic Utilization of Glucose by the Fecal Acetogen Clostridium bovifaecis.

Acetogenic bacteria are a diverse group of anaerobes that use the reductive acetyl coenzyme A (acetyl-CoA) (Wood-Ljungdahl) pathway for CO2 fixation and energy conservation. The conversion of 2 mol CO2 into acetyl-CoA by using the Wood-Ljungdahl pathway as the terminal electron accepting process is the most prominent metabolic feature for these microorganisms. However, here, we describe that the fecal acetogen Clostridium bovifaecis strain BXX displayed poor metabolic capabilities of autotrophic acetogenesis, and acetogenic utilization of glucose occurred only with the supplementation of formate. Genome analysis of Clostridium bovifaecis revealed that it contains almost the complete genes of the Wood-Ljungdahl pathway but lacks the gene encoding formate dehydrogenase, which catalyzes the reduction of CO2 to formate as the first step of the methyl branch of the Wood-Ljungdahl pathway. The lack of a gene encoding formate dehydrogenase was verified by PCR, reverse transcription-PCR analysis, enzyme activity assay, and its formate-dependent acetogenic utilization of glucose on DNA, RNA, protein, and phenotype level, respectively. The lack of a formate dehydrogenase gene may be associated with the adaption to a formate-rich intestinal environment, considering the isolating source of strain BXX. The formate-dependent acetogenic growth of Clostridium bovifaecis provides insight into a unique metabolic feature of fecal acetogens.IMPORTANCE The acetyl-CoA pathway is an ancient pathway of CO2 fixation, which converts 2 mol of CO2 into acetyl-CoA. Autotrophic growth with H2 and CO2 via the acetyl-CoA pathway as the terminal electron accepting process is the most unique feature of acetogenic bacteria. However, the fecal acetogen Clostridium bovifaecis strain BXX displayed poor metabolic capabilities of autotrophic acetogenesis, and acetogenic utilization of glucose occurred only with the supplementation of formate. The formate-dependent acetogenic growth of Clostridium bovifaecis was associated with its lack of a gene encoding formate dehydrogenase, which may result from adaption to a formate-rich intestinal environment. This study gave insight into a unique metabolic feature of fecal acetogens. Because of the requirement of formate for the acetogenic growth of certain acetogens, the ecological impact of acetogens could be more complex and important in the formate-rich environment due to their trophic interactions with other microbes.

Mutants of Escherichia coli specifically deficient in respiratory formate dehydrogenase activity.

Escherichia coli K12 mutants lacking phenazine-methosulphate-linked formate dehydrogenase (FDH-PMS) activity, but still capable of producing normal levels of benzyl-viologen-linked formate dehydrogenase (FDH-BV) and nitrate reductase activities, have been isolated following P1 localized mutagenesis. The relevant mutations mapped with the same cotransduction frequency close to the rhaD gene, at 88 min on the E. coli chromosome. They were further subdivided into two classes. Class I consisted of six fdhD mutants which synthesized an inactive FDH-PMS protein with the same subunit composition as the wild-type enzyme. In contrast, class II contained four fdhE mutants totally devoid of this antigen. Construction of merodiploid strains harbouring various combinations of the mutated alleles, fdhE on the episome and fdhD on the chromosome, led to the restoration of FDH-PMS activity by complementation of the products encoded by the respective wild-type alleles. Difference spectroscopy suggested that both fdhD and fdhE mutants contained normal amounts of the cytochrome b559 associated with FDH-PMS although the cytochrome had lost its capacity for formate-dependent reduction.