Plant growth-promoting rhizobacteria (PGPR) are exogenous bacteria introduced into agricultural ecosystems that act positively upon plant development. However, amendment reproducibility as well as the potential effects of inoculation upon plant root-associated microbial communities can be sources of concern. To address these questions, an understanding of mutual interactions between inoculants and resident rhizosphere microorganisms is required. Mechanisms used by PGPR can be direct or indirect; the former entails the secretion of growth regulators and the latter occurs through the production of antimicrobial compounds that reduce the deleterious effects of phytopathogens. The different modes of action may lead to different relationships between an inoculant and root microbial communities. Rhizobacterial communities are also affected by the plant, engineered genes, environmental stresses and agricultural practices. These factors appear to determine community structure more than an exogenous, active PGPR introduced at high levels.