Rhs NADase effectors and their immunity proteins are exchangeable mediators of inter-bacterial competition in Serratia.

Many bacterial species use Type VI secretion systems (T6SSs) to deliver anti-bacterial effector proteins into neighbouring bacterial cells, representing an important mechanism of inter-bacterial competition. Specific immunity proteins protect bacteria from the toxic action of their own effectors, whilst orphan immunity proteins without a cognate effector may provide protection against incoming effectors from non-self competitors. T6SS-dependent Rhs effectors contain a variable C-terminal toxin domain (CT), with the cognate immunity protein encoded immediately downstream of the effector. Here, we demonstrate that Rhs1 effectors from two strains of Serratia marcescens, the model strain Db10 and clinical isolate SJC1036, possess distinct CTs which both display NAD(P)+ glycohydrolase activity but belong to different subgroups of NADase from each other and other T6SS-associated NADases. Comparative structural analysis identifies conserved functions required for NADase activity and reveals that unrelated NADase immunity proteins utilise a common mechanism of effector inhibition. By replicating a natural recombination event, we show successful functional exchange of CTs and demonstrate that Db10 encodes an orphan immunity protein which provides protection against T6SS-delivered SJC1036 NADase. Our findings highlight the flexible use of Rhs effectors and orphan immunity proteins during inter-strain competition and the repeated adoption of NADase toxins as weapons against bacterial cells.

Antibacterial and antibiofilm activities of ZIF-67.

Currently, antibiotic-resistant bacteria represent a serious threat to public health worldwide. Biofilm formation potentiates both virulence and antibiotic resistance of bacteria. Therefore, the discovery of new antibacterial and antibiofilm compounds is an issue of paramount importance to combat and prevent hard-to-treat bacterial infections. Zeolitic-imidazolate-frameworks (ZIFs) are metallo-organic compounds known to have various interesting chemical and biological applications, including antibacterial properties. In this study, we synthesized ZIF-67 nanoparticles, formed by imidazolate anions and cobalt cations, and found that they inhibit the growth of Acinetobacter baumannii, Pseudomonas aeruginosa, and Staphylococcus aureus. Sub-inhibitory concentrations of ZIF-67 were also able to significantly reduce the biomass of pre-established biofilms of these pathogenic bacteria. On the other hand, the ZIF-67 nanoparticles had null or low cytotoxicity in mammalian cells at those concentrations showing antibacterial or antibiofilm activities. Thus, our results reveal the potential of ZIF-67 nanoparticles to be used against pathogenic bacteria.

Phenotypic plasticity: The role of a phosphatase family Rap in the genetic regulation of Bacilli.

In the last two decades, an increasing number of bacterial species have been recognized that are able to generate a phenotypically diverse population that shares an identical genotype. This ability is dependent on a complex genetic regulatory network that includes cellular and environmental signals, as well as stochastic elements. Among Bacilli, a broadly distributed family of Rap (Response-regulator aspartyl phosphate) phosphatases is known to modulate the function of the main phenotypic heterogeneity regulators by controlling their phosphorylation. Even more, their related extracellular Phr (Phosphatase regulator) peptides function as signals, creating a cell-cell communication network that regulates the phenotypic development of the entire population. In this review, we examine the role that the Rap phosphatases and their Phr peptides play in the regulation of Bacillus subtilis phenotypic differentiation, and in other members of the Bacillus genus. We also highlight the contribution of these regulatory elements to the fitness of bacterial cells and mobile genetic elements, for example, prophages and conjugative vectors.

Deletion of Rap-Phr systems in Bacillus subtilis influences in vitro biofilm formation and plant root colonization.

