Abstract
Bacillus subtilis is long known to produce poly-γ-glutamic acids (γ-PGA) as one of the major secreted polymeric substances. In B. subtilis, the regulation of γ-PGA production and its physiological role are still unclear. B. subtilis is also capable of forming structurally complex multicellular communities, or biofilms, in which an extracellular matrix consisting of secreted proteins and polysaccharides holds individual cells together. Biofilms were shown to facilitate B. subtilis–plant interactions. In this study, we show that different environmental isolates of B. subtilis, all capable of forming biofilms, vary significantly in γ-PGA production. This is possibly due to differential regulation of γ-PGA biosynthesis genes. In many of those environmental isolates, γ-PGA seems to contribute to robustness and complex morphology of the colony biofilms, suggesting a role of γ-PGA in biofilm formation. Our evidence further shows that in selected B. subtilis strains, γ-PGA also plays a role in root colonization by the bacteria, pinpointing a possible function of γ-PGA in B. subtilis–plant interactions. Finally, we found that several pathways co-regulate both γ-PGA biosynthesis genes and genes for the biofilm matrix in B. subtilis, but in an opposing fashion. We discussed potential biological significance of that.
Highlights
The soil bacterium Bacillus subtilis is well-known for its ability to form structurally complex multicellular communities, known as biofilms (Branda et al, 2001; Hamon and Lazazzera, 2001; Kolter and Greenberg, 2006; Aguilar et al, 2007; Vlamakis et al, 2013)
We showed that environmental isolates of B. subtilis were capable of robust biofilm formation and that there was a correlation between biofilm robustness and the biocontrol efficacy of those isolates (Chen et al, 2012, 2013)
Past studies on the putative role of γ-PGA in biofilm formation in B. subtilis led to contradictory conclusions due to usage of different strains, media, and biofilm assays in those studies (Stanley and Lazazzera, 2005; Branda et al, 2006; Morikawa et al, 2006)
Summary
The soil bacterium Bacillus subtilis is well-known for its ability to form structurally complex multicellular communities, known as biofilms (Branda et al, 2001; Hamon and Lazazzera, 2001; Kolter and Greenberg, 2006; Aguilar et al, 2007; Vlamakis et al, 2013). A number of environmental strains of B. subtilis are able to form biofilms on solid agar media and floating pellicles at the air-liquid interface (Branda et al, 2001; Hamon and Lazazzera, 2001). Both types of biofilms demonstrate sophisticated surface architectures. In the field of agriculture, a number of B. subtilis strains have been engineered and widely used as the so-called biological control agent (BCA) for plant protection (Emmert and Handelsman, 1999; Aliye et al, 2008; Ji et al, 2008)
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