Bacterial degradation of chlorophenols and their derivatives

Abstract

Chlorophenols (CPs) and their derivatives are persistent environmental pollutants which are used in the manufacture of dyes, drugs, pesticides and other industrial products. CPs, which include monochlorophenols, polychlorophenols, chloronitrophenols, chloroaminophenols and chloromethylphenols, are highly toxic to living beings due to their carcinogenic, mutagenic and cytotoxic properties. Several physico-chemical and biological methods have been used for removal of CPs from the environment. Bacterial degradation has been considered a cost-effective and eco-friendly method of removing CPs from the environment. Several bacteria that use CPs as their sole carbon and energy sources have been isolated and characterized. Additionally, the metabolic pathways for degradation of CPs have been studied in bacteria and the genes and enzymes involved in the degradation of various CPs have been identified and characterized. This review describes the biochemical and genetic basis of the degradation of CPs and their derivatives.

Figure 1

Chemical structures of chlorophenols and their derivatives.

Figure 2

Bacterial degradation pathways for 4-chlorophenol. (a) 4-Chlorophenol degradation via modified ortho-cleavage, (b) 4-chlorophenol degradation via the meta-cleavage, (c) two pathways of degradation of 4-chlorophenol [4-Chlorocatechol-Benzenetriol pathway (lower) and Hydroquinone pathway (upper)].

Figure 3

Bacterial degradation pathways for 2-chlorophenol via 3-chlorocatechol. (a) Modified ortho cleavage pathway, (b) new modified ortho cleavage pathway, and (c)meta-cleavage pathway of 3-chlorocatechol.

Figure 4

Bacterial degradation pathways for 2,4-dichlorophenol via ortho -cleavage (a) and the distal meta -cleavage (b).

Figure 5

Bacterial degradation pathway for 2,4,6-trichlorophenol (a), and 2,4,5-trichlorophenol (b).

Figure 6

Bacterial degradation pathways for pentachlorophenol in Sphingomonas chlorophenolicum L-1 (a), and Mycobacterium strains (b).

Figure 7

Bacterial degradation pathways for 2-chloro-4-nitrophenol in (a) Burkholderia sp. SJ98, (b) Burkholderia sp. RKJ 800 and Rhodococcus imtechensis RKJ300, and (c) Arthrobacter nitrophenolicus SJCon.

Figure 8

Bacterial degradation pathways for 4-chloro-2-nitrophenol.

Figure 9

Bacterial degradation pathway for 2-chloro-5-nitrophenol (a), and 2-chloro-3-nitrophenol (b).

Figure 10

Bacterial degradation pathways for 4-chloro-2-aminophenol (a), 2-chloro-4-aminophenol (b), and 4-chloro-2-methylphenol (c).

Figure 11

Anaerobic degradation of CPs and poly-CPs. (a) Anaerobic dehalogenation and degradation of pentachlorophenol, (b) Two biotransformation pathways for 2-chlorophenol, and (c) Anaerobic mineralization for 2,4-dichlorophenol.