Biodegradation of chlorinated biphenyls and benzoic acids by a Pseudomonas strain

Mitigation of tobacco bacteria wilt with microbial degradation of phenolic allelochemicals

Long-term continuous monoculture cropping of tobacco leads to high incidence of tobacco bacterial wilt (TBW) caused by Ralstonia solanacearum, which threatening world tobacco production and causing great economy loss. In this study, a safe and effective way to control TBW by microbial degradation of phenolic allelochemicals (PAs) was explored. Eleven kinds of PAs were identified from continuous tobacco cropping soil. These PAs exhibited various effects on the growth, chemotaxis and biofilm formation of R. solanacearum. Then we isolated eight strains of Bacillus, one strain of Brucella, one strain of Enterobacter and one strain of Stenotrophomonas capable of degrading these PAs. The results of degradation assay showed that these isolated strains could degrade PAs both in culture solutions and soil. Besides, the incidence of TBW caused by R. solanacearum and deteriorated by PAs were significantly decreased by treating with these degrading strains. Furthermore, six out of eleven isolated strains were combined to degrade all the identified PAs and ultimately sharply reduced the incidence of TBW by 61.44% in pot experiment. In addition, the combined degrading bacteria could promote the plant growth and defense response. This study will provide a promising strategy for TBW control in tobacco production.

Effects of salinity on the treatment of synthetic petroleum-industry wastewater in pilot vertical flow constructed wetlands under simulated hot arid climatic conditions

Petroleum-industry wastewater (PI-WW) is a potential source of water that can be reused in areas suffering from water stress. This water contains various fractions that need to be removed before reuse, such as light hydrocarbons, heavy metals and conditioning chemicals. Constructed wetlands (CWs) can remove these fractions, but the range of PI-WW salinities that can be treated in CWs and the influence of an increasing salinity on the CW removal efficiency for abovementioned fractions is unknown. Therefore, the impact of an increasing salinity on the removal of conditioning chemicals benzotriazole, aromatic hydrocarbon benzoic acid, and heavy metal zinc in lab-scale unplanted and Phragmites australis and Typha latifolia planted vertical-flow CWs was tested in the present study. P. australis was less sensitive than T. latifolia to increasing salinities and survived with a NaCl concentration of 12 g/L. The decay of T. latifolia was accompanied by a decrease in the removal efficiency for benzotriazole and benzoic acid, indicating that living vegetation enhanced the removal of these chemicals. Increased salinities resulted in the leaching of zinc from the planted CWs, probably as a result of active plant defence mechanisms against salt shocks that solubilized zinc. Plant growth also resulted in substantial evapotranspiration, leading to an increased salinity of the CW treated effluent. A too high salinity limits the reuse of the CW treated water. Therefore, CW treatment should be followed by desalination technologies to obtain salinities suitable for reuse. In this technology train, CWs enhance the efficiency of physicochemical desalination technologies by removing organics that induce membrane fouling. Hence, P. australis planted CWs are a suitable option for the treatment of water with a salinity below 12 g/L before further treatment or direct reuse in water scarce areas worldwide, where CWs may also boost the local biodiversity. Graphical abstract.

Comparative Genomics of Pandoraea, a Genus Enriched in Xenobiotic Biodegradation and Metabolism

