RUNOFF LOSSES OF PSEUDOMONAS-AUREOFACIENS (LACZY) FROM SOIL

Leaching of genetically modified microorganisms (GMMs) through soil pr ofiles is generally not a significant concern, since GMMs typically re main near the soil surface following application. The presence of high numbers of GMMs at the soil surface, however, suggests that losses vi a runoff may occur. Traditional methods of plating nonlabeled bacteria lack precision and are thus seldom used to monitor runoff losses. To examine whether lacZY, a common genetic marker, could be used to evalu ate bacterial runoff from soil, a lacZY(+) strain of Pseudomonas aureo faciens 3732 RN-L11 was used at three different pH levels, with and wi thout wheat (Triticum aestivum L.) cover in a greenhouse experiment. T welve times over a 245 day period, soils were subjected to simulated r ainfall of 84 mm h(-1) for a 15 min duration. Runoff losses and surviv al were quantified at each time. Pseudomonas aureofaciens survived for the longest period at a soil pH of 7; survival was reduced at lower p Hs. The number of cells in runoff were usually related to the number o f cells surviving in the soil. When high soil populations were present , runoff losses often exceeded 10(10) cfu event(-1). When the soil pop ulation declined to low or undetectable levels, the runoff contained f ewer organisms. Runoff losses of 10(8) cfu event(-1), however, were ob served during one runoff event even when the soil population was below the detection limit. This study indicates that lacZY is an effective marker, and that runoff of GMMs may be an important mechanism for move ment to nontarget environments.