Soil Inoculation and Blocker-Mediated Sequencing Show Effects of the Antibacterial T6SS on Agrobacterial Tumorigenesis and Gallobiome

Type VI secretion system drives bacterial diversity and functions in multispecies biofilms

Type VI secretion system (T6SS) plays an essential role in interspecies interactions and provides an advantage for a strain with T6SS in multispecies biofilms. However, how T6SS drives the bacterial community structure and functions in multispecies biofilms still needs to be determined. Using gene deletion and Illumina sequencing technique, we estimated bacterial community responses in multispecies biofilms to T6SS by introducing T6SS-containing Pseudomonas putida KT2440. Results showed that the niche structure shifts of multispecies biofilms were remarkably higher in the presence of T6SS than in the absence of T6SS. The presence of T6SS significantly drove the variation in microbial composition, reduced the alpha-diversity of bacterial communities in multispecies biofilms, and separately decreased and increased the relative abundance of Proteobacteria and Bacteroidota. Co-occurrence network analysis with inferred putative bacterial interactions indicated that P. putida KT2440 mainly displayed strong negative associations with the genera of Psychrobacter, Cellvibrio, Stenotrophomonas, and Brevundimonas. Moreover, the function redundancy index of the bacterial community was strikingly higher in the presence of T6SS than in the absence of T6SS, regardless of whether relative abundances of bacterial taxa were inhibited or promoted. Remarkably, the increased metabolic network similarity with T6SS-containing P. putida KT2440 could enhance the antibacterial activity of P. putida KT2440 on other bacterial taxa. Our findings extend knowledge of microbial adaptation strategies to potential bacterial weapons and could contribute to predicting biodiversity loss and change in ecological functions caused by T6SS.

Influence of Co-occurring Weakly Pathogenic Bacterial Species on Bacterial Spot Disease Dynamics on Tomato

Mixed infections caused by multiple pathogenic and weakly pathogenic strains inhabiting the same host plants are common in nature and may modify pathogen dynamics. However, traditional plant pathogen studies have mostly focused on the binary interaction between a single host and a single pathogen. In this study, we have looked beyond this binary interaction and evaluated the impact of coinfection on disease dynamics on tomato using the bacterial spot pathogen Xanthomonas perforans (Xp), the co-occurring weakly pathogenic strain of X. arboricola (Xa), and the co-occurring potential weak pathogenic strain of Pseudomonas capsici (Pc). Time-series coinfection experiments monitoring disease severity and within-host population dynamics revealed higher disease severity in coinfection by three species compared with infection by Xp alone. However, coinfection by dual species, Xp and Pc, or Xa resulted in lower disease severity compared with Xp alone. Thus, coinfection outcomes depend on interacting species. Weak pathogens could exploit Xp to colonize the host plant as indicated by their higher populations in coinfection. However, Xp population dynamics were dependent on the coinfecting partner. While resource competition might be a possible explanation for lower Xp population in dual coinfection, interaction of Pc with the host was found to influence Xp population. Interestingly, Xp population was higher in the presence of three-species interaction compared with Xp and Xa coinfection, suggesting potential modulation of cooperative interactions among Xp and Xa in three-species coinfection rather than competitive interactions. Humidity played a significant role in population dynamics of the three species. Overall, this study highlighted the importance of coinfection dynamics in studying plant disease outbreaks.

Virulence and Ecology of Agrobacteria in the Context of Evolutionary Genomics

Among plant-associated bacteria, agrobacteria occupy a special place. These bacteria are feared in the field as agricultural pathogens. They cause abnormal growth deformations and significant economic damage to a broad range of plant species. However, these bacteria are revered in the laboratory as models and tools. They are studied to discover and understand basic biological phenomena and used in fundamental plant research and biotechnology. Agrobacterial pathogenicity and capability for transformation are one and the same and rely on functions encoded largely on their oncogenic plasmids. Here, we synthesize a substantial body of elegant work that elucidated agrobacterial virulence mechanisms and described their ecology. We review findings in the context of the natural diversity that has been recently unveiled for agrobacteria and emphasize their genomics and plasmids. We also identify areas of research that can capitalize on recent findings to further transform our understanding of agrobacterial virulence and ecology.