The ability to move on solid surfaces provides ecological advantages for bacteria, yet many bacterial species lack this trait. We found that Xanthomonas spp. overcome this limitation by making use of proficient motile bacteria in their vicinity. Using X. perforans and Paenibacillus vortex as models, we show that X. perforans induces surface motility, attracts proficient motile bacteria and ‘rides’ them for dispersal. In addition, X. perforans was able to restore surface motility of strains that lost this mode of motility under multiple growth cycles in the lab. The described interaction occurred both on agar plates and tomato leaves and was observed between several xanthomonads and motile bacterial species. Thus, suggesting that this motility induction and hitchhiking strategy might be widespread and ecologically important. This study provides an example as to how bacteria can rely on the abilities of their neighboring species for their own benefit, signifying the importance of a communal organization for fitness.

Toxin–antitoxin systems are commonly found on plasmids and chromosomes of bacteria and archaea. These systems appear as biscystronic genes encoding a stable toxin and a labile antitoxin, which protects the cells from the toxin’s activity. Under specific, mostly stressful conditions, the unstable antitoxin is degraded, the toxin becomes active and growth is arrested. Using genome analysis we identified a putative toxin–antitoxin encoding system in the genome of the plant pathogen Acidovorax citrulli. The system is homologous to vapB-vapC systems from other bacterial species. PCR and phylogenetic analyses suggested that this locus is unique to group II strains of A. citrulli. Using biochemical and molecular analyses we show that A. citrulli VapBC module is a bona- fide toxin–antitoxin module in which VapC is a toxin with ribonuclease activity that can be counteracted by its cognate VapB antitoxin. We further show that the transcription of A. citrulli vapBC locus is induced by amino acid starvation, chloramphenicol and during plant infection. Due to the possible role of TA systems in both virulence and dormancy of human pathogenic bacteria, studies of these systems are gaining a lot of attention. Conversely, studies characterizing toxin–antitoxin systems in plant pathogenic bacteria are lacking. The study presented here validates the activity of VapB and VapC proteins in A. citrulli and suggests their involvement in stress response and host-pathogen interactions.

Bacteria have evolved a diverse array of signaling pathways that enable them to quickly respond to environmental changes. Understanding how these pathways reflect environmental conditions and produce an orchestrated response is an ongoing challenge. Herein, we present a role for collective modifications of environmental pH carried out by microbial colonies living on a surface. We show that by collectively adjusting the local pH value, Paenibacillus spp., specifically, regulate their swarming motility. Moreover, we show that such pH-dependent regulation can converge with the carbon repression pathway to down-regulate flagellin expression and inhibit swarming in the presence of glucose. Interestingly, our results demonstrate that the observed glucose-dependent swarming repression is not mediated by the glucose molecule per se, as commonly thought to occur in carbon repression pathways, but rather is governed by a decrease in pH due to glucose metabolism. In fact, modification of the environmental pH by neighboring bacterial species could override this glucose-dependent repression and induce swarming of Paenibacillus spp. away from a glucose-rich area. Our results suggest that bacteria can use local pH modulations to reflect nutrient availability and link individual bacterial physiology to macroscale collective behavior. Significance Individual cells within a community often need to respond to changing environmental conditions at the population level. Accordingly, bacteria have evolved a large array of regulatory mechanisms that enable them to quickly reprogram behavior in a coordinated manner. Unraveling these mechanisms provides valuable information regarding collective behavior in the bacterial world. Herein, we show that by modulating the environmental pH levels, bacterial colonies reflect the nutritional status of the environment and accordingly regulate a collective form of surface motility. We further show that in mixed populations, neighboring bacterial species can change the outcome of such regulation through pH modification and thus affect the community’s spatial organization.

In this study we identify a volatile compound produced by Pseudomonas aeruginosa, which can specifically activate the LuxR quorum-sensing response regulator of Vibrio fischeri. Comparative gas-chromatography analysis between P. aeruginosa wild type and a ΔlasR mutant strain implied that the active volatile is 2-aminoacetophenone. The use of synthetic 2-aminoacetophenone and in silico docking analyses verified the activity of the molecule and provided putative interacting residues within the binding site.