We used a combination of genomic, transcriptional and enzymatic analyses to determine the mechanism of interspecies electron transfer by two model syntrophic microorganisms, Syntrophomonas wolfei and Syntrophus aciditrophicus. Both organisms contain multiple hydrogenase and formate dehydrogenase genes, but lack genes for outer membrane cytochromes and nanowire formation. Syntrophically grown cells and cell-free extracts of S. aciditrophicus and S. wolfei had both hydrogenase and formate dehydrogenase activities. Butyrate metabolism and CH4 production by washed cell suspensions of S. wolfei and Methanospirillum hungatei were inhibited by hydrogenase inhibitors (cyanide and carbon monoxide), but not by a formate dehydrogenase inhibitor (hypophosphite). Syntrophic benzoate oxidation and CH4 production by washed cell suspensions of S. aciditrophicus and M. hungatei were inhibited by hypophosphite, but not cyanide and carbon monoxide. All three inhibitors halted syntrophic cyclohexane-1-carboxylate metabolism. Two hydrogenase genes, hydA1 and hydA2, were more highly expressed when S. wolfei was grown syntrophically. S. aciditrophicus expressed multiple hydrogenase and formate dehydrogenase genes during syntrophic benzoate and cyclohexane-1-carboxylate growth, one of which (fdhA2) was highly differentially expressed during syntrophic benzoate growth. Thus, these syntrophic microorganisms have flexible metabolisms that allow them to use either H2 or formate transfer depending on the substrate involved.

The cholesterol-dependent cytolysins (CDCs) are pore-forming toxins that have been exclusively associated with a wide variety of bacterial pathogens and opportunistic pathogens from the Firmicutes and Actinobacteria, which exhibit a Gram-positive type of cell structure. We have characterized the first CDCs from Gram-negative bacterial species, which include Desulfobulbus propionicus type species Widdel 1981 (DSM 2032) (desulfolysin [DLY]) and Enterobacter lignolyticus (formerly Enterobacter cloacae) SCF1 (enterolysin [ELY]). The DLY and ELY primary structures show that they maintain the signature motifs of the CDCs but lack an obvious secretion signal. Recombinant, purified DLY (rDLY) and ELY (rELY) exhibited cholesterol-dependent binding and cytolytic activity and formed the typical large CDC membrane oligomeric pore complex. Unlike the CDCs from Grampositive species, which are human-and animal-opportunistic pathogens, neither D. propionicus nor E. lignolyticus is known to be a pathogen or commensal of humans or animals: the habitats of both organisms appear to be restricted to anaerobic soils and/or sediments. These studies reveal for the first time that the genes for functional CDCs are present in bacterial species that exhibit a Gram-negative cell structure. These are also the first bacterial species containing a CDC gene that are not known to inhabit or cause disease in humans or animals, which suggests a role of these CDCs in the defense against eukaryote bacterial predators.

Syntrophomonas wolfei syntrophically oxidizes short-chain fatty acids (four to eight carbons in length) when grown in coculture with a hydrogen- and/or formate-using methanogen. The oxidation of 3-hydroxybutyryl-coenzyme A (CoA), formed during butyrate metabolism, results in the production of NADH. The enzyme systems involved in NADH reoxidation in S. wolfei are not well understood. The genome of S. wolfei contains a multimeric [FeFe]-hydrogenase that may be a mechanism for NADH reoxidation. The S. wolfei genes for the multimeric [FeFe]-hydrogenase (hyd1ABC; SWOL_RS05165, SWOL_RS05170, SWOL_RS05175) and [FeFe]-hydrogenase maturation proteins (SWOL_RS05180, SWOL_RS05190, SWOL_RS01625) were coexpressed in Escherichia coli, and the recombinant Hyd1ABC was purified and characterized. The purified recombinant Hyd1ABC was a heterotrimer with an αβγ configuration and a molecular mass of 115 kDa. Hyd1ABC contained 29.2 ± 1.49 mol of Fe and 0.7 mol of flavin mononucleotide (FMN) per mole enzyme. The purified, recombinant Hyd1ABC reduced NAD+ and oxidized NADH without the presence of ferredoxin. The HydB subunit of the S. wolfei multimeric [FeFe]-hydrogenase lacks two iron-sulfur centers that are present in known confurcating NADH- and ferredoxin-dependent [FeFe]-hydrogenases. Hyd1ABC is a NADH-dependent hydrogenase that produces hydrogen from NADH without the need of reduced ferredoxin, which differs from confurcating [FeFe]-hydrogenases. Hyd1ABC provides a mechanism by which S. wolfei can reoxidize NADH produced during syntrophic butyrate oxidation when low hydrogen partial pressures are maintained by a hydrogen-consuming microorganism.IMPORTANCE Our work provides mechanistic understanding of the obligate metabolic coupling that occurs between hydrogen-producing fatty and aromatic acid-degrading microorganisms and their hydrogen-consuming partners in the process called syntrophy (feeding together). The multimeric [FeFe]-hydrogenase used NADH without the involvement of reduced ferredoxin. The multimeric [FeFe]-hydrogenase would produce hydrogen from NADH only when hydrogen concentrations were low. Hydrogen production from NADH by Syntrophomonas wolfei would likely cease before any detectable amount of cell growth occurred. Thus, continual hydrogen production requires the presence of a hydrogen-consuming partner to keep hydrogen concentrations low and explains, in part, the obligate requirement that S. wolfei has for a hydrogen-consuming partner organism during growth on butyrate. We have successfully expressed genes encoding a multimeric [FeFe]-hydrogenase in E. coli, demonstrating that such an approach can be advantageous to characterize complex redox proteins from difficult-to-culture microorganisms.

The microbial metabolism of hydrocarbons is increasingly associated with the corrosion of carbon steel in sulfate-rich marine waters. However, how such transformations influence metal biocorrosion in the absence of an electron acceptor is not fully recognized. We grew a marine alkane-utilizing, sulfate-reducing bacterium, Desulfoglaeba alkanexedens, with either sulfate or Methanospirillum hungatei as electron acceptors, and tested the ability of the cultures to catalyze metal corrosion. Axenically, D. alkanexedens had a higher instantaneous corrosion rate and produced more pits in carbon steel coupons than when the same organism was grown in syntrophic co-culture with the methanogen. Since anaerobic hydrocarbon biodegradation pathways converge on fatty acid intermediates, the corrosive ability of a known fatty acid-oxidizing syntrophic bacterium, Syntrophus aciditrophicus was compared when grown in pure culture or in co-culture with a H2-utilizing sulfate-reducing bacterium (Desulfovibrio sp., strain G11) or a methanogen (M. hungatei). The instantaneous corrosion rates in the cultures were not substantially different, but the syntrophic, sulfate-reducing co-culture produced more pits in coupons than other combinations of microorganisms. Lactate-grown cultures of strain G11 had higher instantaneous corrosion rates and coupon pitting compared to the same organism cultured with hydrogen as an electron donor. Thus, if sulfate is available as an electron acceptor, the same microbial assemblages produce sulfide and low molecular weight organic acids that exacerbated biocorrosion. Despite these trends, a surprisingly high degree of variation was encountered with the corrosion assessments. Differences in biomass, initial substrate concentration, rates of microbial activity or the degree of end product formation did not account for the variations. We are forced to ascribe such differences to the metallurgical properties of the coupons.

Glutamate is usually synthesized from acetyl coenzyme A (acetyl-CoA) via citrate, isocitrate, and 2-oxoglutarate. Genome analysis revealed that in Syntrophus aciditrophicus, the gene for Si-citrate synthase is lacking.