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International audience; Microbial transformation of arsenic species and their interaction with the carbon cycle play a major role in the mobility of this toxic metalloid in the environment. The influence of simple or complex organic substrates on arsenic bio-oxidation was studied using two bacterial strains: one – the arsenivorans strain of Thiomonas delicata – is able to use AsIII as sole energy source; the other, Herminiimonas arsenicoxydans, is not. Experiments were performed at two AsIII concentrations (75 and 2 mg/L). At 75 mg/L As, for both strains, expression of aioA gene decreased when yeast extract concentration was raised from 0.2 to 1 g/L. At 2 mg/L As, the presence of either yeast extract or simple (succinate or acetate) organic substrates in the medium during bacterial growth decreased the AsIII-oxidation rate by both strains. When added specifically during oxidation test, yeast extract but not simple organic substrates seems to have a negative effect on AsIII oxidation. Taken together, results confirm the negative influence of simple or complex organic substrates on the kinetics of microbial AsIII oxidation and suggest that this effect results from different mechanisms depending on the type of organic substrate. Further, for the first time, the influence of a complex organic substrate, yeast extract, on aioA gene expression has been evidenced.

Several studies have suggested the existence of a close relationship between antibiotic-resistant phenotypes and resistance to other toxic compounds such as heavy metals, which involve co-resistance or cross-resistance mechanisms. A metagenomic library was previously constructed in Escherichia coli with DNA extracted from the bacterial community inhabiting an acid mine drainage (AMD) site highly contaminated with heavy metals. Here, we conducted a search for genes involved in antibiotic resistance using this previously constructed library. In particular, resistance to antibiotics was observed among five clones carrying four different loci originating from CARN5 and CARN2, two genomes reconstructed from the metagenomic data. Among the three CARN2 loci, two carry genes homologous to those previously proposed to be involved in antibiotic resistance. The third CARN2 locus carries a gene encoding a membrane transporter with an unknown function and was found to confer bacterial resistance to rifampicin, gentamycin, and kanamycin. The genome of Thiomonas delicata DSM 16361 and Thiomonas sp. X19 were sequenced in this study. Homologs of genes carried on these three CARN2 loci were found in these genomes, two of these loci were found in genomic islands. Together, these findings confirm that AMD environments contaminated with several toxic metals also constitute habitats for bacteria that function as reservoirs for antibiotic resistance genes.

The published sequence of the 16S rRNA gene of Thiomonas cuprina strain Hö5 (=DSM 5495T) (GenBank accession no. U67162) was found to be erroneous. The 16S rRNA genes from the type strain held by the DSMZ since 1990 (DSM 5495T =NBRC 102145T) and strain Hö5 maintained frozen in the Universität Regensburg for 23 years (=NBRC 102094) were sequenced and found to be identical, but to show no significant similarity to the U67162 sequence. This also casts some doubt on the previously published 5S and 23S rRNA gene sequences (GenBank accession nos U67171 and X75567). The correct 16S rRNA gene sequence showed 99.8 % identity to those from Thiomonas delicata NBRC 14566T and ‘Thiomonas arsenivorans’ DSM 16361. The properties of these three species are re-evaluated, and emended descriptions are provided for the genus Thiomonas and the species Thiomonas cuprina.

The transfer of Thiobacillus delicatus to the genus Thiomonas as a distinct species, Thiomonas delicata (type strain NBRC 14566T), is confirmed by its morphological and physiological properties, DNA–DNA hybridization and the grouping of its 16S rRNA gene sequence with those of other species of the genus. An emended formal description of Thiomonas delicata is given. The status of Thiomonas cuprina DSM 5495T as a member of the genus is reconsidered.

The three As(III)-oxidizing members of the class Betaproteobacteria Thiomonas delicata, Thiomonas cuprina and ‘Thiomonas arsenivorans’ were isolated from mining sites in geographically distinct areas, namely Japan, Germany and France, respectively. They are all able to oxidize As(III) but only ‘T. arsenivorans’ and T. cuprina show efficient autotrophic growth with As(III) and are able to grow on a sole carbon source. These two organisms are also motile, whereas T. delicata is not. Only T. cuprina can grow autotrophically on chalcopyrite. The three strains share >99 % gene sequence similarity with each other based on their 16S rRNA genes and 16S–23S ITS regions. DNA–DNA hybridization results are above, or close to, the threshold value of 70 % recommended for the definition of bacterial species. The three taxa show very similar fatty acid profiles with differences only in five minor fatty acid components. They possess phylogenetic and chemotaxonomic similarities supporting the reclassification of these taxa as a single species. We propose that ‘T. arsenivorans’ and T. cuprina be reassigned as strains of T. delicata (type strain DSM 17897T).