Population dynamics of an introduced bacterium degrading chlorinated benzenes in a soil column and in sewage sludge

The capacity of the beta-Proteobacterium Pseudomonas sp. strain P51, which degrades chlorinated benzenes, to metabolize 1,2,4-trichlorobenzene (TCB) u nder environmental conditions was tested by its release into two experiment al systems. The first system consisted of laboratory scale microcosms which were operated with and without the addition of TCB and which were inoculat ed with sludge from a wastewater treatment plant. The second system consist ed of a non sterile, water saturated soil column. We determined survival of strain P51 after its introduction and its ability to degrade TCB. The popu lation dynamics was followed by selective plating and applying the polymera se chain reaction (PCR) to detect strain P51 and the chlorobenzene (tcb) ge nes on catabolic plasmid pP51. The results showed a completely different be haviour of strain P51 in the two habitats under the applied conditions. In the soil column the P51 bacteria inoculated the entire area and their popul ation reached 2 x 10(6) cells/g soil. The population remained active since TCB was degraded to concentrations below the detection limit of 30 mu g/l. In the sludge microcosms, the number of strain P51 cells immediately decrea sed from 4 x 10(7) cells/ml to 10(5) cells/ml over a period of 2 days after inoculation, and then the strain disappeared to levels below our detection limit (10(3)-10(4) cells/ml). In the reactor without TCB the population of P51 maintained a stable value of 10(5) cells/ml during 8 days but then als o decreased to levels below the detection limit. In addition, no significan t TCB degradation was found in the sludge reactors. The influence of presen ce of TCB on maintenance of strain P51 in the two habitats is discussed. Th is work demonstrates the possibility to successfully apply preselected stra ins to degrade otherwise poorly degradable substances in complex mixed micr obial communities. However, survival and activity may depend strongly on th e type of system into which the strain is introduced.