Comparative analysis of partial gyrB, recA, and gltB gene sequences of 84 Pandoraea reference strains and field isolates revealed several clusters that included no taxonomic reference strains. The gyrB, recA, and gltB phylogenetic trees were used to select 27 strains for whole-genome sequence analysis and for a comparative genomics study that also included 41 publicly available Pandoraea genome sequences. The phylogenomic analyses included a Genome BLAST Distance Phylogeny approach to calculate pairwise digital DNA–DNA hybridization values and their confidence intervals, average nucleotide identity analyses using the OrthoANIu algorithm, and a whole-genome phylogeny reconstruction based on 107 single-copy core genes using bcgTree. These analyses, along with subsequent chemotaxonomic and traditional phenotypic analyses, revealed the presence of 17 novel Pandoraea species among the strains analyzed, and allowed the identification of several unclassified Pandoraea strains reported in the literature. The genus Pandoraea has an open pan genome that includes many orthogroups in the ‘Xenobiotics biodegradation and metabolism’ KEGG pathway, which likely explains the enrichment of these species in polluted soils and participation in the biodegradation of complex organic substances. We propose to formally classify the 17 novel Pandoraea species as P. anapnoica sp. nov. (type strain LMG 31117^T = CCUG 73385^T), P. anhela sp. nov. (type strain LMG 31108^T = CCUG 73386^T), P. aquatica sp. nov. (type strain LMG 31011^T = CCUG 73384^T), P. bronchicola sp. nov. (type strain LMG 20603^T = ATCC BAA-110^T), P. capi sp. nov. (type strain LMG 20602^T = ATCC BAA-109^T), P. captiosa sp. nov. (type strain LMG 31118^T = CCUG 73387^T), P. cepalis sp. nov. (type strain LMG 31106^T = CCUG 39680^T), P. commovens sp. nov. (type strain LMG 31010^T = CCUG 73378^T), P. communis sp. nov. (type strain LMG 31110^T = CCUG 73383^T), P. eparura sp. nov. (type strain LMG 31012^T = CCUG 73380^T), P. horticolens sp. nov. (type strain LMG 31112^T = CCUG 73379^T), P. iniqua sp. nov. (type strain LMG 31009^T = CCUG 73377^T), P. morbifera sp. nov. (type strain LMG 31116^T = CCUG 73389^T), P. nosoerga sp. nov. (type strain LMG 31109^T = CCUG 73390^T), P. pneumonica sp. nov. (type strain LMG 31114^T = CCUG 73388^T), P. soli sp. nov. (type strain LMG 31014^T = CCUG 73382^T), and P. terrigena sp. nov. (type strain LMG 31013^T = CCUG 73381^T).

Microbial transformation and degradation of polychlorinated biphenyls

This paper reviews the potential of microorganisms to transform polychlorinated biphenyls (PCBs). In anaerobic environments, higher chlorinated biphenyls can undergo reductive dehalogenation. Meta- and para-chlorines in PCB congeners are more susceptible to dechlorination than ortho-chlorines. Anaerobes catalyzing PCB dechlorination have not been isolated in pure culture but there is strong evidence from enrichment cultures that some Dehalococcoides spp. and other microorganisms within the Chloroflexi phylum can grow by linking the oxidation of H(2) to the reductive dechlorination of PCBs. Lower chlorinated biphenyls can be co-metabolized aerobically. Some aerobes can also grow by utilizing PCB congeners containing only one or two chlorines as sole carbon/energy source. An example is the growth of Burkholderia cepacia by transformation of 4-chlorobiphenyl to chlorobenzoates. The latter compounds are susceptible to aerobic mineralization. Higher chlorinated biphenyls therefore are potentially fully biodegradable in a sequence of reductive dechlorination followed by aerobic mineralization of the lower chlorinated products.

Degradation and mineralization of 2-chloro-, 3-chloro- and 4-chlorobiphenyl by a newly characterized natural bacterial strain isolated from an electrical transformer fluid-contaminated soil

A bacterium classified as Achromobacter xylosoxidans strain IR08 by phenotypic typing coupled with 16S rRNA gene analysis was isolated from a soil contaminated with electrical transformer fluid for over sixty years using Aroclor 1221 as an enrichment substrate. The substrate utilization profiles revealed that IR08 could grow on all three monochlorobiphenyls (CBs), 2,4'- and 4,4'-dichlorobiphenyl as well as 2-chlorobenzoate (2-CBA), 3-CBA, 4-CBA, and 2,3-dichlorobenzoate. Unusually, growth was poorly sustained on biphenyl and benzoate. In growth experiments, IR08 degraded all CBs (0.27 mmol/L) in less than 96 h with concomitant stoichiometric release of inorganic chloride and growth yields were 2-3 times higher than those observed on biphenyl. In contrast to most of the chlorobiphenyl-degrading strains described in the literature, which are reported to form CBA, no metabolite was identified in the culture broth by HPLC analysis. When co-incubated with respective CBs and biphenyl, strain IR08 preferentially utilized the chlorinated analogues in less than 96 h while it took another 264 h before 90% of the initially supplied biphenyl could be degraded. The promotion of co-metabolic transformation of halogenated substrates by the inclusion of their non-halogenated derivatives may not therefore, result in universal benefits.