Natural isolates of the soil-dwelling bacterium Bacillus subtilis form robust biofilms under laboratory conditions and colonize plant roots. B. subtilis biofilm gene expression displays phenotypic heterogeneity that is influenced by a family of Rap-Phr regulatory systems. Most Rap-Phr systems in B. subtilis have been studied independently, in different genetic backgrounds and under distinct conditions, hampering true comparison of the Rap-Phr systems' impact on bacterial cell differentiation. Here, we investigated each of the 12 Rap-Phr systems of B.subtilis NCIB 3610 for their effect on biofilm formation. By studying single ∆rap-phr mutants, we show that despite redundancy between the cell-cell communication systems, deletion of each of the 12 Rap-Phr systems influences matrix gene expression. These Rap-Phr systems therefore enable fine-tuning of the timing and level of matrix production in response to specific conditions. Furthermore, some of the ∆rap-phr mutants demonstrated altered biofilm formation in vitro and colonization of Arabidopsis thaliana roots, but not necessarily similarly in both processes, indicating that the pathways regulating matrix gene expression and other factors important for biofilm formation may be differently regulated under these distinct conditions.

The ecological impact of a bacterial weapon: microbial interactions and the Type VI secretion system.

Bacteria inhabit all known ecological niches and establish interactions with organisms from all kingdoms of life. These interactions are mediated by a wide variety of mechanisms and very often involve the secretion of diverse molecules from the bacterial cells. The Type VI secretion system (T6SS) is a bacterial protein secretion system that uses a bacteriophage-like machinery to secrete a diverse array of effectors, usually translocating them directly into neighbouring cells. These effectors display toxic activity in the recipient cell, making the T6SS an effective weapon during inter-bacterial competition and interactions with eukaryotic cells. Over the last two decades, microbiology research has experienced a shift towards using systems-based approaches to study the interactions between diverse organisms and their communities in an ecological context. Here, we focus on this aspect of the T6SS. We consider how our perspective of the T6SS has developed and examine what is currently known about the impact that bacteria deploying the T6SS can have in diverse environments, including niches associated with plants, insects and mammals. We consider how T6SS-mediated interactions can affect host organisms by shaping their microbiota, as well as the diverse interactions that can be established between different microorganisms through the deployment of this versatile secretion system.

Impact of Rap-Phr system abundance on adaptation of Bacillus subtilis.

Microbes commonly display great genetic plasticity, which has allowed them to colonize all ecological niches on Earth. Bacillus subtilis is a soil-dwelling organism that can be isolated from a wide variety of environments. An interesting characteristic of this bacterium is its ability to form biofilms that display complex heterogeneity: individual, clonal cells develop diverse phenotypes in response to different environmental conditions within the biofilm. Here, we scrutinized the impact that the number and variety of the Rap-Phr family of regulators and cell-cell communication modules of B. subtilis has on genetic adaptation and evolution. We examine how the Rap family of phosphatase regulators impacts sporulation in diverse niches using a library of single and double rap-phr mutants in competition under 4 distinct growth conditions. Using specific DNA barcodes and whole-genome sequencing, population dynamics were followed, revealing the impact of individual Rap phosphatases and arising mutations on the adaptability of B. subtilis.

Type VI secretion system effector proteins: Effective weapons for bacterial competitiveness.

The Type VI secretion system (T6SS) is a protein translocation nanomachine widespread among Gram-negative bacteria and used as a means to deliver effectors directly into target bacterial or eukaryotic cells. These effectors have a wide variety of functions within target cells that ultimately help the secreting cell gain a competitive fitness advantage. Here, we discuss the different ways in which these effectors can be delivered by the T6SS and the diverse mechanisms by which they exert their noxious action upon recipient cells. We also highlight the existence of roles for T6SS effectors beyond simply the killing of neighbouring cells.

The Ectomycorrhizospheric Habitat of Norway Spruce and Tricholoma vaccinum: Promotion of Plant Growth and Fitness by a Rich Microorganismic Community.

