Phenazines are a class of bacterially produced redox-active natural antibiotics that have demonstrated potential as a sustainable alternative to traditional pesticides for the biocontrol of fungal crop diseases. However, the prevalence of bacterial resistance to agriculturally relevant phenazines is poorly understood, limiting both the understanding of how these molecules might shape rhizosphere bacterial communities and the ability to perform a risk assessment for off-target effects. Here, we describe profiles of susceptibility to the antifungal agent phenazine-1-carboxylic acid (PCA) across more than 100 bacterial strains isolated from a wheat field where PCA producers are indigenous and abundant. We found that Gram-positive bacteria are typically more sensitive to PCA than Gram-negative bacteria, and there was significant variability in susceptibility both within and across phyla. Phenazine-resistant strains were more likely to be isolated from the wheat rhizosphere, where PCA producers were also more abundant, compared to bulk soil. Furthermore, PCA toxicity was pH-dependent for most susceptible strains and broadly correlated with PCA reduction rates, suggesting that uptake and redox-cycling were important determinants of phenazine toxicity. Our results shed light on which classes of bacteria are most likely to be susceptible to phenazine toxicity in acidic or neutral soils. In addition, the taxonomic and phenotypic diversity of our strain collection represents a valuable resource for future studies on the role of natural antibiotics in shaping wheat rhizosphere communities. IMPORTANCE Microbial communities contribute to crop health in important ways. For example, phenazine metabolites are a class of redox-active molecules made by diverse soil bacteria that underpin the biocontrol of diseases of wheat and other crops. Their physiological functions are nuanced. In some contexts, they are toxic. In others, they are beneficial. While much is known about phenazine production and the effect of phenazines on producing strains, our ability to predict how phenazines might shape the composition of environmental microbial communities is poorly constrained. In addition, phenazine prevalence in the rhizosphere has been predicted to increase in arid soils as the climate changes, providing an impetus for further study. As a step toward gaining a predictive understanding of phenazine-linked microbial ecology, we document the effects of phenazines on diverse bacteria that were coisolated from a wheat rhizosphere and identify conditions and phenotypes that correlate with how a strain will respond to phenazines.
Keywords: correlates; dryland wheat; efflux pumps; phenazines; resistance; rhizosphere; toxicity.