Growth on dichlorobiphenyls with chlorine substitution on each ring by bacteria isolated from contaminated African soils

Until recently, it was generally believed that the presence of more than one chlorine substituent prevented chlorinated biphenyls from serving as a sole source of carbon and energy for aerobic bacteria. In this study, we report the isolation of three aerobic strains, identified as Enterobacter sp. SA-2, Ralstonia sp. SA-4, and Pseudomonas sp. SA-6 from Nigerian polluted soils, that were able to grow on a wide range of dichlorobiphenyls (diCBs). In addition to growing on all monochlorobiphenyls (monoCBs), the strains were all able to utilize 2,2'-, 2,4'-, and 2,3-diCB as a sole source of carbon and energy. With the exception of strain SA-2, growth was also sustainable on 3,3'-, and 3,5-diCB. Washed benzoate-grown cells were typically able to degrade 68 to 100% of the diCB (100 ppm) within 188 h, concomitant with a cell number increase of up to three orders-of-magnitude and elimination of varying amounts of chloride. In many cases, stoichiometric production of a chlorobenzoate (CBA) as a product was observed. During growth on 2,2'-, and 2,4'-diCB, organisms exclusively attacked an o-chlorinated ring resulting in the production of 2-CBA and 4-CBA, respectively. A gradual decline in the concentration of the latter was observed, which suggested that the product was being degraded further. In the case of 2,3-diCB, the unsubstituted ring was preferentially metabolized. Initial diCB degradation rates were greatest for 2,4'-diCB (11.2 +/- 0.91 to 30.3 +/- 7.8 nmol/min per 10(9) cells) and lowest for 2,2'-diCB (0.37 +/- 0.12 to 2.7 +/- 1.2 nmol/min per 10(9) cells).

Isolation and preliminary characterization of a 3-chlorobenzoate degrading bacteria

A study was conducted to compare the diversity of 2-, 3-, and 4-chlorobenzoate degraders in two pristine soils and one contaminated sewage sludge. These samples contained strikingly different populations of mono-chlorobenzoate degraders. Although fewer cultures were isolated in the uncontaminated soils than contaminated one, the ability of microbial populations to mineralize chlorobenzoate was widespread. The 3- and 4-chlorobenzoate degraders were more diverse than the 2-chlorobenzoate degraders. One of the strains isolated from the sewage sludge was obtained. Based on its phenotype, chemotaxonomic properties and 16S rRNA gene, the organism S-7 was classified as Rhodococcus erythropolis. The strain can grow at temperature from 4 to 37 degrees C. It can utilize several (halo)aromatic compounds. Moreover, strain S-7 can grow and use 3-chlorobenzoate as sole carbon source in a temperatures range of 10-30 degrees C with stoichiometric release of chloride ions. The psychrotolerant ability was significant for bioremediation in low temperature regions. Catechol and chlorocatechol 1,2-dioxygenase activities were present in cell free extracts of the strain, but no (chloro)catechol 2,3-dioxygenase activities was detected. Spectral conversion assays with extracts from R. erythropolis S-7 showed accumulation of a compound with a similar UV spectrum as chloro-cis,cis-muconate from 3-chlorobenzoate. On the basis of these results, we proposed that S-7 degraded 3-chlorobenzoate through the modified ortho-cleave pathway.

From PCBs to highly toxic metabolites by the biphenyl pathway

The degradation of polychlorobiphenyls (PCBs) by diverse bacteria, including Burkholderia sp. LB400, is incomplete with a concomitant accumulation of metabolic intermediates. In this study, the toxicity of diverse (chloro)biphenyls and of their biotransformation into the first two metabolic intermediates of the biphenyl pathway, were determined for the model bacterium Escherichia coli. Recombinant E. coli strains expressing different subsets of bph genes of strain LB400 accumulated metabolic intermediates from (chloro)biphenyls. During biotransformation of these compounds into metabolic intermediates, the viability and metabolic kinetics were determined. The toxicity of biotransformation of (chloro)biphenyls into different metabolic intermediates of (chloro)biphenyls varied. Dihydrodiols and dihydroxybiphenyls are very toxic metabolites for bacteria even after short incubation times, affecting the cell viability much more than (chloro)biphenyls. When bacteria transformed 2-CB into dihydrodiol or dihydroxybiphenyl, a great decrease of intact cells and abundant cell lysis was observed by transmission electronic microscopy. Cell viability of Burkholderia sp. LB400 and of E. coli exposed directly to 2,3-dihydroxybiphenyl decreased also drastically. The toxicity of metabolites generated during oxidation of PCBs may partly explain the recalcitrance to biodegradation of these pollutants. Conversion of less toxic compounds into products with increased toxicity resembles the bioactivation of xenobiotics in higher organisms.