The contribution of the mycorrhizospheric microbes in a stand of ectomycorrhizal Norway spruce (Picea abies) featuring mycorrhiza with the basidiomycete Tricholoma vaccinum was addressed by microbiome analysis and in vitro reconstruction of microbial as well as plant-microbe interactions. The protective role of the mycorrhizal fungus with respect to pathogen attack could be validated against Botrytis cinerea and Heterobasidion annosum in co-cultures revealing reduced pathogen growth, higher survival rate of the spruce trees and reduced symptoms on needles upon symbiosis with T. vaccinum. The community structure was shown to yield a high diversity in ECM forming basidiomycetes of Thelephorales and Agaricales associated with a rich bacterial diversity dominated by Rhizobiales with the most abundant Nitrobacter winogradski (3.9%). Isolated bacteria were then used to address plant growth promoting abilities, which included production of the phytohormone indole-3-acetic acid (performed by 74% of the bacterial isolates), siderophores (22%), and phosphate mobilization (23%). Among the isolates, mycorrhiza helper bacteria (MHB) were identified, with Bacillus cereus MRZ-1 inducing hyperbranching in T. vaccinum, supporting tree germination, shoot elongation, and root formation as well as higher mycorrhization rates. Thus, a huge pool of potential MHB and fungal community with widely distributed auxin-production potential extended the ability of T. vaccinum to form ectomycorrhiza. The forest community profited from the mycorrhizal fungus T. vaccinum, with spruce survival enhanced by 33% in microcosms using soil from the native habitat. A higher fungal abundance and diversity in cases where the tree had died during the experiment, showing that decomposition of plant litter from a dead tree supported a different community. T. vaccinum thus actively structured the community of microorganisms in its habitat.

Dissimilar pigment regulation in Serpula lacrymans and Paxillus involutus during inter-kingdom interactions.

Production of basidiomycete atromentin-derived pigments like variegatic acid (pulvinic acid-type) and involutin (diarylcyclopentenone) from the brown-rotter Serpula lacrymans and the ectomycorrhiza-forming Paxillus involutus, respectively, is induced by complex nutrition, and in the case of S. lacrymans, bacteria. Pigmentation in S. lacrymans was stimulated by 13 different bacteria and cell-wall-damaging enzymes (lytic enzymes and proteases), but not by lysozyme or mechanical damage. The use of protease inhibitors with Bacillus subtilis or heat-killed bacteria during co-culturing with S. lacrymans significantly reduced pigmentation indicating that enzymatic hyphal damage and/or released peptides, rather than mechanical injury, was the major cause of systemic pigment induction. Conversely, no significant pigmentation by bacteria was observed from P. involutus. We found additional putative transcriptional composite elements of atromentin synthetase genes in P. involutus and other ectomycorrhiza-forming species that were absent from S. lacrymans and other brown-rotters. Variegatic and its precursor xerocomic acid, but not involutin, in return inhibited swarming and colony biofilm spreading of Bacillus subtilis, but did not kill B. subtilis. We suggest that dissimilar pigment regulation by fungal lifestyle was a consequence of pigment bioactivity and additional promoter motifs. The focus on basidiomycete natural product gene induction and regulation will assist in future studies to determine global regulators, signalling pathways and associated transcription factors of basidiomycetes.

Lysinibacillus fusiformis M5 Induces Increased Complexity in Bacillus subtilis 168 Colony Biofilms via Hypoxanthine.

