Plasmid-mediated biodegradative fate of monohalogenated biphenyls in facultatively anaerobic sediments

Biodegradation of hexachlorocyclohexane isomers in soil and food environment

Persistence of chlorinated hydrocarbon insecticides in the environment is well documented. One early introduced insecticide, hexachlorocyclohexane (HCH), popularly called BHC, was used in large quantities all over the world until recently. In India, even today, technical grade HCH is being used extensively. Theoretically, HCH has eight possible stereoisomers of which four (alpha, beta, gamma, and delta) predominate in the technical product. These isomers significantly differ between themselves with respect to their persistence and toxicity toward insects, birds, mammals, and other nontarget organisms. The relative proportion of HCH isomers is, therefore, crucial from a toxicology standpoint. This problem assumes importance in light of reports that the HCH isomers undergo interconversion in soil, water, animals, plants, insects, etc. The persistence of HCH can be attributed in part to the interconversion of HCH isomers, which also restrict their solubility. In the present review, biotransformation of HCH isomers, both under aerobic and anaerobic conditions, and their degradation pathways have been described. In addition, emphasis is given to the interconversion of HCH isomers, including interconversion mechanisms, as this area has not received adequate coverage in earlier reviews on HCH.

Biodegradation of halogenated organic compounds

In this review we discuss the degradation of chlorinated hydrocarbons by microorganisms, emphasizing the physiological, biochemical, and genetic basis of the biodegradation of aliphatic, aromatic, and polycyclic compounds. Many environmentally important xenobiotics are halogenated, especially chlorinated. These compounds are manufactured and used as pesticides, plasticizers, paint and printing-ink components, adhesives, flame retardants, hydraulic and heat transfer fluids, refrigerants, solvents, additives for cutting oils, and textile auxiliaries. The hazardous chemicals enter the environment through production, commercial application, and waste. As a result of bioaccumulation in the food chain and groundwater contamination, they pose public health problems because many of them are toxic, mutagenic, or carcinogenic. Although synthetic chemicals are usually recalcitrant to biodegradation, microorganisms have evolved an extensive range of enzymes, pathways, and control mechanisms that are responsible for catabolism of a wide variety of such compounds. Thus, such biological degradation can be exploited to alleviate environmental pollution problems. The pathways by which a given compound is degraded are determined by the physical, chemical, and microbiological aspects of a particular environment. By understanding the genetic basis of catabolism of xenobiotics, it is possible to improve the efficacy of naturally occurring microorganisms or construct new microorganisms capable of degrading pollutants in soil and aquatic environments more efficiently. Recently a number of genes whose enzyme products have a broader substrate specificity for the degradation of aromatic compounds have been cloned and attempts have been made to construct gene cassettes or synthetic operons comprising these degradative genes. Such gene cassettes or operons can be transferred into suitable microbial hosts for extending and custom designing the pathways for rapid degradation of recalcitrant compounds. Recent developments in designing recombinant microorganisms and hybrid metabolic pathways are discussed.

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.

Transduction of linked chromosomal genes between Pseudomonas aeruginosa strains during incubation in situ in a freshwater habitat

Both transduction of single chromosomal loci and cotransduction of closely linked loci were observed between lysogenic and nonlysogenic strains of Pseudomonas aeruginosa in a freshwater habitat. Transductants were recovered at frequencies of 10(-6) to 10(-5) transductants per CFU. Transductants of lysogenized strains were recovered 10- to 100-fold more frequently than were transductants of nonlysogenic parents. Lysogens are thus capable of introducing phages which mediate generalized transduction into the natural microbial community and serving as recipients of transduced DNA. It would appear that lysogeny has the potential of increasing the size and flexibility of the gene pool available to natural populations of bacteria. The ability to generate and select new genetic combinations through phage-mediated exchange can be significant in the face of a continually changing environment and may contribute to the apparent fitness of the lysogenic state in natural ecosystems.

Characteristics and restriction analysis of the 4-chlorobiphenyl catabolic plasmid, pSS50

The plasmid pSS50 is a 53-kilobase self-transmissible plasmid of broad host range that has been isolated from several Alcaligenes and Acinetobacter species. This plasmid has previously been shown to mediate the mineralization of 4-chlorobiphenyl to carbon dioxide and water. Physical characterization of this plasmid by restriction analysis indicates that most hexanucleotide cleavage sites are clustered in a 5-kilobase region, leaving large regions without restriction sites. The paucity of restriction sites is not due to DNA methylation.