Factors affecting the movement of bacterial inocula through soil
An understanding of the movement of bacteria in soil is of importance in many areas of microbial ecology. The study of bacterial dispersal is of fundamental importance in understanding the dissemination of soil-borne plant pathogens and symbionts as well as human pathogens introduced into soil. In addition, the increasing interest in the use of bacterial inocula for improved plant nutrition, biological control of plant pathogens and bioremediation of contaminated soils, necessitates the study of movement of such inocula, both to optimise their function and determine their fate in the environment. The fate of inocula is of particular interest when such inocula are non-indigenous (genetically-modified or otherwise), where their impact on the environment outwith the target site is uncertain. Intact soil microcosms were used in the study of factors affecting the movement of bacterial inocula through soils in the presence of percolating water. Two ecophysiologically contrasting bacterial species (Pseudomonas fluorescens and Bacillus subtilis) were used as inocula. The strains were genetically marked with lux genes encoding bioluminescence and with antibiotic resistance markers (chromosomal integrations), these traits were used in the selective enumeration of the introduced bacteria against the indigenous soil populations. The effect of soil type on the leaching of P. fluorescens inocula was investigated using intact soil microcosms sampled from contrasting soils: Craibstone (loamy sand), Insch (sandy loam) and Cruden Bay (clay loam). It was found that cells of the inoculum were leached more rapidly, in greater numbers and over a longer period from the clay loam, than the two lighter textured soils. These differences were attributed to the interaction of the inocula with percolating water, as determined by the respective soil characteristics. Leaching of B. subtilis differed from that of P. fluorescens, in that the rate of movement through soil was slower and the number of colony forming units leached was less. It was found that the colony forming units of B. subtilis leached were predominantly in the form of spores. The differences in leaching of B. subtilis and P. fluorescens were attributed to the greater adsorption of vegetative cells of B. subtilis to the soils and the requirement for formation of spores prior to leaching.