In recent years, biofilms have become a central subject of research in the fields of microbiology, medicine, agriculture, and systems biology, among others. The sociomicrobiology of multispecies biofilms, however, is still poorly understood. Here, we report a screening system that allowed us to identify soil bacteria which induce architectural changes in biofilm colonies when cocultured with Bacillus subtilis We identified the soil bacterium Lysinibacillus fusiformis M5 as an inducer of wrinkle formation in B. subtilis colonies mediated by a diffusible signaling molecule. This compound was isolated by bioassay-guided chromatographic fractionation. The elicitor was identified to be the purine hypoxanthine using mass spectrometry and nuclear magnetic resonance (NMR) spectroscopy. We show that the induction of wrinkle formation by hypoxanthine is not dependent on signal recognition by the histidine kinases KinA, KinB, KinC, and KinD, which are generally involved in phosphorylation of the master regulator Spo0A. Likewise, we show that hypoxanthine signaling does not induce the expression of biofilm matrix-related operons epsABCDEFGHIJKLMNO and tasA-sipW-tapA Finally, we demonstrate that the purine permease PbuO, but not PbuG, is necessary for hypoxanthine to induce an increase in wrinkle formation of B. subtilis biofilm colonies. Our results suggest that hypoxanthine-stimulated wrinkle development is not due to a direct induction of biofilm-related gene expression but rather is caused by the excess of hypoxanthine within B. subtilis cells, which may lead to cell stress and death.IMPORTANCE Biofilms are a bacterial lifestyle with high relevance regarding diverse human activities. Biofilms can be beneficial, for instance, in crop protection. In nature, biofilms are commonly found as multispecies communities displaying complex social behaviors and characteristics. The study of interspecies interactions will thus lead to a better understanding and use of biofilms as they occur outside laboratory conditions. Here, we present a screening method suitable for the identification of multispecies interactions and showcase L. fusiformis as a soil bacterium that is able to live alongside B. subtilis and modify the architecture of its biofilms.

Unphosphorylated EIIANtr induces ClpAP-mediated degradation of RpoS in Azotobacter vinelandii.

The nitrogen-related phosphotransferase system (PTSNtr ) is composed of the EINtr , NPr and EIIANtr proteins that form a phosphorylation cascade from phosphoenolpyruvate. PTSNtr is a global regulatory system present in most Gram-negative bacteria that controls some pivotal processes such as potassium and phosphate homeostasis, virulence, nitrogen fixation and ABC transport activation. In the soil bacterium Azotobacter vinelandii, unphosphorylated EIIANtr negatively regulates the expression of genes related to the synthesis of the bioplastic polyester poly-ß-hydroxybutyrate (PHB) and cyst-specific lipids alkylresorcinols (ARs). The mechanism by which EIIANtr controls gene expression in A. vinelandii is not known. Here, we show that, in presence of unphosphorylated EIIANtr , the stability of the stationary phase sigma factor RpoS, which is necessary for transcriptional activation of PHB and ARs synthesis related genes, is reduced, and that the inactivation of genes coding for ClpAP protease complex in strains that carry unphosphorylated EIIANtr , restored the levels and in vivo stability of RpoS, as well as the synthesis of PHB and ARs. Taken together, our results reveal a novel mechanism, by which EIIANtr globally controls gene expression in A. vinelandii, where the unphosphorylated EIIANtr induces the degradation of RpoS by the proteolytic complex ClpAP.

Monitoring Spatial Segregation in Surface Colonizing Microbial Populations.

Microbes provide an intriguing system to study social interaction among individuals within a population. The short generation times and relatively simple genetic modification procedures of microbes facilitate the development of the sociomicrobiology field. To assess the fitness of certain microbial species, selected strains or their genetically modified derivatives within one population, can be fluorescently labelled and tracked using microscopy adapted with appropriate fluorescence filters. Expanding colonies of diverse microbial species on agar media can be used to monitor the spatial distribution of cells producing distinctive fluorescent proteins. Here, we present a detailed protocol for the use of green- and red-fluorescent protein producing bacterial strains to follow spatial arrangement during surface colonization, including flagellum-driven community movement (swarming), exopolysaccharide- and hydrophobin-dependent growth mediated spreading (sliding), and complex colony biofilm formation. Non-domesticated isolates of the Gram-positive bacterium, Bacillus subtilis can be utilized to scrutinize certain surface spreading traits and their effect on two-dimensional distribution on the agar-solidified medium. By altering the number of cells used to initiate colony biofilms, the assortment levels can be varied on a continuous scale. Time-lapse fluorescent microscopy can be used to witness the interaction between different phenotypes and genotypes at a certain assortment level and to determine the relative success of either.

