Bacteria are social creatures that are able to interact and coordinate behaviors with each other in a multitude of ways. The study of such group behaviors in microbes was coined “sociomicrobiology” in 2005. Two such group behaviors in bacteria are quorum sensing (QS) and biofilm formation. At a very basic level, QS is the ability to sense bacterial density via cell-to-cell signaling using self-produced signals called autoinducers, and biofilms are aggregates of cells that are attached to one another via a self-produced, extracellular matrix. Since cells in biofilm aggregates are in close proximity, biofilms represent an ecologically relevant environment for QS. While QS is known to affect biofilm formation in both Gram-negative and Gram-positive species, in this review, we will focus exclusively on Gram-negative bacteria, with an emphasis on Pseudomonas aeruginosa. We will begin by describing QS systems in P. aeruginosa and how they affect P. aeruginosa biofilm formation. We then expand our review to other Gram-negative bacteria and conclude with interesting questions with regard to the effect of biofilms on QS.

There is an increasing interest in studying interspecies bacterial interactions in diseases of animals and plants as it is believed that the great majority of bacteria found in nature live in complex communities. Plant pathologists have thus far mainly focused on studies involving single species or on their interactions with antagonistic competitors. A bacterial disease used as model to study multispecies interactions is the olive knot disease, caused by Pseudomonas savastanoi pv. savastanoi (Psv). Knots caused by Psv in branches and other aerial parts of the olive trees are an ideal niche not only for the pathogen but also for many other plant-associated bacterial species, mainly belonging to the genera Pantoea, Pectobacterium, Erwinia and Curtobacterium. The non-pathogenic bacterial species Erwinia toletana, Pantoea agglomerans and Erwinia oleae, which are frequently isolated inside the olive knots, cooperate with Psv in modulating the disease severity. Co-inoculations of these species with Psv result in bigger knots and better bacterial colonization when compared to single inoculations. Moreover, harmless bacteria co-localize with the pathogen inside the knots, indicating the formation of stable bacterial consortia that may facilitate the exchange of quorum sensing signals and metabolites. Here we discuss the possible role of bacterial communities in the establishment and development of olive knot disease, which we believe could be taking place in many other bacterial plant diseases.

Although the great majority of bacteria found in nature live in multispecies communities, microbiological studies have focused historically on single species or competition and antagonism experiments between different species. Future directions need to focus much more on microbial communities in order to better understand what is happening in the wild. We are using olive knot disease as a model to study the role and interaction of multispecies bacterial communities in disease establishment/development. In the olive knot, non-pathogenic bacterial species (e.g.Erwinia toletana) co-exist with the pathogen (Pseudomonas savastanoipv.savastanoi); we have demonstrated cooperation among these two species via quorum sensing (QS) signal sharing. The outcome of this interaction is a more aggressive disease when co-inoculations are made compared with single inoculations.In plantaexperiments show that these two species co-localize in the olive knot, and this close proximity most probably facilitates exchange of QS signals and metabolites.In silicorecreation of their metabolic pathways showed that they could have complementing pathways also implicating sharing of metabolites. Our microbiome studies of nine olive knot samples have shown that the olive knot community possesses great bacterial diversity; however. the presence of five genera (i.e.Pseudomonas,Pantoea,Curtobacterium,PectobacteriumandErwinia) can be found in almost all samples.

The ability of diazotrophic Burkholderia to produce two exopolysaccharides that differ by the presence of a terminal glucosyl residue provides insight into polysaccharide function with potentially significant biological consequences in the endophytic-host plant interaction.

Burkholderia kururiensis M130 is one of the few characterized rice endophytes and was isolated from surface-sterilized rice roots. This bacterium shows strong growth-promoting effects, being able to increase rice yields. Here we present its draft genome sequence, which contains important traits for endophytic life and plant growth promotion.