Sulfur (S), nitrogen (N) and oxygen (O) heterocycles areamong the most potent environmental pollutants.Microbial degradation of these pollutants is attractingmore and more attention because such bioprocesses areenvironmentally friendly. The biotechnological potentialof these processes is being investigated, for example, toachieve better sulfur removal by immobilized biocatalystswith magnetite nanoparticles or by solvent-tolerantbacteria, and to obtain valuable intermediates fromthese heterocycles. Other recent advances have demonstratedthe mechanisms of angular dioxygenation ofnitrogen heterocycles by microbes. However, these technologiesare not yet available for large-scale applicationsso future research must investigate proper modificationsfor industrial applications of these processes. Thisreview focuses on recent progress in understanding howmicrobes degrade S, N and O heterocycles.

A new technology for 6-hydroxy-3-succinoyl-pyridine(HSP) production from (S)-nicotine in tobacco waste bywhole cells of a Pseudomonas sp. has been developed.Whendeionized water was used in the transformation reactionas a medium and the initial pH value of reaction mixturewas adjusted to 7.0, 1.45 g/L HSP was produced from 3 g/Lof nicotine in 5 h with 3.4 g/L of cells in a 5-L flask at 30°C. HSP could be easily purified from the reaction withoutperplexing separation steps. A quantity of 1.3 g of HSPwas recovered without impurity, and the overall yield of HSPwas 43.8% (w/w), based on an initial concentration of3.0 g/L of nicotine in reaction. This biotransformation madeit possible to convert nicotine in tobacco wastes withhigh nicotine content into valuable compounds.

A carbazole-utilizing bacterium was isolated by enrichment from petroleum-contaminated soil. The isolate,designated Sphingomonas sp. strain XLDN2-5, could utilize carbazole (CA) as the sole source of carbon,nitrogen, and energy. Washed cells of strain XLDN2-5 were shown to be capable of degrading dibenzofuran(DBF) and dibenzothiophene (DBT). Examination of metabolites suggested that XLDN2-5 degraded DBF to2-hydroxy-6-(2-hydroxyphenyl)-6-oxo-2,4-hexadienic acid and subsequently to salicylic acid through the angulardioxygenation pathway. In contrast to DBF, strain XLDN2-5 could transform DBT through the ringcleavage and sulfoxidation pathways. Sphingomonas sp. strain XLDN2-5 could cometabolically degrade DBFand DBT in the growing system using CA as a substrate. After 40 h of incubation, 90% of DBT was transformed,and CA and DBF were completely removed. These results suggested that strain XLDN2-5 might be useful inthe bioremediation of environments contaminated by these compounds.

Previous research suggested that Pseudomonas spp. may attack the pyrrolidine ring of nicotine in a waysimilar to mammalian metabolism, resulting in the formation of pseudooxynicotine, the direct precursor of apotent tobacco-specific lung carcinogen. In addition, the subsequent intermediates, 6-hydroxy-3-succinoylpyridine(HSP) and 2,5-dihydroxypyridine (DHP) in the Pseudomonas nicotine degradation pathway are twoimportant precursors for drug syntheses. However, there is little information on the molecular mechanism fornicotine degradation via the pyrrolidine pathway until now. In this study we cloned and sequenced a 4,879-bpgene cluster involved in nicotine degradation. Intermediates N-methylmyosmine, pseudooxynicotine, 3-succinoylpyridine,HSP, and DHP were identified from resting cell reactions of the transformant containing the genecluster and shown to be identical to those of the pyrrolidine pathway reported in wild-type strain Pseudomonasputida S16. The gene for 6-hydroxy-3-succinoylpyridine hydroxylase (HSP hydroxylase) catalyzing HSP directlyto DHP was cloned, sequenced, and expressed in Escherichia coli, and the purified HSP hydroxylase (38 kDa)is NADH dependent. DNA sequence analysis of this 936-bp fragment reveals that the deduced amino acidshows no similarity with any protein of known function.

Polycyclic aromatic heterocycles, such as carbazole, are environmental contaminants suspected of posinghuman health risks. In this study, we investigated the degradation of carbazole by immobilized Sphingomonassp. strain XLDN2-5 cells. Four kinds of polymers were evaluated as immobilization supports for Sphingomonassp. strain XLDN2-5. After comparison with agar, alginate, and carrageenan, gellan gum was selected as theoptimal immobilization support. Furthermore, Fe3O4 nanoparticles were prepared by a coprecipitation method, andthe average particle size was about 20 nm with 49.65-electromagnetic-unit (emu) g-1 saturation magnetization.When the mixture of gellan gel and the Fe3O4 nanoparticles served as an immobilization support, the magneticallyimmobilized cells were prepared by an ionotropic method. The biodegradation experiments were carriedout by employing free cells, nonmagnetically immobilized cells, and magnetically immobilized cells in aqueousphase. The results showed that the magnetically immobilized cells presented higher carbazole biodegradationactivity than nonmagnetically immobilized cells and free cells. The highest biodegradation activity was obtainedwhen the concentration of Fe3O4 nanoparticles was 9 mg·ml-1 and the saturation magnetization ofmagnetically immobilized cells was 11.08 emu g-1. Additionally, the recycling experiments demonstrated thatthe degradation activity of magnetically immobilized cells increased gradually during the eight recycles. Theseresults support developing efficient biocatalysts using magnetically immobilized cells and provide a promisingtechnique for improving biocatalysts used in the biodegradation of not only carbazole, but also other hazardousorganic compounds.