Draft Genome Sequence of the Soil Isolate Lysinibacillus fusiformis M5, a Potential Hypoxanthine Producer.

Lysinibacillus fusiformis strain M5 is a potential hypoxanthine producer that was isolated from clay soil. Here, we present the draft genome sequence that was annotated in order to facilitate future studies of L. fusiformis M5.

Specific Bacillus subtilis 168 variants form biofilms on nutrient-rich medium.

Bacillus subtilis is an intensively studied Gram-positive bacterium that has become one of the models for biofilm development. B. subtilis 168 is a well-known domesticated strain that has been suggested to be deficient in robust biofilm formation. Moreover, the diversity of available B. subtilis laboratory strains and their derivatives have made it difficult to compare independent studies related to biofilm formation. Here, we analysed numerous 168 stocks from multiple laboratories for their ability to develop biofilms in different set-ups and media. We report a wide variation among the biofilm-forming capabilities of diverse stocks of B. subtilis 168, both in architecturally complex colonies and liquid-air interface pellicles, as well as during plant root colonization. Some 168 variants are indeed unable to develop robust biofilm structures, while others do so as efficiently as the non-domesticated NCIB 3610 strain. In all cases studied, the addition of glucose to the medium dramatically improved biofilm development of the laboratory strains. Furthermore, the expression of biofilm matrix component operons, epsA-O and tapA-sipW-tasA, was monitored during colony biofilm formation. We found a lack of direct correlation between the expression of these genes and the complexity of wrinkles in colony biofilms. However, the presence of a single mutation in the exopolysaccharide-related gene epsC correlates with the ability of the stocks tested to form architecturally complex colonies and pellicles, and to colonize plant roots.

Unraveling the predator-prey relationship of Cupriavidus necator and Bacillus subtilis.

Cupriavidus necator is a non-obligate bacterial predator of Gram-negative and Gram-positive bacteria. In this study, we set out to determine the conditions, which are necessary to observe predatory behavior of C. necator. Using Bacillus subtilis as a prey organism, we confirmed that the predatory performance of C. necator is correlated with the available copper level, and that the killing is mediated, at least in part, by secreted extracellular factors. The predatory activity depends on the nutrition status of C. necator, but does not require a quorum of predator cells. This suggests that C. necator is no group predator. Further analyses revealed that sporulation enables B. subtilis to avoid predation by C. necator. In contrast to the interaction with predatory myxobacteria, however, an intact spore coat is not required for resistance. Instead resistance is possibly mediated by quiescence.

The impact of manganese on biofilm development of Bacillus subtilis.

Bacterial biofilms are dynamic and structurally complex communities, involving cell-to-cell interactions. In recent years, various environmental signals that induce the complex biofilm development of the Gram-positive bacterium Bacillus subtilis have been identified. These signalling molecules are often media components or molecules produced by the cells themselves, as well as those of other interacting species. The responses can also be due to depletion of certain molecules in the vicinity of the cells. Extracellular manganese (Mn2+) is essential for proper biofilm development of B. subtilis. Mn2+ is also a component of practically all laboratory biofilm-promoting media used for B. subtilis. Comparison of complex colony biofilms in the presence or absence of supplemented Mn2+ using microarray analyses revealed that genes involved in biofilm formation are indeed downregulated in the absence of Mn2+. In addition, Mn2+ also affects the transcription of several other genes involved in distinct differentiation pathways of various cellular processes. The effects of Mn2+ on other biofilm-related traits like motility, antimicrobial production, stress and sporulation were followed using fluorescent reporter strains. The global transcriptome and morphology studies highlight the importance of Mn2+ during biofilm development and provide an overview on the expressional changes in colony biofilms in B. subtilis.