Metabolic pathways of polychlorinated biphenyls degradation by Pseudomonas sp. 2

Polychlorinated biphenyls (PCBs) included with the commercial mixture Delor 103 were degraded by immobilized cells of aerobic bacterial strain Pseudomonas sp. 2. The ability of the strain to metabolise selected tri- and tetrachlorobiphenyls, and the site of primary attack of the biphenyl skeleton were investigated. It was observed that the amount of residual PCBs was 1-48% of the original PCBs after three weeks of incubation. Identified metabolites indicate that the used bacterial strain attacks the biphenyl skeleton at the 2,3- and 3,4-positions, and it is also able to dehalogenate PCBs. Metabolic pathways of degradation of individual congeners were proposed. Transformation of 2,4- and 2,5-dichlorobenzoic acids by Pseudomonas sp. 2 was also observed.

Occurrence of Tn4371-related mobile elements and sequences in (chloro)biphenyl-degrading bacteria

Tn4371, a 55-kb transposable element involved in the degradation and biphenyl or 4-chlorobiphenyl identified in Ralstonia eutropha A5, displays a modular structure including a phage-like integrase gene (int), a Pseudomonas-like (chloro)biphenyl catabolic gene cluster (bph), and RP4- and Ti-plasmid-like transfer genes (trb) (C. Merlin, D. Springael, and A. Toussaint, Plasmid 41:40-54, 1999). Southern blot hybridization was used to examine the presence of different regions of Tn4371 in a collection of (chloro)biphenyl-degrading bacteria originating from different habitats and belonging to different bacterial genera. Tn4371-related sequences were never detected on endogenous plasmids. Although the gene probes containing only bph sequences hybridized to genomic DNA from most strains tested, a limited selection of strains, all beta-proteobacteria, displayed hybridization patterns similar to the Tn4371 bph cluster. Homology between Tn4371 and DNA of two of those strains, originating from the same area as strain A5, extended outside the catabolic genes and covered the putative transfer region of Tn4371. On the other hand, none of the (chloro)biphenyl degraders hybridized with the outer left part of Tn4371 containing the int gene. The bph catabolic determinant of the two strains displaying homology to the Tn4371 transfer genes and a third strain isolated from the A5 area could be mobilized to a R. eutropha recipient, after insertion into an endogenous or introduced IncP1 plasmid. The mobilized DNA of those strains included all Tn4371 homologous sequences previously identified in their genome. Our observations show that the bph genes present on Tn4371 are highly conserved between different (chloro)biphenyl-degrading hosts, isolated globally but belonging mainly to the beta-proteobacteria. On the other hand, Tn4371-related mobile elements carrying bph genes are apparently only found in isolates from the environment that provided the Tn4371-bearing isolate A5.

A novel bacterium that utilizes monochlorobiphenyls and 4-chlorobenzoate as growth substrates

A bacterial isolate, tentatively named SK-3, was isolated from tertiary lagoon sludge contaminated with polychlorinated biphenyls for over 25 years. SK-3 was able to grow on 2-chlorobiphenyl, 3-chlorobiphenyl, 4-chlorobiphenyl (4-CB), chloroacetate and 4-chlorobenzoic acid (4-CBA) as sole carbon and energy sources. During growth on 4-CB, a stoichiometric amount of chloride ion was produced and growth yields were comparable to those observed during growth on biphenyl. The production of 4-CBA as a metabolite was not observed. Protein yields during growth on 4-CB and biphenyl suggested that the presence of the chlorine did not impede use of carbons on both aromatic rings. Growth on 4-CBA also resulted in nearly stoichiometric production of chloride ion. Benzoate-grown SK-3 was also able to degrade several Aroclor 1242 congeners without the need for a primary substrate of previous growth on biphenyl.

A kinetic distribution model of evaporation, biosorption and biodegradation of polychlorinated biphenyls (PCBs) in the suspension of Pseudomonas stutzeri

Kinetics of distribution of PCBs in an active bacterial suspension of Pseudomonas stutzeri was studied by monitoring the evaporated amounts and the concentration remaining in the liquid medium with the biomass. To determine the biodegradation rate constants of the individual congeners of the PCB formulation Delor 103, a model considering biosorption, evaporation, and primary biodegradation constructed previously was used. Rate constants of biodegradation imply that biodegradation of individual congeners is structure-dependent process. Biodegradability decreases with increasing number of chlorine substituents in the molecule, especially if they are in the ortho and para positions. On the other hand, the increasing number of free ortho and meta positions in the biphenyl molecule leads to better biodegradability. For a simple empirical determination of the influence of the chlorine substitution pattern on biodegradability, the di- and trichlorobiphenyl rate constants of biodegradation were analysed.

