Phylogenetic Distribution and Evolution of Type VI Secretion System in the Genus Xanthomonas

Genetic and Functional Diversity Help Explain Pathogenic, Weakly Pathogenic, and Commensal Lifestyles in the Genus Xanthomonas

The genus Xanthomonas has been primarily studied for pathogenic interactions with plants. However, besides host and tissue-specific pathogenic strains, this genus also comprises nonpathogenic strains isolated from a broad range of hosts, sometimes in association with pathogenic strains, and other environments, including rainwater. Based on their incapacity or limited capacity to cause symptoms on the host of isolation, nonpathogenic xanthomonads can be further characterized as commensal and weakly pathogenic. This study aimed to understand the diversity and evolution of nonpathogenic xanthomonads compared to their pathogenic counterparts based on their cooccurrence and phylogenetic relationship and to identify genomic traits that form the basis of a life history framework that groups xanthomonads by ecological strategies. We sequenced genomes of 83 strains spanning the genus phylogeny and identified eight novel species, indicating unexplored diversity. While some nonpathogenic species have experienced a recent loss of a type III secretion system, specifically the hrp2 cluster, we observed an apparent lack of association of the hrp2 cluster with lifestyles of diverse species. We performed association analysis on a large data set of 337 Xanthomonas strains to explain how xanthomonads may have established association with the plants across the continuum of lifestyles from commensals to weak pathogens to pathogens. Presence of distinct transcriptional regulators, distinct nutrient utilization and assimilation genes, transcriptional regulators, and chemotaxis genes may explain lifestyle-specific adaptations of xanthomonads.

Transcriptome profiling of type VI secretion system core gene tssM mutant of Xanthomonas perforans highlights regulators controlling diverse functions ranging from virulence to metabolism

IMPORTANCE

T6SS has received attention due to its significance in mediating interorganismal competition through contact-dependent release of effector molecules into prokaryotic and eukaryotic cells. Reverse-genetic studies have indicated the role of T6SS in virulence in a variety of plant pathogenic bacteria, including the one studied here, Xanthomonas. However, it is not clear whether such effect on virulence is merely due to a shift in the microbiome-mediated protection or if T6SS is involved in a complex virulence regulatory network. In this study, we conducted in vitro transcriptome profiling in minimal medium to decipher the signaling pathways regulated by tssM-i3* in X. perforans AL65. We show that TssM-i3* regulates the expression of a suite of genes associated with virulence and metabolism either directly or indirectly by altering the transcription of several regulators. These findings further expand our knowledge on the intricate molecular circuits regulated by T6SS in phytopathogenic bacteria.

Exopolysaccharides Producing Bacteria: A Review

Bacterial exopolysaccharides (EPS) are essential natural biopolymers used in different areas including biomedicine, food, cosmetic, petroleum, and pharmaceuticals and also in environmental remediation. The interest in them is primarily due to their unique structure and properties such as biocompatibility, biodegradability, higher purity, hydrophilic nature, anti-inflammatory, antioxidant, anti-cancer, antibacterial, and immune-modulating and prebiotic activities. The present review summarizes the current research progress on bacterial EPSs including their properties, biological functions, and promising applications in the various fields of science, industry, medicine, and technology, as well as characteristics and the isolation sources of EPSs-producing bacterial strains. This review provides an overview of the latest advances in the study of such important industrial exopolysaccharides as xanthan, bacterial cellulose, and levan. Finally, current study limitations and future directions are discussed.

A Pan-Global Study of Bacterial Leaf Spot of Chilli Caused by Xanthomonas spp

Bacterial Leaf Spot (BLS) is a serious bacterial disease of chilli (Capsicum spp.) caused by at least four different Xanthomonas biotypes: X. euvesicatoria pv. euvesicatoria, X. euvesicatoria pv. perforans, X. hortorum pv. gardneri, and X. vesicatoria. Symptoms include black lesions and yellow halos on the leaves and fruits, resulting in reports of up to 66% losses due to unsalable and damaged fruits. BLS pathogens are widely distributed in tropical and subtropical regions. Xanthomonas is able to survive in seeds and crop residues for short periods, leading to the infections in subsequent crops. The pathogen can be detected using several techniques, but largely via a combination of traditional and molecular approaches. Conventional detection is based on microscopic and culture observations, while a suite of Polymerase Chain Reaction (PCR) and Loop-Mediated Isothermal Amplification (LAMP) assays are available. Management of BLS is challenging due to the broad genetic diversity of the pathogens, a lack of resilient host resistance, and poor efficacy of chemical control. Some biological control agents have been reported, including bacteriophage deployment. Incorporating stable host resistance is a critical component in ongoing integrated management for BLS. This paper reviews the current status of BLS of chilli, including its distribution, pathogen profiles, diagnostic options, disease management, and the pursuit of plant resistance.