A Duo of Potassium-Responsive Histidine Kinases Govern the Multicellular Destiny of Bacillus subtilis.

UNLABELLED: Multicellular biofilm formation and surface motility are bacterial behaviors considered mutually exclusive. However, the basic decision to move over or stay attached to a surface is poorly understood. Here, we discover that in Bacillus subtilis, the key root biofilm-controlling transcription factor Spo0A~Pi (phosphorylated Spo0A) governs the flagellum-independent mechanism of social sliding motility. A Spo0A-deficient strain was totally unable to slide and colonize plant roots, evidencing the important role that sliding might play in natural settings. Microarray experiments plus subsequent genetic characterization showed that the machineries of sliding and biofilm formation share the same main components (i.e., surfactin, the hydrophobin BslA, exopolysaccharide, and de novo-formed fatty acids). Sliding proficiency was transduced by the Spo0A-phosphorelay histidine kinases KinB and KinC. We discovered that potassium, a previously known inhibitor of KinC-dependent biofilm formation, is the specific sliding-activating signal through a thus-far-unnoticed cytosolic domain of KinB, which resembles the selectivity filter sequence of potassium channels. The differential expression of the Spo0A~Pi reporter abrB gene and the different levels of the constitutively active form of Spo0A, Sad67, in Δspo0A cells grown in optimized media that simultaneously stimulate motile and sessile behaviors uncover the spatiotemporal response of KinB and KinC to potassium and the gradual increase in Spo0A~Pi that orchestrates the sequential activation of sliding, followed by sessile biofilm formation and finally sporulation in the same population. Overall, these results provide insights into how multicellular behaviors formerly believed to be antagonistic are coordinately activated in benefit of the bacterium and its interaction with the host. IMPORTANCE: Alternation between motile and sessile behaviors is central to bacterial adaptation, survival, and colonization. However, how is the collective decision to move over or stay attached to a surface controlled? Here, we use the model plant-beneficial bacterium Bacillus subtilis to answer this question. Remarkably, we discover that sessile biofilm formation and social sliding motility share the same structural components and the Spo0A regulatory network via sensor kinases, KinB and KinC. Potassium, an inhibitor of KinC-dependent biofilm formation, triggers sliding via a potassium-perceiving cytosolic domain of KinB that resembles the selectivity filter of potassium channels. The spatiotemporal response of these kinases to variable potassium levels and the gradual increase in Spo0A~Pi levels that orchestrates the activation of sliding before biofilm formation shed light on how multicellular behaviors formerly believed to be antagonistic work together to benefit the population fitness.

Motility, Chemotaxis and Aerotaxis Contribute to Competitiveness during Bacterial Pellicle Biofilm Development.

Biofilm formation is a complex process involving various signaling pathways and changes in gene expression. Many of the sensory mechanisms and regulatory cascades involved have been defined for biofilms formed by diverse organisms attached to solid surfaces. By comparison, our knowledge on the basic mechanisms underlying the formation of biofilms at air-liquid interfaces, that is, pellicles, is much less complete. In particular, the roles of flagella have been studied in multiple solid-surface biofilm models but remain largely undefined for pellicles. In this work, we characterize the contributions of flagellum-based motility, chemotaxis and oxygen sensing to pellicle formation in the Gram-positive Bacillus subtilis. We confirm that flagellum-based motility is involved in, but is not absolutely essential for, B. subtilis pellicle formation. Further, we show that flagellum-based motility, chemotaxis and oxygen sensing are important for successful competition during B. subtilis pellicle formation. We report that flagellum-based motility similarly contributes to pellicle formation and fitness in competition assays in the Gram-negative Pseudomonas aeruginosa. Time-lapse imaging of static liquid cultures demonstrates that, in both B. subtilis and P. aeruginosa, a turbulent flow forms in the tube and a zone of clearing appears below the air-liquid interface just before the formation of the pellicle but only in strains that have flagella.