Production of metabolites from chlorobiphenyls by resting cells ofPseudomonasstrain LB400 after growth on different carbon sources

Cells of Pseudomonas strain LB400, grown on biphenyl, glucose, or glycerol, transformed polychlorinated biphenyl (PCB) congeners into chlorobenzoic acid (CBA) metabolites. Transformation of the PCB congeners, 2,3-chlorobiphenyl (CBP), 2,2'-CBP, 2,5,4'-CBP, and 2,4,2',4'-CBP, produced the metabolites, 2,3-CBA, 2-CBA, 4-CBA, and 2,4-CBA, respectively. Rates and extents of PCB transformation and metabolite formation were highest with biphenyl-grown cells. Intermediate rates of metabolite production were observed with glycerol-grown cells, and lowest rates of production were found with glucose-grown cells. Regardless of carbon source, the rate of degradation of congeners was faster than the rate of production of CBAs. Relative rates of PCB transformation and metabolite production from different congeners with cells grown on a particular substrate followed the same general order, 2,3-CBA (from 2,3-CBP) > 2-CBA (from 2,2'-CBP) > 4-CBA (from 2,5,4'-CBP) > 2,4-CBA (from 2,4,2',4'-CBP). Pseudomonas strain LB400 appeared unable to grow on any of the chlorobenzoic acids. However, Pseudomonas strain LB400 cells grown on biphenyl appeared capable of degrading 2-CBA and 2,3-CBA but not 4-CBA nor 2,4-CBA. Cells grown on glycerol appeared unable to metabolize any CBAs.Key words: polychlorinated biphenyls, metabolites, Pseudomonas LB400.

Aerobic degradation of polychlorinated biphenyls by Alcaligenes sp. JB1: metabolites and enzymes

In contrast to the degradation of penta- and hexachlorobiphenyls in chemostat cultures, the metabolism of PCBs by Alcaligenes sp. JB1 was shown to be restricted to PCBs with up to four chlorine substituents in resting-cell assays. Among these, the PCB congeners containing ortho chlorine substituents on both phenyl rings were found to be least degraded. Monochloro-benzoates and dichlorobenzoates were detected as metabolites. Resting cell assays with chlorobenzoates showed that JB1 could metabolize all three monochlorobenzoates and dichlorobenzoates containing only meta and para chlorine substituents, but not dichlorobenzoates possessing an ortho chlorine substituent. In enzyme activity assays, meta cleaving 2,3-dihydroxybiphenyl 1,2-dioxygenase and catechol 2,3-dioxygenase activities were constitutive, whereas benzoate dioxygenase and ortho cleaving catechol 1,2-dioxygenase activities were induced by their substrates. No activity was found for pyrocatechase II, the enzyme that is specific for chlorocatechols. The data suggest that complete mineralization of PCBs with three or more chlorine substituents by Alcaligenes sp. JB1 is unlikely.

A meta cleavage pathway for 4-chlorobenzoate, an intermediate in the metabolism of 4-chlorobiphenyl by Pseudomonas cepacia P166

Bacterial degradation of biphenyl and polychlorinated biphenyls proceeds by a well-studied pathway which produces benzoate and 2-hydroxypent-2,4-dienoate (or, in the case of polychlorinated biphenyls, the chlorinated derivatives of these compounds). Pseudomonas cepacia P166 utilizes 4-chlorobiphenyl for growth and produces 4-chlorobenzoate as a central intermediate. In this study we found that strain P166 further transforms 4-chlorobenzoate to 4-chlorocatechol, which is mineralized by a meta cleavage pathway. Key metabolites which we identified include the meta cleavage product (5-chloro-2-hydroxymuconic semialdehyde), 5-chloro-2-hydroxymuconate, 5-chloro-2-oxopent-4-enoate, 5-chloro-4-hydroxy-2-oxopentanoate, and chloroacetate. Chloroacetate accumulated transiently, and slow but stoichiometric dehalogenation was observed.

