Matric potential and the survival and activity of a Pseudomonas fluorescens inoculum in soil

The survival and activity of a strain of Pseudomonas fluorescens, chromosomally marked with lux genes, was studied following inoculation into microcosms containing autoclaved or non-autoclaved soil adjusted to matric potentials of −30, −750 and −1500 kPa and incubated for up to 3 months. Viable cell concentrations were determined by dilution plate counting and population activity was measured by luminometry radiorespirometry and dehydrogenase activity. Although increased matric stress appeared to reduce survival, measured by viable cell concentration, effects were not statistically significant (P > 0.05) when variability within replicates was taken into account. Similarly, when all data were taken into account, changes in viable cell concentration were statistically insignificant (P > 0.05) during the incubation. Survival was, however, significantly (P < 0.05) greater in autoclaved soil at −30 and −750 kPa, presumably because of reduced competition and predation. Survival at −1500 kPa was not affected by autoclaving of soil, possibly due to reduced protozoan mobility at this matric potential. Metabolic activity measured by luminescence decreased significantly with time (P < 0.05) and was a sensitive indicator of differences in matric potential, autoclaving and the duration of the incubation. Luminescence following amendment with substrates (potential luminescence) demonstrated the time required for activation of the starved inoculum. Reactivation decreased greatly with prolonged starvation and was not detectable in non-autoclaved soil at −1500 kPa after 28 days. Potential luminesence also indicated the amount of biomass capable of reactivation, which decreased with starvation. Luminescence was more sensitive, reproducible and convenient as a measure of population activity than radiorespirometry and dehydrogenase activity and, in non-autoclaved soil, enabled selective assessment of inoculum activity in the presence of the indigenous microflora. In addition, luminescence was the only activity technique which demonstrated the effects of matric potential and time on population activity. Our study demonstrates the use of luminescence-based marker systems in tracking genetically-modified bacteria in soil, and in selective and sensitive assessment of their metabolic activity. It also highlights the importance of measurement of both viable cell concentration and population activity when assessing the potential environmental effects and efficiency of microbial inocula.