Klaus Michalke

Production of Volatile Derivatives of Metal(loid)s by Microflora Involved in Anaerobic Digestion of Sewage Sludge

ABSTRACT Gases released from anaerobic wastewater treatment facilities contain considerable amounts of volatile methyl and hydride derivatives of metals and metalloids, such as arsine (AsH3), monomethylarsine, dimethylarsine, trimethylarsine, trimethylbismuth (TMBi), elemental mercury (Hg0), trimethylstibine, dimethyltellurium, and tetramethyltin. Most of these compounds could be shown to be produced by pure cultures of microorganisms which are representatives of the anaerobic sewage sludge microflora, i.e., methanogenic archaea (Methanobacterium formicicum,Methanosarcina barkeri, Methanobacterium thermoautotrophicum), sulfate-reducing bacteria (Desulfovibrio vulgaris, D. gigas), and a peptolytic bacterium (Clostridium collagenovorans). Additionally, dimethylselenium and dimethyldiselenium could be detected in the headspace of most of the pure cultures. This is the first report of the production of TMBi, stibine, monomethylstibine, and dimethylstibine by a pure culture of M. formicicum.

Reconstruction of the Central Carbohydrate Metabolism of Thermoproteus tenax by Use of Genomic and Biochemical Data

The hyperthermophilic, facultatively heterotrophic crenarchaeum Thermoproteus tenax was analyzed using a low-coverage shotgun-sequencing approach. A total of 1.81 Mbp (representing 98.5% of the total genome), with an average gap size of 100 bp and 5.3-fold coverage, are reported, giving insights into the genome of T. tenax. Genome analysis and biochemical studies enabled us to reconstruct its central carbohydrate metabolism. T. tenax uses a variant of the reversible Embden-Meyerhof-Parnas (EMP) pathway and two different variants of the Entner-Doudoroff (ED) pathway (a nonphosphorylative variant and a semiphosphorylative variant) for carbohydrate catabolism. For the EMP pathway some new, unexpected enzymes were identified. The semiphosphorylative ED pathway, hitherto supposed to be active only in halophiles, is found in T. tenax. No evidence for a functional pentose phosphate pathway, which is essential for the generation of pentoses and NADPH for anabolic purposes in bacteria and eucarya, is found in T. tenax. Most genes involved in the reversible citric acid cycle were identified, suggesting the presence of a functional oxidative cycle under heterotrophic growth conditions and a reductive cycle for CO2 fixation under autotrophic growth conditions. Almost all genes necessary for glycogen and trehalose metabolism were identified in the T. tenax genome.

Volatilisation of metals and metalloids: an inherent feature of methanoarchaea?

As shown by recent studies, anaerobic members of Archaea and Bacteria are involved in processes that transform ionic species of metals and metalloids (arsenic, antimony, bismuth, selenium, tellurium and mercury) into volatile and mostly toxic derivatives (mainly methyl derivatives or hydrides). Since the fact that these transformations proceed in both environmental settings and in parts of the human body, we have to consider that these processes also interfere directly with human health. The diversity of the volatile derivatives produced and their emission rates were significantly higher in methanoarchaeal than in bacterial strains, which supports the pivotal role of methanoarchaea in transforming metals and metalloids (metal(loid)s) into their volatile derivatives. Compared with methanoarchaea, 14 anaerobic bacterial strains showed a significantly restricted spectrum of volatilised derivatives and mostly lower production rates of volatile bismuth and selenium derivatives. Since methanoarchaea isolated from the human gut (Methanosphaera stadtmanae, Methanobrevibacter smithii) showed a higher potential for metal(loid) derivatisation compared to bacterial gut isolates, we assume that methanoarchaea in the human gut are mainly responsible for the production of these volatile derivatives. The observation that trimethylbismuth ((CH(3))(3)Bi), the main volatile derivative of bismuth produced in human feces, inhibited growing cultures of Bacteroides thetaiotaomicron, a representative member of the human physiological gut flora, suggests that these volatiles exert their toxic effects on human health not only by direct interaction with host cells but also by disturbing the physiological gut microflora.

Role of intestinal microbiota in transformation of bismuth and other metals and metalloids into volatile methyl and hydride derivatives in humans and mice.

ABSTRACT The present study shows that feces samples of 14 human volunteers and isolated gut segments of mice (small intestine, cecum, and large intestine) are able to transform metals and metalloids into volatile derivatives ex situ during anaerobic incubation at 37°C and neutral pH. Human feces and the gut of mice exhibit highly productive mechanisms for the formation of the toxic volatile derivative trimethylbismuth [(CH3)3Bi] at rather low concentrations of bismuth (0.2 to 1 μmol kg−1 [dry weight]). An increase of bismuth up to 2 to 14 mmol kg−1 (dry weight) upon a single (human volunteers) or continuous (mouse study) administration of colloidal bismuth subcitrate resulted in an average increase of the derivatization rate from approximately 4 pmol h−1 kg−1 (dry weight) to 2,100 pmol h−1 kg−1 (dry weight) in human feces samples and from approximately 5 pmol h−1 kg−1 (dry weight) to 120 pmol h−1 kg−1 (dry weight) in mouse gut samples, respectively. The upshift of the bismuth content also led to an increase of derivatives of other elements (such as arsenic, antimony, and lead in human feces or tellurium and lead in the murine large intestine). The assumption that the gut microbiota plays a dominant role for these transformation processes, as indicated by the production of volatile derivatives of various elements in feces samples, is supported by the observation that the gut segments of germfree mice are unable to transform administered bismuth to (CH3)3Bi.

