Interactions between soil, toxicant, and a lux-marked bacterium during solid phase-contact toxicity testing

Bioluminescence-based, solid-contact toxicity assays allow test bacterium a nd toxicant to interact at the solid-solution interface. A (lux-marked bact erium, Burkholderia sp. RASC, and 2,4-dichlorophenol (2,4-DCP) were used to characterize these interactions. In the basic bioassay, cells were added t o soil slurries containing 2,4-DCP (0-120 mu g ml(-1)). After 15 min, soil was removed by centrifugation, and bioluminescence in the supernatant was d etermined. Investigation of 2,4-DCP adsorption to soil revealed that sorpti on was linear and not significantly (p > 0.1) affected by the presence of B urkholderia cells. The numbers of culturable Burkholderia cells in the assa y supernatant were 48.2 to 64.8% of the inoculum and independent of the soi l weight. The effect of soil on 2,4-DCP toxicity was investigated by compar ing soil aqueous extract and contact assays. The percentage bioluminescence for the contact assay was consistently higher than the extract assay at al l test concentrations, and counts of viable Burkholderia cells were enhance d by the presence of 2,4-DCP in the contact assay. Expressing results as sp ecific bioluminescence decreased the variability in response and the discre pancy in results between the two protocols. We suggest that solid-contact a ssays need improvement to ensure defined contact between cells and solid ph ase, and that the reporting of specific activity should be emphasized.