Heavy metal co-contamination in crude oil-polluted environments may inhibit microbial bioremediation of hydrocarbons. The model heavy metal-resistant bacterium Cupriavidus metallidurans CH34 possesses cadmium and mercury resistance, as well as genes related to the catabolism of hazardous BTEX aromatic hydrocarbons. The aims of this study were to analyze the aromatic catabolic potential of C. metallidurans CH34 and to determine the functionality of the predicted benzene catabolic pathway and the influence of cadmium and mercury on benzene degradation. Three chromosome-encoded bacterial multicomponent monooxygenases (BMMs) are involved in benzene catabolic pathways. Growth assessment, intermediates identification, and gene expression analysis indicate the functionality of the benzene catabolic pathway. Strain CH34 degraded benzene via phenol and 2-hydroxymuconic semialdehyde. Transcriptional analyses revealed a transition from the expression of catechol 2,3-dioxygenase (tomB) in the early exponential phase to catechol 1,2-dioxygenase (catA1 and catA2) in the late exponential phase. The minimum inhibitory concentration to Hg (II) and Cd (II) was significantly lower in the presence of benzene, demonstrating the effect of co-contamination on bacterial growth. Notably, this study showed that C. metallidurans CH34 degraded benzene in the presence of Hg (II) or Cd (II).

Plant growth-promoting bacteria (PGPB) play a role in stimulating plant growth through mechanisms such as the synthesis of the phytohormone indole-3-acetic acid (IAA). The aims of this study were the characterization of IAA synthesis and degradation by the model aromatic-degrading bacterium Paraburkholderia xenovorans LB400, and its growth promotion of the Nicotiana tabacum plant. Strain LB400 was able to synthesize IAA (measured by HPLC) during growth in the presence of tryptophan and at least one additional carbon source; synthesis of anthranilic acid was also observed. RT-PCR analysis indicates that under these conditions, strain LB400 expressed the ipdC gene, which encodes indole-3-pyruvate decarboxylase, suggesting that IAA biosynthesis proceeds through the indole-3-pyruvate pathway. In addition, strain LB400 degraded IAA and grew on IAA as a sole carbon and energy source. Strain LB400 expressed the iacC and catA genes, which encode the α subunit of the aromatic-ring-hydroxylating dioxygenase in the IAA catabolic pathway and the catechol 1,2-dioxygenase, respectively, which may suggest a peripheral IAA pathway leading to the central catechol pathway. Notably, P. xenovorans LB400 promoted the growth of tobacco seedlings, increasing the number and the length of the roots. In conclusion, this study indicates that the versatile bacterium P. xenovorans LB400 is a PGPB.

Benzene, toluene, ethylbenzene and the three xylene isomers are monoaromatic contaminants widely distributed on polluted sites. Some microorganisms have developed mechanisms to degrade these compounds, but their aerobic and anaerobic degradation is inhibited in presence of heavy metals, such as mercury or lead. In this report, the degradation of benzene and other aromatic compounds catalyzed by the metal resistant bacterium Cupriavidus metallidurans CH34 was characterized. A metabolic reconstruction of aromatic catabolic pathways was performed based on bioinformatics analyses. Functionality of the predicted pathways was confirmed by growing strain CH34 on benzene, toluene, o- xylene, p- cymene, 3-hydroxybenzoate, 4-hydroxybenzoate, 3-hydroxyphenylacetate, 4-hydroxyphenylacetate, homogentisate, catechol, naphthalene, and 2-aminophenol as sole carbon and energy sources. Benzene catabolic pathway was further characterized. Results showed that firstly benzene is transformed into phenol and, thereafter, into catechol. Benzene is degraded under aerobic conditions via a combined pathway catalyzed by three Bacterial Multicomponent Monooxygenases: a toluene-2-monoxygenase (TomA012345), a toluene-4-monooxygenase (TmoABCDEF) and a phenol-2-hydroxylase (PhyZABCDE). A catechol-2,3-dioxygenase (TomB) expressed at early exponential phase cleaves the catechol ring in meta -position; an ortho -cleavage of catechol is accomplished by a catechol-1,2-dioxygenase (CatA) at late exponential phase instead. This study additionally shows that C. metallidurans CH34 is capable of degrading benzene in presence of heavy metals, such as Hg(II) or Pb(II). This capability of degrading aromatic compounds in presence of heavy metals is rather unusual among environmental bacteria; therefore, C. metallidurans CH34 seems to be a promising candidate for developing novel bioremediation process for multi-contaminated environments. HIGHLIGHTS: The strain Cupriavidus metallidurans CH34 is capable to degrade benzene aerobically Benzene oxydation is mediated by bacterial multicomponent monoxygenases Strain CH34 is able to grow using a broad range of aromatic compounds as sole carbon and energy source Benzene degradation occurs even in presence of heavy metals such as mercury and lead