Degradation of 2,5-dichlorobenzoic acid by Pseudomonas aeruginosa JB2 at low oxygen tensions

From long-term chemostat experiments, variants of Pseudomonas aeruginosa JB2 were obtained which exhibited altered properties with respect to the metabolism of 2,5-dichlorobenzoic acid (2,5-DBA). Thus, unlike the original strain JB2-WT, strain JB2-var1 is able to grow in continuous culture on 2,5-DBA as the sole limiting carbon and energy source. Yet, at a dilution rate of 0.07 h-1 and a dissolved oxygen concentration of < or = 12 microM, even with this strain no steady states with 2,5-DBA alone could be established in continuous cultures. Yet another strain was obtained after prolonged continuous growth of JB2-var1 in the chemostat. It has improved 2,5-DBA degrading capabilities which become apparent only during growth in continuous culture: a lower apparent Km for 2,5-DBA and lowered steady-state residual concentrations of 2,5 DBA. Although with this strain steady states were obtained at oxygen concentrations as low as 11 microM, at further lowered concentrations this was no longer possible. In C-limited continuous cultures of JB2-var1 or JB2-var2, addition of benzoic acid (BA) to the feed reduced the amounts of 2,5-DBA degraded, which was most apparent at low oxygen concentrations (< 30 microM). At higher dissolved oxygen concentrations the addition of BA resulted in increasing cell-densities but did not affect the residual steady state concentration of 2,5-DBA. Indeed, whole cell suspensions from chemostat cultures grown on BA plus 2,5-DBA did show a lower apparent affinity for 2,5-DBA than those from cultures grown on 2,5-DBA alone.(ABSTRACT TRUNCATED AT 250 WORDS)

Transfer and expression of PCB-degradative genes into heavy metal resistant Alcaligenes eutrophus strains

Sites polluted with organic compounds frequently contain inorganic pollutants such as heavy metals. The latter might inhibit the biodegradation of the organics and impair bioremediation. Chromosomally located polychlorinated biphenyl (PCB) catabolic genes of Alcaligenes eutrophus A5, Achromobacter sp. LBS1C1 and Alcaligenes denitrificans JB1 were introduced into the heavy metal resistant Alcaligenes eutrophus strain CH34 and related strains by means of natural conjugation. Mobile elements containing the PCB catabolic genes were transferred from A. eutrophus A5 and Achromobacter sp. LB51C1 into A. eutrophus CH34 after transposition onto their endogenous IncP plasmids pSS50 and pSS60, respectively. The PCB catabolic genes of A. denitrificans JB1 were transferred into A. eutrophus CH34 by means of RP4::Mu3A mediated prime plasmid formation. The A. eutrophus CH34 transconjugant strains expressed both catabolic and metal resistance markers. Such constructs may be useful for the decontamination of sites polluted by both organics and heavy metals.

Genetic construction of PCB degraders

Genetic construction of recombinant strains with expanded degradative abilities may be useful for bioremedation of recalcitrant compounds, such as polychlorinated biphenyls (PCBs). Some degradative genes have been found either on conjugative plasmids or on transposons, which would facilitate their genetic transfer. The catabolic pathway for the total degradation of PCBs is encoded by two different sets of genes that are not normally found in the same organism. The bphABCD genes normally reside on the chromosome and encode for the four enzymes involved in the production of benzoate and chlorobenzoates from the respective catabolism of biphenyl and chlorobiphenyls. The genes encoding for chlorobenzoate catabolism have been found on both plasmids and the chromosome, often in association with transposable elements. Ring fission of chlorobiphenyls and chlorobenzoates involves the meta-fission pathway (3-phenylcatechol 2,3-dioxygenase) and the ortho-fission pathway (chlorocatechol 1,2-dioxygenase), respectively. As the catecholic intermediates of both pathways are frequently inhibitory to each other, incompatibilities result. Presently, all hybrid strains constructed by in vivo matings metabolize simple chlorobiphenyls through complementary pathways by comprising the bph, benzoate, and chlorocatechol genes of parental strains. No strains have yet been verified which are able to utilize PCBs having at least one chlorine on each ring as growth substrates. The possible incompatibilities of hybrid pathways are evaluated with respect to product toxicity, and the efficiency of both in vivo and in vitro genetic methods for the construction of recombinant strains able to degrade PCBs is discussed.

