Previous studies of anaerobic biocorrosion have suggested that microbial sulfur and phosphorus products as well as cathodic hydrogen consumption may accelerate anaerobic metal oxidation. Methanogenic bacteria, which normally use molecular hydrogen (H(2)) and carbon dioxide (CO(2)) to produce methane (CH(4)) and which are major inhabitants of most anaerobic ecosystems, use either pure elemental iron (Fe(0)) or iron in mild steel as a source of electrons in the reduction of CO(2) to CH(4). These bacteria use Fe(0) oxidation for energy generation and growth. The mechanism of Fe(0) oxidation is cathodic depolarization, in which electrons from Fe(0) and H(+) from water produce H(2), which is then released for use by the methanogens; thermodynamic calculations show that significant Fe(0) oxidation will not occur in the absence of H(2) consumption by the methanogens. The data suggest that methanogens can be significant contributors to the corrosion of iron-containing materials in anaerobic environments.

Samples of human dental plaque were examined for the presence of methanogenic bacteria. Of 54 samples from 36 patients, 20 yielded H2/CO2-using methanogenic enrichment cultures. All methanogen-positive samples were from patients with some degree of periodontal disease. The predominant populations in the enrichments had morphologies characteristic of Methanobrevibacter spp. In six enrichments derived from three patients, the common methanogen was antigenically similar to Methanobrevibacter smithii. The same was true for the three methanogenic isolates obtained in axenic culture from a fourth patient. The six enrichments and two of the three isolates were antigenically closer to strain ALI than to PS. Two of the enrichments also had subpopulations with weak antigenic similarity to Methanosphaera stadtmanae. The data indicate that methanogens in the oral cavity of humans are antigenically close to those found in the intestinal tract. Methanogenic bacteria belong to the group of organisms known as the archaebacteria (2, 9). They are strict anaerobes characterized by the ability to produce methane from H2/CO2 and, in some cases, from formate, acetate, methanol, or methylamine. Methanogenic bacteria have been isolated from sewage sludge, lake sediments, hot springs, and the intestinal tracts of animals, including humans. About one-third of the adult human population in the United States is reported to carry methanogens in the large intestine, with Methanobrevibacter smithii being the predominant methanogen present (5, 6, 18). However, the presence of methanogens in the human oral cavity has not been examined. Recently, the presence of methanogens in dental-plaque samples from monkeys was reported (13). We describe in this report our examination for methanogens in subgingival dental-plaque samples from patients at the

Ornithine transcarbamylase (OTCase) deficiency, the most common inherited urea cycle disorder, is transmitted as an X-linked trait. The clinical phenotype in affected males as well as heterozygous females shows a spectrum of severity ranging from neonatal hyperammonaemic coma to asymptomatic adults. The ornithine transcarbamylase enzyme is a trimer with three active sites per holoenzyme molecule, each of which is composed of an interdomain region of one polypeptide and a polar domain of the adjacent polypeptide. The OTC gene is located on the short arm of the X-chromosome and one of the two alleles undergoes inactivation in female cells. Approximately 140 mutations have been found in families affected with OTCase deficiency, most having their own 'private' mutation. Large deletions of one exon or more are seen in approximately 7% of patients, small deletions or insertions are seen in about 9%, and the remaining mutations are single base substitutions. Approximately 15% of mutations affect RNA splicing sites. The recurrent mutations are distributed equally among CpG dinucleotide hot spots. Generally, mutations causing neonatal disease affect amino acid residues that are 'buried' in the interior of the enzyme, especially around the active site, while those associated with late onset and milder phenotypes tend to be located on the surface of the protein. Very few mutations have been found in the sequence of the leader peptide, proportionally much fewer than in the sequence of the mature enzyme. Only few of the mutations have been expressed in bacteria or mammalian cells for the study of their deleterious mechanisms. Examples of expressed mutations include R277W and R277Q associated with late-onset disease, which markedly increase the Km for ornithine, shift the pH optimum to more alkaline and decrease the thermal stability of the purified mutant enzyme. R141Q (neonatal disease) disrupts the active site, whereas the purified R40H mutant has normal catalytic function and this mutation is likely to affect posttranslational processing such as mitochondrial targeting. It appears that most new mutations occur in male sperm and are then passed on to a transmitting heterozygous female. Uncommonly, mild mutations are transmitted by asymptomatic males to their daughters, subsequently resulting in clinical disease of males in future generations. The causes for variable expressivity of these mutations are currently unknown but are likely to involve a combination of environmental and genetic modifiers.

A variety of sulfur-containing compounds were investigated for use as medium reductants and sulfur sources for growth of four methanogenic bacteria. Sulfide (1 to 2 mM) served all methanogens investigated well. Methanococcus thermolithotrophicus and Methanobacterium thermoautotrophicum Marburg and AH grew well with S, s032-, or thiosulfate as the sole sulfur source. Only Methanococcus thermolithotrophicus was able to grow with s042as the sole sulfur source. 2-Mercaptoethanol at 20 mM was greatly inhibitory to growth of Methanococcus thermolithotrophicus on s042or s032and Methanobacterium thermoautotrophicum Marburg on s032but not to growth of strain AH on s032-. Sulfite was metabolized during growth by Methanococcus thermolithotrophicus. Sulfide was produced in cultures of Methanococcus thermolithotrophicus growing on s042-, s032-, thiosulfate, and So. Methanobacterium thermoautotrophicum Marburg was successfully grown in a 10-liter fermentor with S, s032-, or thiosulfate as the sole sulfur source.

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