Methods for studying root colonization by introduced beneficial bacteria

Some free-living rhizobacteria are considered as potential biocontrol and plant growth-promoting agents. Successful application of beneficial bacteria as microbial inoculants requires their presence and activity at the appropriate level, but even more, at the right time and place. Various markers are described in the literature to differentiate introduced bacteria from indigenous microflora and to visualize them. These markers are presented together with the methods currently applied to quantify bacterial densities and to characterize the distribution of introduced bacteria. The methods to quantify bacterial densities are either based on bacterial cultivation or not. Different types of microscopical observations, allowing the characterization of the bacterial distribution and location in the rhizosphere, are also described. The respective advantages and limitations of these markers and methods are discussed.