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

Abstract

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).