Colonization of contaminated soil by an introduced bacterium: effects of initial pentachlorophenol levels on the survival of Sphingomonas chlorophenolica strain RA2

The survival of a Sphingomonas species that was introduced into pentachloro phenol (PCP)-contaminated soil was monitored with two complementary methods , a respiration-based assay and a most probable number (MPN) technique. Sph ingomonas chlorophenolica strain RA2 is a PCP-mineralizing bacterium that w as introduced into soil contaminated with a range of PCP concentrations (0- 300 mu g PCP g(-1) soil). The population of introduced microorganisms was f ollowed for 170 days using a substrate-induced growth-response method and a MPN assay that specifically targets PCP-mineralizing bacteria. Varying the initial PCP concentration resulted in the emergence of three distinct patt erns of survival. In soil contaminated with 300 mu g PCP g(-1) the populati on of S. chlorophenolica strain RA2 immediately declined following introduc tion, increased by 200-fold and leveled off by the end of the 170-day incub ation. In contrast, populations of S. chlorophenolica strain RA2 declined t o levels below detection limits in uncontaminated soil by the end of the ex periment. Intermediate PCP concentrations (10-100 mu g PCP g(-1) soil) resu lted in the establishment of S. chlorophenolica strain RA2 that slowly decl ined in numbers. These results indicate that Sphingomonas chlorophenolica s train RA2 is an effective colonizer of PCP-contaminated soil but will not p ersist in the absence of PCP.