Screening and characterizing soil isolates for polychlorinated biphenyl-transforming competence

Abstract

Polychlorinated biphenyls (PCBs) are recalcitrant environmental pollutants strictly anthropogenic in origin. The detection of genes responsible for biphenyl catabolism can be used for the detection of potentially PCB-transforming genotypes in microbial communities. The objectives of this project were: (l) to isolate and enumerate bacteria potentially capable of transforming PCBs using bipheny! enrichment media, (ii) to determine for the presence of biphenyl/PCB degrading genes using DNA-DNA hybridization, (iii) to study the biodegradation of PCBs by identifying metabolites produced during a defined-congener assay, and (iv) to study the biodegradation of PCBs in soil slurry microcosms. Results showed that less than 12o/o of the bacterial colonies isolated from soils were biphenyl/PGB degraders when a selective media was used but when a nonselective media was used, less than 1o/o wara biphenyl degraders. Aerobic, gram-negative biphenyl/PCB degraders were isolated from all the soil examined. Characterization of the soil isolate indicates that they belong to the genus Comamonas, Pseudomonas, Alcaligenes, Stenotrcphomonas, Sphingomonas and Acinetobacter. DNA isolated from the tropical soil microbiota subjected to hybridization experiments showed homology to the bipheny! genes from Pseudomonas pseudoalcaligenes KF707 but no homology was detected to the biphenyl genes from Comamonas fesfosferoni 8356 or to the genes coding for the dehalogenation of chlorobenzoic acid from the catabolic plasmid pss70. The DNA probes used belong to biphenyl/pcB degrading strains previously isolated from microorganisms in temperate areas of the world. The presence of the biphenyl genes in our soil isolates was detected on DNA from the main bacterial chromosome but not in plasmid DNA. ln the defined-congener assays some of our bacterial isotates were able to transform all the chlorinated congeners examined, inctuding the tetra- and pentachlorobiphenyls. Mono-, di-, and trichlorobenzoic acid were detected by GC-MS as the final products of pCBs degradation. Some of our soil isolates were able to further metabolize the mono- and dichlorobenzoic acid, as evidenced by its disappearance and the detection of parachlorophenol. other expected and not expected degradation products from the biodegradation of biphenyl/pCB were also detected by GC-MS. ln soil slurry microcosm studies the indigenous bacteria were able to transform PCBs, although with the addition of exogenous bacteriathe time for the biotransformation was decreased.