Phenazines and Bacterial Biofilms

Most bacteria in the environment exist in biofilms—structured, surface-attached multicellular communities that are enmeshed in a self-produced polysaccharide matrix. Biofilms allow bacteria to participate is social interactions, survive under harsh conditions and successfully resist antimicrobials, invasion by competitors, predation, and destruction by components of the immune system. Fluorescent Pseudomonas spp. are prolific biofilm formers and some members of the genus have become model organisms for the study of biofilm biology. Several economically important groups of pseudomonads produce phenazines, pigmented, redox-active metabolites that have long been recognized for their broad-spectrum antibiotic activity. The current chapter focuses on the emerging close link between phenazine production and biofilm formation in Pseudomonas spp., and on the important role of phenazines in biofilms associated with human infectious diseases and highly competitive environmental niches such as soil and the plant rhizosphere

Phenazine antibiotics produced by fluorescent pseudomonads contribute to natural soil suppressiveness to Fusarium wilt

Natural disease-suppressive soils provide an untapped resource for the discovery of novel beneficial microorganisms and traits. For most suppressive soils, however, the consortia of microorganisms and mechanisms involved in pathogen control are unknown. To date, soil suppressiveness to Fusarium wilt disease has been ascribed to carbon and iron competition between pathogenic Fusarium oxysporum and resident non-pathogenic F. oxysporum and fluorescent pseudomonads. In this study, the role of bacterial antibiosis in Fusarium wilt suppressiveness was assessed by comparing the densities, diversity and activity of fluorescent Pseudomonas species producing 2,4-diacetylphloroglucinol (DAPG) (phlD+) or phenazine (phzC+) antibiotics. The frequencies of phlD+ populations were similar in the suppressive and conducive soils but their genotypic diversity differed significantly. However, phlD genotypes from the two soils were equally effective in suppressing Fusarium wilt, either alone or in combination with non-pathogenic F. oxysporum strain Fo47. A mutant deficient in DAPG production provided a similar level of control as its parental strain, suggesting that this antibiotic does not play a major role. In contrast, phzC+ pseudomonads were only detected in the suppressive soil. Representative phzC+ isolates of five distinct genotypes did not suppress Fusarium wilt on their own, but acted synergistically in combination with strain Fo47. This increased level of disease suppression was ascribed to phenazine production as the phenazine-deficient mutant was not effective. These results suggest, for the first time, that redox-active phenazines produced by fluorescent pseudomonads contribute to the natural soil suppressiveness to Fusarium wilt disease and may act in synergy with carbon competition by resident non-pathogenic F. oxysporum