UTILIZATION OF AROMATIC COMPOUNDS BY BACTERIA. II. FLEXIBILITY OF AROMATIC XENOBIOTICS

Abstract

In this review, the peripheral pathways for the decomposition of aromatic compounds by bacteria are considered. Aromatic compounds can be degraded with participation of bacteria under aerobic and anaerobic conditions. In the presence of oxygen, aromatic compounds can be metabolized by bacteria of genera Pseudomonas, Rhodococcus, Nocardia, Micrococcus, Bacillus, Pseudomonas, Arthrobacter and others. Under anaerobic conditions, decomposition of compounds with a benzene core is carried out by sulfate-reducing, nitrate-reducing and fermenting bacteria.
Degradation of the aromatic compounds is a complex long-term process, which in natural conditions depends on biotic and abiotic factors. Peripheral pathways for the expansion of aroma compounds differ according to their structure, however, they mainly lead to the formation of central intermediates: catechol – under aerobic conditions and benzoyl-CoA – under anaerobic.
The aromatic compounds that are converted via benzoyl-CoA pathway should contain a carboxyl group (that is, an aromatic acid) or carboxylate to form an aromatic acid in one of the first steps of the metabolism. In this way, a destruction of phenol, o-cresol, catechol and hydroquinone occurs. All intermediate compounds of the reducing benzoyate pathway are СoA-thioesters. Aromatic compounds with two or more hydroxyl groups are less stable and more easily degraded by microorganisms. A decomposition of these compounds is not always associated with carboxylation as an initial stage, and subsequent hydroxylation or rearrangement processes provide a reduction in the stability of the benzene ring.
The review considers a novel pathway for degradation of the aromatic compounds, described by B. Sсhink et al., іn which hydroxyhydroquinone is a central intermediate. Using this pathway, nitrate-reducing bacteria decompose resorcinol, a-resorcylate, 3-hydroxybenzoate, gentisic acid and possibly hydroquinone in hydroxylation and decarboxylation reactions.
Due to high mobility and ability to form contaminants in aquifers, hydrocarbon oils are among the most common pollutants in the groundwater. The initial stages of transformation of the aromatic compounds – of the benzene, toluene, ethylbenzene and xylene (BTEХ) by various microorganisms lead to a formation of benzoyl-CoA, which further decomposes along the benzoyl-CoA pathway. Toluene is most easily exposed to biodestruction in anaerobic conditions among the components of BTEХ.
The activities of agricultural enterprises and various industries contribute to a continuous flow of xenobiotics, in particular of the aromatic nature into the environment. An important issue that should be addressed is a search for a variety of methods for cleaning the contaminated environment. An effective and environmentally safe way is bioremediation, as a technology for the use of living organisms to decompose pollutants into less toxic compounds or transform them into carbon dioxide and water. Therefore, more studies are being conducted on the ability of different types of microorga^­nisms to detoxify the environment from pollutants.