A model that uses the induction phase of lux gene-dependent bioluminescence in Pseudomonas fluorescens HK44 to quantify cell density in translucent porous media

A cooled charge-coupled device (CCD) camera was used to follow the kinetics of induction of lux gene-dependent bioluminescence in Pseudomonas fluorescens HK44 held either in aqueous suspensions minus sand, saturated or unsaturated translucent sand (0.348 and 0.07 cm3 H2O/cm3 of sand, respectively), and at cell densities ranging between 1×106 and 8.5×108 cells/ml. Before O2 availability became a limiting factor, the rate of light emission (L) increased with the square of time (t) and linearly with increasing cell density (c). A nonlinear model was developed that contains a “rate of increase in light emission” constant, B′, which is determined directly from the slope of a plot of √L/c against t. The model predicted the behavior of lux induction in HK44 under a variety of conditions. Similar B′ values were determined [49.0–57.6×10−10 light units/(cell min2)] for cell suspensions held in aqueous medium minus sand, in saturated or unsaturated 40/50 grade sand (0.36 mm grain diameter) and in two other textural classes of translucent sand. Although both the growth phase, and the presence of glucose during lux induction affected the first detectable time (FDT) of bioluminescence by HK44 in sand, the kinetics of induction of light emission were similar among treatments (stationary phase cells plus glucose, B′=61.6±3.2, log phase cells plus glucose, B′=63.2±7.2). The potential exists to use a combination of a CCD camera system, an inducible lux gene containing bioluminescent bacterium, and a light transmission chamber to nonintrusively visualize and quantify in real time the interactions between bacterial growth and unsaturated flow of water and solutes in porous media.