Bacteria of the genera Pseudomonas and Bacillus can promote plant growth and protect plants from pathogens. However, the interactions between these plant-beneficial bacteria are understudied. Here, we explore the interaction between Bacillus subtilis 3610 and Pseudomonas chlororaphis PCL1606. We show that the extracellular matrix protects B. subtilis colonies from infiltration by P. chlororaphis . The absence of extracellular matrix results in increased fluidity and loss of structure of the B. subtilis colony. The P. chlororaphis type VI secretion system (T6SS) is activated upon contact with B. subtilis cells, and stimulates B. subtilis sporulation. Furthermore, we find that B. subtilis sporulation observed prior to direct contact with P. chlororaphis is mediated by histidine kinases KinA and KinB. Finally, we demonstrate the importance of the extracellular matrix and the T6SS in modulating the coexistence of the two species on melon plant leaves and seeds.

A number of microorganisms have the ability to thrive in the presence of a range of toxic solvents. Tolerance to these chemicals is a multifactorial process, meaning that bacterial cells use a set of physiological and gene expression changes to overcome the damage imparted by these chemicals. This review focuses mainly on issues related to tolerance to aromatic hydrocarbons and butanol in Pseudomonas, although other microorganisms are also discussed. Pseudomonas putida strains contain a circular chromosome of approximately 6 Mbp which encodes about 5300 genes. A combination of physiological and biochemical assays, a genome-wide collection of mutants and several omics approaches have provided useful information to help identify functions involved in solvent tolerance in P. putida. The solvent response involves fine-tuning of lipid fluidity to adjust membrane functions including impermeabilization, activation of a general stress-response system, increased energy generation and induction of specific efflux pumps that extrude solvents to the medium. These responses are modulated at the transcriptional level by local and global regulators as well as by a number of sRNAs whose levels fluctuate with the presence of solvents in the environment. Taken as a whole these regulatory inputs orchestrate the complex network of metabolic responses observed after solvent addition.