Formation of chlorocatechol meta cleavage products by a pseudomonad during metabolism of monochlorobiphenyls

Pseudomonas cepacia P166 was able to metabolize all monochlorobiphenyls to the respective chlorobenzoates. Although they transiently accumulated, the chlorobenzoate degradation intermediates were further metabolized to chlorocatechols, which in turn were meta cleaved. 2- and 3-Chlorobiphenyl both produced 3-chlorocatechol, which was transformed to an acyl halide upon meta cleavage. 3-Chlorocatechol metabolism was toxic to the cells and impeded monochlorobiphenyl metabolism. In the case of 2-chlorobiphenyl, toxicity was manifested as a diminished growth rate, which nevertheless effected rapid substrate utilization. In the case of 3-chlorobiphenyl, which generates 3-chlorocatechol more rapidly than does 2-chlorobiphenyl, toxicity was manifested as a decrease in viable cells during substrate utilization. 4-Chlorobenzoate was transformed to 4-chlorocatechol, which was metabolized by a meta cleavage pathway leading to dehalogenation. Chloride release from 4-chlorocatechol metabolism, however, was slow and did not coincide with rapid 4-chlorocatechol turnover. Growth experiments with strain P166 on monochlorobiphenyls illustrated the difficulties of working with hydrophobic substrates that generate toxic intermediates. Turbidity could not be used to measure the growth of bacteria utilizing monochlorobiphenyls because high turbidities were routinely measured from cultures with very low viable-cell counts.

Molecular diagnostics for polychlorinated biphenyl degradation in contaminated soils

Molecular diagnostic methods using DNA hybridization with specific gene probes are being developed for the monitoring of microbial populations capable of polychlorinated biphenyl (PCB) degradation in contaminated soils. Evaluation of composite samples from contaminated electrical substation soil by gas chromatography (GC) indicated that the PCBs present in the soil (approximately 200 ppm) resulted from contamination with Aroclor 1248. The PCBs have been weathered or degraded so that the lower molecular weight PCB congeners are no longer present. Microbiological and molecular site characterizations are in progress to determine the abundance of PCB degradative organisms and catabolic genes present. Cloned DNA fragments for the bphC gene (2,3-dihydroxybiphenyl dioxygenase) from the biphenyl/chlorobiphenyl degradative pathways of different organisms were used as gene probes to identify indigenous microorganisms with bphC gene sequences. In colony hybridization experiments, positive signals with the pDA251 gene probe were detected in cultures from both contaminated and uncontaminated soils. The degradative abilities of indigenous microorganisms and an added PCB-degradative bacterial strain were also monitored with [14C]4-chlorobiphenyl mineralization assays and gas chromatography of PCB residues extracted from the soils. Enrichment of the contaminated soil with biphenyl and chlorobiphenyls did not stimulate the indigenous microorganisms to degrade the soil PCB. Nevertheless, enrichment of the contaminated soil with biphenyl and chlorobiphenyl and addition of the PCB-degrading strain Alcaligenes eutrophus GG4202 did result in additional degradation of the soil PCB. The results obtained from these experiments should assist in developing and monitoring a remediation plan for these PCB-contaminated soils.

Enhanced mineralization of polychlorinated biphenyls in soil inoculated with chlorobenzoate-degrading bacteria

An Altamont soil containing no measurable population of chlorobenzoate utilizers was examined for the potential to enhance polychlorinated biphenyl (PCB) mineralization by inoculation with chlorobenzoate utilizers, a biphenyl utilizer, combinations of the two physiological types, and chlorobiphenyl-mineralizing transconjugants. Biphenyl was added to all soils, and biodegradation of 14C-Aroclor 1242 was assessed by disappearance of that substance and by production of 14CO2. Mineralization of PCBs was consistently greatest (up to 25.5%) in soils inoculated with chlorobenzoate degraders alone. Mineralization was significantly lower in soils receiving all other treatments: PCB cometabolizer (10.7%); chlorobiphenyl mineralizers (8.7 and 14.9%); and mixed inocula of PCB cometabolizers and chlorobenzoate utilizers (11.4 and 18.0%). However, all inoculated soils had higher mineralization than did the uninoculated control (3.1%). PCB disappearance followed trends similar to that observed with the mineralization data, with the greatest degradation occurring in soils inoculated with the chlorobenzoate-degrading strains Pseudomonas aeruginosa JB2 and Pseudomonas putida P111 alone. While the mechanism by which the introduction of chlorobenzoate degraders alone enhanced biodegradation of PCBs could not be elucidated, the possibility that chlorobenzoate inoculants acquired the ability to metabolize biphenyl and possibly PCBs was explored. When strain JB2, which does not utilize biphenyl, was inoculated into soil containing biphenyl and Aroclor 1242, the frequency of isolates able to utilize biphenyl and 2,5-dichlorobenzoate increased progressively with time from 3.3 to 44.4% between 15 and 48 days, respectively. Since this soil contained no measurable level of chlorobenzoate utilizers yet did contain a population of biphenyl utilizers, the possibility of genetic transfer between the latter group and strain JB2 cannot be excluded.

