Effects of an Introduced Bacterium on Bacterial Communities on Roots

The objective of this work was to determine whether introduction of a bacterium altered microbial communities associated with roots. We conducted experiments in the field and in a growth chamber to determine whether coating soybean seeds with marked strains derived from the biological control agent Bacillus cereus UW85 affected the bacterial community in the rhizosphere of soybeans. We characterized 2651 individual isolates of bacteria from root—free soil and from soybean seedlings based on 43—50 physiological attributes. Discriminant analysis of the bacterial communities according to these attributes showed that the communities of rhizosphere bacteria that developed on non—treated plants and on plants grown from seeds coated with a single strain of bacteria were sometimes dramatically different. This occurred even when the introduced strain did not persist as a common member of the community. In two of four experiments we could as easily differentiate between bacterial communities on roots of UW85—treated and non—treated seedlings as between the communities in the rhizosphere and root—free soil. In the other two experiments we could differentiate only between communities in root—free soil and on roots. In the comparison of bacteria from root—free soil and from the soybean rhizosphere, we found that bacteria from root—free were more likely to degrade complex carbon sources than were rhizosphere bacteria, whereas bacteria from roots were resistant to more antimicrobial substances and were able to grow on a wider range of simple carbon sources than were bacteria isolated from root—free soil.