Submitted to: Annual Review of Phytopathology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 5/26/2012
Publication Date: 6/6/2012
Citation: Raaijmakers, J.M., Mazzola, M. 2012. Diversity and natural functions of antibiotices produced by beneficial and pathogenic soil bacteria. Annual Review of Phytopathology. 50:403-424. Interpretive Summary: Soil and plant-associated environments harbor numerous bacteria that produce antibiotic metabolites with specific or broad-spectrum activities against co-existing microorganisms. The function and ecological importance of antibiotics has long been assumed to yield a survival advantage to the producing bacteria in the highly competitive but resource limited soil environments through direct suppression. Although specific antibiotics may enhance producer persistence when challenged by competitors or predators in soil habitats, at sub-inhibitory concentrations antibiotics exhibit a diversity of other roles in the life history of the producing bacteria. Many processes modulated by antibiotics may be inherently critical to the producing bacterium such as the acquisition of substrates or initiation of developmental changes that will ensure survival under stressful conditions. Antibiotics may also have roles in more complex interactions including in virulence on host plants or in shaping the outcomes of multitrophic interactions. The innate functions of antibiotics to producing bacteria in their native ecosystem are just beginning to emerge, but current knowledge already reveals a breadth of activities well beyond the historical perspective of antibiotics as weaponry in microbial conflicts.
Technical Abstract: Soil and plant-associated environments harbor numerous bacterial species that produce antibiotic metabolites. Many of these bacteria have been exploited for the discovery of clinical antibiotics and other therapeutics. In the field of plant pathology, antibiotic-producing bacteria are used as a resource for new antifungal compounds and for biological control of pre- and postharvest diseases. Despite their use as weapons/shields against human, animal and plant pathogenic microorganisms, there is still limited evidence that antibiotics also have this function in situ. In the 1980s -1990s, antibiotics and other secondary metabolites have been considered as ‘overflow or waste products’ or ‘evolutionary leftovers’ (Davies, 1990; Kell et al. 1995). Since then our views of the natural functions of antibiotics have shifted from weapons/shields in microbial interactions to regulators and signaling molecules in microbial communities (Figure 1). In this context, Davies et al. (2006) and Martinez (2008) hypothesized that the principal role of antibiotic resistance genes in microorganisms may be to fine-tune the regulatory roles of antibiotics and to protect microorganisms from overexpressing regulatory molecules. The transition from the signaling function to antibiotic activity is clearly dependent on concentration. Furthermore, antibiotics most likely do not act in isolation but are working together with other metabolites produced by the same cell. This latter aspect should also be considered in interpreting the biological significance of antibiotic production by beneficial and plant pathogenic bacteria.