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Growth Characteristics of Methanomassiliicoccus luminyensis and expression of Methyltransferase En 时间:2017-11-07 12:03 来源:未知作者:admin 点击: 次 Abstract:DNA sequence analysis of the human gut revealed the presence a seventh order of methanogens referred to as Methanomassiliicoccales. Methanomassiliicoccus luminyensis is the only member of this order that grows in pure culture. Here, we show that the organism has a doubling time of 1.8 d with methanol + H2 and a growth yield of 2.4 g dry weight/mol CH4. M. luminyensis also uses methylamines + H2 (monomethylamine, dimethylamine, and trimethylamine) with doubling times of 2.1–2.3 d. Similar cell yields were obtained with equimolar concentrations of methanol and methylamines with respect to their methyl group contents. The transcript levels of genes encoding proteins involved in substrate utilization indicated increased amounts of mRNA from the mtaBC2 gene cluster in methanol-grown cells. When methylamines were used as substrates, mRNA of the mtb/mtt operon and of the mtmBC1 cluster were found in high abundance. The transcript level of mtaC2 was almost identical in methanol- and methylamine-grown cells, indicating that genes for methanol utilization were constitutively expressed in high amounts. The same observation was made with resting cells where methanol always yielded the highest CH4 production rate independently from the growth substrate. Hence, M. luminyensis is adapted to habitats that provide methanol + H2 as substrates.
1. Introduction
Among living organisms, methane formation is restricted to methanogenic organisms of the domain Archaea. These organisms are important players of the global carbon cycle due to their capability to use fermentation products of organic matter as substrates [1]. While methanogenic archaea have been studied extensively with respect to biogas production and other man-made habitats, little is known about human-associated methanogens [2]. One of these organisms is Methanomassiliicoccus luminyensis, which was originally isolated from human feces [3]. This organism is the only member of the seventh order of methanogens, the Methanomassiliicoccales, that is available in pure culture. DNA sequence analysis indicated that members of the Methanomassiliicoccales are also found in gastrointestinal tracts of higher termites and millipedes, the digestive tracts of animals, rice paddy fields, natural wetlands, subseafloor and freshwater sediments [4, 5] and in anaerobic digester sludge [6]. These findings highlight the wide distribution and diversity of members of the order Methanomassiliicoccales which are phylogenetically related to the Thermoplasmatales but only distantly related to other methanogens [4, 7].
Methanogenic archaea are usually divided into three major groups based on their substrate spectrum. While hydrogenotrophic archaea use H2 + CO2 or formate as substrates, methylotrophic organisms form methane from methylated compounds, and aceticlastic archaea use acetate as substrate [8]. M. luminyensis is distinct from these common groups because it uses H2 as electron donor but is not able to reduce CO2 to CH4 [3]. In the first description of the strain, it was stated that methanogenesis is possible by methanol reduction with H2 as the electron donor [3]. In this respect, M. luminyensis resembled Methanosphaera stadtmanae, another methanogenic archaeon that was isolated from human feces and can use methanol in the presence of H2 [9, 10]. Genome analysis revealed that besides the genes for methanol utilization, members of the genus Methanomassiliicoccus contain complete sets of genes for the degradation of methylamines to CH4 and NH3 [5, 11, 12]. Lang et al. [12] showed that methane accumulated in the culture headspace of M. luminyensis containing methanol, monomethylamine (MMA), dimethylamine (DMA), or trimethylamine (TMA) in the presence of H2. In addition, it was found that M. luminyensis grows on TMA in the presence of H2 but not on TMA in the absence of H2 [13, 14]. Due to the ability to use TMA as substrate, M. luminyensis is also discussed in the context of the so-called “Archaebiotics” [2, 14]. These therapeutic agents were proposed as a treatment for patients suffering from trimethylaminuria. The patients hold a genetic disposition that prevents oxidation of TMA to trimethylamine oxide (TMAO) in the human liver. TMA is a product of degradation processes in the human gut and can be spread throughout the body by being absorbed from the intestinal lumen and transferred into the bloodstream. When the mechanism to oxidize TMA to TMAO is absent, TMA rapidly accumulates and results in a fish-like odor coming from patients’ sweat and breath [15]. The cultivation of M. luminyensis in the intestines of patients and the simultaneous metabolization of TMA could reduce the accumulation of TMA in the intestines and could prevent the fish-like odor [14].
Another aspect of “Archaebiotics” based on M. luminyensis is the general conversion of TMA into TMAO in the human body. TMAO is thought to raise the risk for developing cardiovascular diseases, for example, arteriosclerosis. Therefore, a low TMAO level is generally desirable in healthy individuals. Due to the fact that M. luminyensis produces methane from TMA and TMA oxidation to TMAO does not take place, the occurrence of this archaeon in the human intestinal tract could decrease accumulation of TMAO and consequently the risk for cardiovascular diseases caused by this compound [2].