Specific removal of chlorine from the ortho-position of halogenated benzoic acids by reductive dechlorination in anaerobic enrichment cultures

Anaerobic enrichment cultures catalysing the reductive dechlorination of chlorinated benzoic acids were obtained from three fresh-water sediments collected from seven different locations. Sub-cultures from these enrichments specifically removed ortho-substituted chlorine from 2,3,6-, 2,3,5- and 2,4,6-trichlorobenzoic acid, yielding chloride and 2,5-, 3,5-, and 2,4-dichlorobenzoic acids, respectively. These reductive dehalogenations were stimulated by the addition of benzoate and/or volatile organic acids. In one of these enrichments dehalogenation of ortho- and/or para-chlorine substituents was also observed from 2,3-, 2,4-, 2,5-, and 3,4-dichlorobenzoic acid, yielding 3- and 4-chlorobenzoate. Removal of meta-chlorines was not observed in any of the enrichments.

Degradation of halogenated aromatic compounds

Due to their persistence, haloaromatics are compounds of environmental concern. Aerobically, bacteria degrade these compounds by mono- or dioxygenation of the aromatic ring. The common intermediate of these reactions is (halo)catechol. Halocatechol is cleaved either intradiol (ortho-cleavage) or extradiol (meta-cleavage). In contrast to ortho-cleavage, meta-cleavage of halocatechols yields toxic metabolites. Dehalogenation may occur fortuitously during oxygenation. Specific dehalogenation of aromatic compounds is performed by hydroxylases, in which the halo-substituent is replaced by a hydroxyl group. During reductive dehalogenation, haloaromatic compounds may act as electron-acceptors. Herewith, the halosubstituent is replaced by a hydrogen atom.

Degradation and mineralization of 3-chlorobiphenyl by a mixed aerobic bacterial culture

A mixed bacterial culture obtained from polychlorinated-biphenyl-contaminated river sediments proved capable of degrading 3-chlorobiphenyl (3-CB) under aerobic laboratory conditions. Almost total mineralization of 150 mg/l of 3-CB occurred when, after 3 days of incubation, the mineral medium was supplied with benzoic acid as a carbon source. Two strains of Pseudomonas capable of degrading the substrate to 3-chlorobenzoic acid and a strain of Pseudomonas fluorescens capable of co-metabolizing this metabolite were selected from the mixed culture. A nearly stoichiometric amount of chloride, which defines the percentage of total mineralization, was eliminated during mixed culture growth.

Chlorinated biphenyl mineralization by individual populations and consortia of freshwater bacteria

Comparative studies were performed to investigate the contribution of microbial consortia, individual microbial populations, and specific plasmids to chlorinated biphenyl biodegradation among microbial communities from a polychlorinated biphenyl-contaminated freshwater environment. A bacterial consortium, designated LPS10, was shown to mineralize 4-chlorobiphenyl (4CB) and dehalogenate 4,4'-dichlorobiphenyl. The LPS10 consortium involved three isolates: Pseudomonas testosteroni (LPS10A), which mediated the breakdown of 4CB and 4,4'-dichlorobiphenyl to 4-chlorobenzoic acid; an isolate tentatively identified as an Arthrobacter sp. (LPS10B), which mediated 4-chlorobenzoic acid degradation; and Pseudomonas putida bv. A (LPS10C), whose role in the consortium has not been determined. None of these isolates contained detectable plasmids or sequences homologous to the 4CB-degradative plasmid pSS50. A freshwater isolate, designated LBS1C1, was found to harbor a 41-megadalton plasmid that was related to the 35-megadalton plasmid pSS50, and this isolate was shown to mineralize 4CB. In chemostat enrichments with biphenyl and 4CB as primary carbon sources, the LPS10 consortium was found to outcomplete bacterial populations harboring plasmids homologous to pSS50. These results demonstrate that an understanding of the biodegradative capacity of individual bacterial populations as well as interacting populations of bacteria must be considered in order to gain a better understanding of polychlorinated biphenyl biodegradation in the environment.