Phylogenetic Evidence for Transfer of Pentachlorophenol-Mineralizing Rhodococcus chlorophenolicus PCP-IT to the Genus Mycobacterium

Abstract

We determined the nucleotide sequence of a 16s rRNA gene of Rhodococcus chlorophenolicus PCP-IT (= DSM 43826T) (T = type strain). Sequence comparisons revealed that there was a close relationship between strain PCP-IT and strains belonging to the genus Mycobucteriurn. The sequence data were used to construct a phylogenetic tree, which showed that Mycobucteriurn chubuense is the closest relative of strain PCP-IT. We propose that strain PCP-IT should be transferred to the genus Mycobucteriurn and renamed Mycobacterium chlorophenolicum PCP-IT comb. nov.Historically, the genus Rhodococcus has encompassed a group of aerobic, nonmotile, gram-positive actinomycetes that exhibit considerable morphological diversity, contain mycolic acids, and can be acid fast (18). This genus used to be heterogeneous and included species that contain menaquinone MK-8(H2) and menaquinone MK-9(H2) with eight and nine isoprenoid units, respectively, and one hydrogenated double bond and also contain long-chain mycolic acids with 48 to 66 C atoms. In a subsequent phylogenetic study of this genus in which 16s rRNA sequence data were used, Stackebrandt et al. showed that the species containing menaquinone MK-9( H2) and long-chain mycolic acids with 63 to 69 C atoms clustered together on a phylogenetic tree on a branch that was separate from the branch that contained the other rhodococci (31). Therefore, these species (Rhodococcus bronchialis, Rhodococcus terrae, Rhodococcus rubropertinctus, and Rhodococcus sputi) appeared not to belong to the genus Rhodococcus. The transfer of these taxa to the revived genus Gordona (31) made the genus Rhodococcus more homogeneous chemotaxonomically. Most members of the genus Rhodococcus have type IV cell walls and contain menaquinone MK-8(H2) and mycolic acids with 34 to 64 C atoms. The only exception is the pentachlorophenol-mineralizing actinomycete Rhodococcus chlorophenolicus PCP-IT (T = type strain), which was isolated from lake sediment (1-3). This strain contains predominantly menaquinone MK-9(H2), which is found in strains belonging to the genera Gordona, Tsukamurella, and Mycobacterium, and mycolic acids that have 33 to 43 C atoms, which is typical of the genus Rhodococcus. R. chlorophenolicus PCP-IT has been shown to mineralize pentachlorophenol in liquid cultures, as well as in polluted soils (3, 6,23,29). Moreover, this pentachlorophenol-degrading bacterium has been found to survive longer than the other known pentachlorophenol-mineralizing microorganism, Flavobacterium sp., when it is introduced into natural soil; the reasons for this are not known (7). Determining the phylogenetic location of strain PCP-IT in more detail could provide new insights into the physiological properties of this organism. This could lead to a better understanding of the behavior of strain PCP-IT in environments that are more complex than liquid culture (soil, sediment, sludge, etc.), and it could result in more efficient use of this bacterium in bioremediation processes (6,23,29,36,37).To obtain more information about ...

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