Sulfophobococcus zilligii gen. nov., spec. nov. a Novel Hyperthermophilic Archaeum Isolated from Hot Alkaline Springs of Iceland

Summary The phenotypic and genotypic properties of a new coccoid hyperthermophilic archaeum (3–5 μm in diameter) which has been isolated from a hot alkaline spring near Hveragerdi, Iceland, are described. The new strain is a strict anaerobe and grows fermentatively on yeast extract. Growth is inhibited by elemental sulfur. Under laboratory conditions, growth is observed between 70 and 95 °C at pH 6.5 to 8.5. Its membrane lipids consist exclusively of dibiphytanyldiglycerol tetraethers. Respiratory lipoquinones are absent. The G+C content of the DNA has been determined to be 54.7 ± 0.8 mol%. On the basis of its 16S rDNA sequence the strain is affiliated to the crenarchaeotal branch of Archaea. The next relatives are Desulfurococcus mobilis (4.5% dissimilarity) and Pyrodictium occultum (7.0% dissimilarity). Due to the phenotypic and genotypic properties of this new isolate which distinguish it from other currently known Archaea, we propose to placing it in a new genus, Sulfophobococcus with the first species S. zilligii in honour of Wolfram Zillig , a pioneer in the field of hyperthermophilic Archaea.

Determination of Trimethylbismuth in the Human Body after Ingestion of Colloidal Bismuth Subcitrate

Biological methylation and hydride formation of metals and metalloids are ubiquitous environmental processes that can lead to the formation of chemical species with significantly increased mobility and toxicity. Whereas much is known about the interaction of metal(loid)s with microorganisms in environmental settings, little information has been gathered on respective processes inside the human body as yet. Here, we studied the biotransformation and excretion of bismuth after ingestion of colloidal bismuth subcitrate (215 mg of bismuth) to 20 male human volunteers. Bismuth absorption in the stomach and upper intestine was very low, as evidenced by the small quantity of bismuth eliminated via the renal route. Total bismuth concentrations in blood increased rapidly in the first hour after ingestion. Most of the ingested bismuth was excreted via feces during the study period. Trace levels of the metabolite trimethylbismuth [(CH3)3Bi] were detected via low temperaturegas chromatography/inductively coupled plasma-mass spectrometry in blood samples and in exhaled air samples. Concentrations were in the range of up to 2.50 pg/ml (blood) and 0.8 to 458 ng/m3 (exhaled air), with high interindividual variation being observed. Elimination routes of bismuth were exhaled air (up to 0.03‰), urine (0.03–1.2%), and feces. The site of (CH3)3Bi production could not be identified in the present study, but the intestinal microflora seems to be involved in this biotransformation if accompanying ex vivo studies are taken into consideration.

Organometal(loid) compounds associated with human metabolism

Biomethylation of metals and metalloids is a well-known process ubiquitously occurring in the environment, which leads to the formation of chemical species with significantly higher mobility and altered toxicology. There are only a few historical reports, e.g. about “bismuth breath” or “Gosio gas” dealing with the association of humans with methylated metal(loid)s. Although the toxicity of the latter [later identified as trimethyl arsine (Challenger 1945)] has not been conclusively demonstrated, this gas produced by fungi in wet wallpaper was considered to be the reason for the illness of people living there (Gosio 1897). Amongst other observations, dimethyltellurium in “bismuth breath” of mine workers, dimethylselenium in the upper ng/m3 range in human breath, as well as the detection of at least twenty-two different organometal(loid) species in human urine are indications for the methylation of metal(loid)s occurring in humans (Cai et al. 1995; Feldmann et al. 1996; Kresimon et al. 2001).

Effect of octamethylcyclotetrasiloxane on methylation of bismuth by Methanosarcina barkeri

ABSTRACT Octamethylcyclotetrasiloxane (OMCTS), a common constituent of household products, triggers the transformation of bismuth to the volatile toxic derivative trimethylbismuth by Methanosarcina barkeri, which is a representative member of the sewage sludge microflora. Comparative studies with the ionophores monensin and lasalocid, which induce effects similar to those observed for OMCTS, indicated that the stimulation of bismuth methylation is not specific for the siloxane and suggested that the stimulation observed is mainly due to facilitated membrane permeation of the metal ion.

Fragwürdige Produkte unserer Darm-Mikrobiota

Numerous members of our physiological gut microbiota support our health by enhancing food digestion, regulating barrier function, and inducing immunity. However, methanoarchaea, which are also common inhabitants of our gut, transform trace elements, introduced into our gut via food and pharmaceuticals, into toxic methylmetal(loid) derivatives. Here, we give an overview of our knowledge of metal(loid) methylation by methanoarchaea and how this phenomenon may compromise our health.